Y - A Q I e N 02 1 SYRAC’JSE UN IV N Y DEPT oc ELECTRICAL A’Y ‘~PUTtR E—ETC F/s 9/5TWO—DTM [NSIONM. OIRCCT rLECTRONIC FOURIER “iSFORM (flEFt) DEvz——rTc (tJUN 77 S I KOWEL’ p 5 KORNREICH, K • LON OAAG5~~76 C—OI62

.I’JCLASSIFIEfl TR~ t7e5

0 h NE _U

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______ I 8

__________

~~~~~~~~~~~~~

Report DAAG53—76 —C—0]62

TWO-DIMENSIONAL DIRECT ELECTRONIC FOURIER

TRANSFORM (DEFT ) DEVICES:

ANALYSIS, FABRICATION , AND EVALUATION

Stephen T. Kowel

Philipp C. KornreichK. V. Loh

Amaresh MahapatraMoosa MehterBruce Eimner

Paul ReckWilliam A. Penn

Dept. of Electrical and Computer Engineering111 Link Hall

Syracuse UniversitySyracuse , New York 13210

30 June 1977

Interim Report for Period10 June 76 — 9 June 77

approved for public release; distribution unlimited

Prepared for:

U.S. Army Night Vision LaboratoryFort Belvoir, Virginia, 22060

>-D D C

~~

________________ ________ _______ a— .~~~~~~~~~~- a - k . , - r T ‘—rn’ - — - - - - - -— ----—— C - V N

S E C U R I T Y C L A S S I F I C A T I O N ~~F T I ? ~ 0* ‘- ~Wher, U~.r, ~~~.,.d)

REPORT DOCUMENTATION PAGE HEJ-DRE COMPLETING FORMI . R E P O R T NUU RF P ~2 GOV T ACC E SSION NO. 3 RE,C IP IEN T ’S C A T A L O G N U M B E R

DAAGS3-76 ~~WI: —_________________________

4, Tlj~~~j~~~~~ cbL j~j,J — Y PE OF REPORT & PERIOD COV~~~~~D

k~’ o — l ) i m e f l S i O f l a I ) I r e c t E cc t ron i Four icr ‘I r ; i n s— 1r~t er i m ,~ep& t.f o r m (OEFT ) M c v i c c s : A n a ly s i s , F a h r i c a t i o n , 10 June 76 —

q j ~rn 77)

~m n I Eva lu at ion ~ ~~~~~~ BER

j 6. —~~3 N T RA CT OR GRANT NUMBER(.)- Step hen T . / K ow e l , P h i i l pp G . / K o r n r e l c h , - ——

K. W .j Loh , A r i a r e s h/ M a h a p a t ra 1 M sa/ Mt ’h t e r , ’ ~~4t~e I ) A A G 5 3 — 7 6 — C — 0 16 2 -.

t~imer , &~~~~~~~~4 W ~~ J4am -A~-- Pau~ -

S PERFORMING O R G A N I Z A T I O N NAME AND ADDRESS & O. PROGRAM ELEM ENT , PROJE CT , TASK

Syracuse U n i v e r s i ty ~ A~~E~ & WOR K W4L T NUMBERS

E l e c t r i c a l and Co m p u t e r Eng in eer ~ ng~” 11j L ink : ia l l ~ i57 627 0 9DH Q S ,_,

Sv r , j ~~t c-~t ’ , NY 13210 DA On 3426

1 1 CO NT R O , L~~N , ‘ ) F F I C E N A M E AND ADDRESS 12. REPORT

I ’S A rm ’.’ si ght V i s i o n L a b o r a t o ry , 30 .Jun ~~ ~~ 77OR S El—N V— SD ‘

~~~~~~~~ 15- NUMBER OF PAGES T7 - -

F t . B e l v o i r , VA 22060 92 ~~~ / ~~iD

14 M O N I T O R I N G A G E N C Y NAME 6 ADDRE SS( I I d i f fe ren t from Controulng Off ice) IS S E C U R I T Y I hII tu ort5J I -

u n c l a s s i f i e dISe . DECLASS I FI C A T I O N / D O W N G R A D I N G

SCHEDUL E

16. D I S T R I B U T I O N S T A T E M E N T (of thu Repor t ) cm r -

Approved for public release ; distribution unlinited . L-’ .‘

• I~z~’~ SEP 13 Lit

IT LIc~~ RI OUT I( ,N S T A T E M E N T (of t he .b.trect entered In f l o c k 70 , If d l f t o r r n t f rom Repor t ) ~~

~~~~ u u’ ~

IS - S u P P L F M E N T A R Y NOTES

Princi pil Invest igators: ‘ tephen T. Kowe l , P h l l ipp (‘, . K o r n r e i ch

19 FF V wO RDS (Co nt ,n.~e on rerere. aid. if n.c.eaa.’y ,d Ide n t i f y by bl o ck numb.r)

optics spectral analysisacoustooptics image transducerssu rf ace a coust ic wav es image convolversFourier theoryim age analysis

20 A B S T R A C T (ContSno • an ,•.‘•ra• .14. If n.c..Iary id id.nSil y b~ block rn..,b r)

We have fabricated an acoustooptical sensor capable of two—dimensional Im-age or signal processing . It Is a monolithic , optically addressed convolver ,

• except that the two transducers propagate orthog nal , ra ther than coilnear ,surface waves. The interaction between Image , acoustics , and electrons is ob-tained in a thin , polycry stalline film of CdS deposited on z—cut LINbO3. Adouble—layer techni que produ ces r epr oduc ib le , stable films with a 20% modulationof the photoconductiv ity proportional to the square of the SAW electric field .‘I’he signal current is detected by an interdi&itpl co_ntact Dattern. madg rom an

DO I jA $ 73 1473 LOITION OF I N0V SS I$ O•SOLETI U n c l a s s i f i e d ~~~~~~~ , a

E SECUR ITY CLASSIFICA TION OP tNI$ PAGE (~~~.n D~~ .d~—

- ~~~‘

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-~~~

• II ’ n c l a s s i 1 1Cc!

SECURITY CLASSIF ICATION OF THIS PAGE(W1,W Dci. SsiI .r.d)

(20 co n t . )

In/-I l film sandw i cit (lepos i t &‘~~ ~n t h e CdSW i t h s i n u so i d al i np u t s , we ( I h t i in a d i f f ‘r en c e f r e qu e n c y L i goal p r o p o r t i o n a l

t o t h e spa t I t i Four i cr I r u i s f o r m of t i l e opt I ( 1 1 pa t t e r n focussed on the f i l m . B yv a r y i n g t h e d r i i n~ f r c q i i ‘ w i CS , Wt ’ p er f o r m a v w ’ t e r s can n i n g in Four ie r s p a c e .Imag e • t I t 1 1 ) 1 1 cat i i l l , t r a c k e d L i t r o n i c i I l y ev er a f u l l 360 ° . The ;en ~~~r cand e t e ct t h e spat i ;I l f r e q u e n c y , o r l e n t ; I t i on , and ;I r ’ lp l I t ude of v a r i a t i o n s in t h eo p t i c a l I n t e n s i t v . P h i a s & ne ; ,s l lr ’m ’n t s ( i f t i e F o u r i e r c o m p o n e n t s , made b y com— -

p a r i n g t he s i gna l wi t i t 1111 e l e c t r on i c a l l v sy n t h e s i z e d I i f f e re n c e f r e q u e n c y , sh owt h e c x ; i ( ’ c t e i l i n ear v a r i a t i o n w i t h f r e q u e n c y , and t h e l i n e a r v a r i a t i o n w i t h d i s —p l a c e m c n t .

Other u~~’s f o r t i l e sensor w ou l d inc lude line scanning, w i t h pulses rep lacing -

s i n u s o i d s , and memory c o n v o l u t i o n w i t h (‘dS CCDs made f rom the CdS f i l m ,

IIpIIIII

‘ T ‘ ‘

~~~~~

- - -------~~ I

p

T ! ric I ai-isi f [edS EC U R IT Y C L A S S I F I C A T I O N OF THIS PAGE(W 1l.n D.Im Fn t.r .d I

Report DAA G53-76-C—0 162

TWO—DIMENSIONAL 1)1 RECT El EC’f RON IC FOURIER

TRANSFORM (DEPI ’) DEVICES:

ANALYSIS , FABRICATION , AND EVALUATION

Stephen T . Kowel

Phl l i pp C. Kornrelch

K . 14 . Loh

Amaresh Mahapat ra

Moosa Mehter

Bruce Enmier

Paul Reck

Wil l iam A. Penn

Dept. of Electrical and Computer Engineering111 Link Hall

Syracuse Universi tySyrac use , New York 13210

30 June 1977

Interim Report for Period

10 June 76 — 9 June 77

approved for public release; distribution unlimited

Prepared for:

U.S. Army Night Vision LaboratoryFor t Belvoir , Virginia , 22060

II

_ _ _ . _ _ _

i

TABLE OF CONTENTS

I. In t roduc t i on 1

I I . Elec t rophotoconduc t ive Sensor 4

11.1. Pr inc ip les cf Operation 7

11.2. Layered ~~~ Fil m Deposi tion and Curin g 1611.3. Measurements of CdS Film Properties 19

11.3.1. Film Thickness 19

11.3.2. Surface Proper ties 20

11.3.3. Film Conductivity 21

11.4. Experimental Confi rmat ion of Pseudo Beam Steering andFourier Imaging 25

11.4 .1. UnidIrect ional Sensor 29

11.4 .2 . The Two—Dimensional Sensor 3411.4.3. Conclusions 46

I I I . DEFT Li gh t Valve 48

IV. A Discussion of the Application of DEFT Spec tral Anal yzers . . . 50

IV .1. Image Reconstruction from DEFT Spectra 51IV.2. Incoherent Reconstruction 51

IV .3. Coherent Optic al Reconstruetors 52IV.4. Conclus ions Regarding DEFT Spectral Analyzers 55

V. Future Work 56

V .1. Near Term Efforts 56

V .2. New Ap plications 59

V I . B ib l iogra ph y 66

Appendix A — Reprint of “J~ o Dimensional—Fourier Imaging of Light UsingAcoustic Pseudo Beam Steering” A—i

Appendix B — Reprint of “Experimental Confirmation of Two DimensionalAcoustic Processing of Images” B—i

Appendix C — Phenomenological Calculation of the Conductance of CdS C—i

p Appendix D — CdS Film Fabrication D—i

Appendix B - Mechanism of the Electropho toconductiv i ty B—i

I— - •- _ _

~~~~~~~~~~~~ ‘ ~~- —.—- -- - — — - - - — ‘a

III I.• INTRODUCTION

I This document serves as the interim report for the period 10 June 1976

to 9 June 1977 on the project entitled , Direct Electronic Fourier Trans—

forms of Images ,~~ funded by the U.S. Army Night Vision Laboratory) The

I ori ginal proposal was submitted to and approved by the Advan,pd~~oncep t

Team, U.S. Army .

I An extensive l i t e r a t u r e alread y exis ts in t roducing DEFT technology

to readers interested in both fundamental device development and appli—

I cations .[1_9] Two recent papers on two—dimensional DEFT ~ensors are in—

I chided as Append ices A and B for reference .

During this year ’s e f f o r t , DEFT technology made very significant

strides . We substantially added to our knowledge of the fundamental na—

‘ture of acoustooptical interactions in CdS. A successful method of fab—

I rica t ing h igh quality CdS films on LiNbO3 s’as developed . This new method ,

I involving layered films , yields stable films even on Z—cut LiNbO3, whose

temperature coefficient of expansion is much larger than other piezoelec—

\ tric materials . Thus the films can be cured at high temperatures with-

ou t cracking and peeling.

I 4 l~~~~~nonstrated that tt~~~ f i lms possess a lar ge surface acoustic

I wave (SAW)—rela ted cone ~tivity modulation proportional to the square

of the SAW—associa ted e lec t r ic f ie ld in the ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ as expected .

I Thus the modulation provided the pseudo—beam steering effect on which

the two—dimensional Fourier transformation property rests.

L ~~~~

‘

~~

.

~~~~~~

._________________ a

1 2

I With appropriate housing, coup ling , and ins trumen tation , we were

I then able to obtain arbitrary two—dimensional Fourier components of

images by f u l l y elec tronic means. Measurements of spatial frequencies ,

their Fourier amplitudes and phase behavior , are reported here.

These are the first devices capable of two—dimensional signal pro—

I cessing using acoustic waves and emp loying no physical motion of the

I device or image . Of c~-urse , the devices on wh ich this repor t is based

need extensive improvements before they can be manufactured for practical

I use. Most of the sensor improvements needed to increase bandwidth , sig-

nal— to—noise ratio , and f i lm uniform ity , are already under way . Our

I analysis of these new sensors indicates that they could be used as cor—

I reiators and convoivers. Indeed , it would seem tha t memory convoivers

and correlators could be fabricated by forming an array of CdS charge—

coupled devices (CCD) whose stored charges can interact with the travel-

ling surface acoustic waves (SAW). Thus our current work on imaging

I could have an impac t on signal processing , yielding signal/signal , image!—

I signal , and image/image correlation and convolution .

-

We also enhanced our understanding of the nature of the reconstruc—

I tion problem . While several options exist , reconstruction of the pic-

ture from the transform signals appears to be the more complex task be—

I cause of the memory function required. Devices , such as coherent light

I valves can use DEFT generated transforms to obtain the inverse transform

optically but they are somewhat cumbersome and costly at present .

I Experiments aimed at demonstrating pseudo—beam steering in a LiNbO3

light valve sandwich structure are continuing. A number of problems

I have been indentified and corresponding improvements are being Implemented.

II

~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - ~~~~•

1 3

I The authors wish to extend their appreciation to Mr. David Helm of

the Night Vision Laboratory , the project monitor , for his encouragement

and active interest. We also wish to acknowledge the participation of

J Mr. Joseph Hannigan of the U.S. Army Engineer Topographic Laboratories,

Fort Beivoir .

II

IIIIIIII

I FJr

- - _ _-

1I

_____________________________II. ELECTROPHOTOCONDUCTIVE SENSOR

I In order to take full advant.’ige of SAW Fourier imaging, it is es—

I sen tial to be able to e lec t ron ica l ly select a rb i t r a ry , two—dimensional ,

Fourier components of arbitrary , two—dimensional images. Previous work

I has been largely confined to obtaining one—dimensional transforms of one—

[10 11] [1 6 7 1dimensional ‘ or two—dimensional images. ‘ ‘ Vari ous techniques

f have been proposed~~~12]

for achieving full two—dimensional capability,

culmina ting in the development of the pseudo beam steering technique

I discussed here.

The direct electronic Fourier transform (DEFT) devices derive sig-

nals representative of Fourier components by acoustically modulating

I photoelectrons . In the electrophotoconductive device , two surface acous-

tic waves, travelling perpendicularly , cross the image , focused on a thin

( photoconductive film (CdS in the current model). It has been demonstrated

that the absorption of light is strongly dependent on app lied elec tric

field , with the generated photocurrent having a component proportional

to the square of the electric fleld. 113 ’~~4’’51 We pr ov ide the ele ctric

fields by propagating the two crossed surf;ii’&’ acoustic waves on a piezo—

F electric substrate (LiNb O3) on which the CdS has been deposited. A full

tensor treatment of the interaction reveals t h a t t h ~ depos i t ed con tac t s

detect a current proportional t o

I i ( t ) ff dx dy F~ (x, v , ~) ! ( x , v i (2 .1)

F where I(x, y) is the image intensit y , x and v ir e the c o o r d i n a t e s on the

film , and E is the electric field perpendicul ar to the p lane .

Since

II.

5

E = E1

cos ( 1t — k1

x) + E 2 cos(w 2 t + k 2 y) (2 . 2 )

where E 1, w1, k

1, refer to the x— dlrected acoustic wave and E2 , e2 , k 2 ,

refer to the y—directed a c o u s t i c wave , E 2 coota ins a term of the form

2~ E

1 E 2 c o s [ ( -

1 —

2) t — (k

1x + k 2 yfl

(2.3)

= E1 E2

c ( ) s [ — ~7 ) t — .

By vary ing the d r i v i n g f r equ e n c i e s , we can vary k , y ie ld ing a signal

term p ropor t iona l to the two—dim ens iona l Four i e r t r a n s f o r m,

2-~ -~ -÷1 (t) exp[j(~~1

— o2

) t ] f d r I ( r ) e x p ( — l k r) ( 2 . 4 )

whi le the other terms from (2.1) can be ignored because they are at d i f -

ferent frequencies. That is , the si gnal behaves as if a new acoustic

wave ha s been crea ted wi th wavevec to r eq ual to the sum of the exci ted

wavevectors. Thus we call this effect “nseudo beam steering. ”

From theore t ica l cons ide ra t ions alone , pseudo beam s tee r ing is

l ikely to be far superior to other proposed techniques f or accomplishing

two—dimens iona l Imag ing . For examp le , the actual creation of a third

sur face wave from non l inear mix ing appears to he a much smaller and less

controllable e f f e c t . [ 1 2 ]

The use of high frequency surface wave transducers requires that

the image be p lac ed on a high spatial frequency carrier. This is ac—

complished in one direction by the Interdigital contact pattern used to

de tec t the signal , and in the perpendicular direc tion by e ither etching

the CdS in to st r ips , or by projecting the image through a grid.

F In thi s chap ter , we first give a complete description of the opera-

tion of the sensor , re ferring to a phoriomenologfcal stud y of the conductivity

I

6

mechanisms . This is fol lowed b y a review of the many a t t empts to make

adequate f i lms of CdS on LiNb O 3, and a f ull descri pt ion of the success-

f ul me thod f ina l ly developed. Included are the measured static and

dynamic f i l m p rope r t i e s and e l ec t ron microscope s tid ies of the su r face .

A f t e r a b r i e f discussion of the c ir c u i t and sh i e ld ing problems ,

we present ex tens ive exper imenta l evidence f r o m two o p e r a t i n g sensors ;

one unidirectional and one fully two—dimensional . This includes measure-

ment of spatial frequencies , amp l it ude and phase behavior. Convincing

evidence of the pseudo—beam steering behavior is found in the data on

image rotation . In these experiments , the physical rotation of an ob-

ject , focused on the sensor , results in the expected disappearance of

the Fourier component signal. The reaquisition of the signal by appro-

priate change of the transducer frequencies permitted the component to

be tracked dur ing a f u l l 3600 r o t a t i on .

The chapter concludes wi th p lans fo r improvemen t in sensor design

and electronic instrumentation .

I

7

11.1. P r i n c i p les of Op er a t i o n

The image sensor c o n si s t s of a Cd S f i l m depos i t ed on a z—cut LIN b O 3

s u b s t r a t e , as shown in F ig . 2 . 1 . 1 . T h i n f i l m m e t a l - o n t o t s , in the form

of an in t e r d i~~i t ; i 1 p a t t e rn , are l e p n s i t e d on to the CdS f i l m . This pat-

t e r n p e r m i t s the c o l l e c t i o n of the t o t a l ph o t o cu r r en t o a r the area of

the CdS f i lm. Indium i ,s used as the c o n t a c t m a t e r i a l s i nce jr is our

experience that i n d i u m makes ohmic contact to CdS.

Be ides p r o v i d i n g e l e c t r i c a l c o n t a c t , t h e c o n t a c t s a lso lorrl a coarse

grating across t he image . ‘I’his , in effect , shifts the Fourier transform

of the image along the k direction to h i~~lier spatial frequencies. It

allows us to generate the Fourier transform of the image with SAW ’s hay—

1mg a limited bandwidth . A similar effect is obtained by etching the CdS

film into stri ps along t h e’ x direction . We here use , for simp li city ,

gratings with equal (lark and light spacings . Unfortunatel y , this type

of grating only exposes 1/4 of the CdS film to the image . Larger film

exposures are possible by making the dark areas of the masks narrower.

Since the photoconductivity of CdS has a term that depends on the

square of the electric fie ld , it Is possible t o obtain a mixing of the

• - electric fields due to the ’ two a co u st i c waves travelling perpendicularl y

to each other. This effect provides the effective st t ’~’ r i n c of the wave

vector corresponding to the spatial frequency ot the ransform .

Now let io~ i ’~~’ t i n ’ f o l l o w i n g p h e n o m e n o l o g i t - a l approach t o show how

the two —dimensional Fourier transform of t h e Image is obtained. In

Appendix C we h ’ r i ’,’t ’ an expression t o r v~irr ent den~ i t v In the CdS film as

a function of the ligh t intensit y I(X , y ) , the T).C. t i o u t I c f i e l d E0 ap-

plied to t b . e l e c t r i . - i l contact , and the el e ctric field E due to the

SAWs in the substrate.

I

~

- - -

~

- - ----

~

- .

~

- - - - -- -~~~~~~~~~~~~ - -~~~~~~~~~~~ _ _

8

Ia: U)0 _J o_ W Z W

0 lii 00~~~~~~~~ -~~ z i.-. F-’— 0 ~ 0~~ Z

~~~~~~~~ _ _ _ _ _ _a) CC ~~ j— () U) W i~i

—*~---—- —— - -—--________________ ——- —

J o W ~~~U) CC Z 0czl~ ~~~~~~~~

tu ci .J 1 0 CL0 C f l~~~X ) lii 00 —(/) 0 (I)

~~~~— CC

~ / \\ ::j- ~~~~/ \ \~~~~ \ \—7 __~c—

~\ \ \ \ U,

~f I ~~~~~~~~~~~ \ \ ~- M ~~~~~ ~:: ~~~~~~~(

ID _ _ _

j L-1~-’ —i --l w -, —j w

a: T~~~ L\ ~~~o 0 ~~~~~~~~ I [

- }-~~~ -.

W O O o~~~~O> 0 > t o >‘4

FI.

H 9

I The d e t a i l s of the s igna l sensed at each small square of CdS are

com p licated by tile fact that the light intensity , l(x , y), may vary with-

in each square . A l s o , the current detected is not proportional to the

I area of each square . For simp licity, let us first assume that the ef—

I fect of the contact pattern and the etching of the CdS (or the project-

ion of the image through a grid perp endicular to the contact lines) is

I to multi p ly the m are intensity T 1(x, y) by a samp l ing grid func ti on ,

S(x, y) , and by an aperture function , A ( x , y), to form a new image , I (x , y ) ,

I(x, y) = I .(x, v)S(x , v ) A ( x , v) (2.1.1)

where S(x, y) depends on tile geometry of the samp l in g. The simp lest

I model for S wou ld he

S1

(x , y) ah~ ~ ~(x- ma)~~(y - nh)

I m — ~-

ab~ S ( x - ma , y — nh)

I m _ c ~

= comb(x/a) cornb(y/b). (2.1.2)

Fi g. (2.1.1) shows tile special case for which h — a , prov idin g un i f o r m

samp l ing in both directions . As we shall see , there are possible advan—

I tages in hav ing h ~ a, h ut all devices built so far do have b = a.

A more realistic sampling function for the sensor Is obtained by

I breaking the image into squares ,

I S2

(x , y) — rect112(x) rect~ /2

(y ) * S1(x, v) (2.1.3)

where * indica tes convo lu tio n and

II

-~ -~~~ - - - _~~~-

10

1 j x ~ a /2

$ rec t12

(x ) = (2 . 1 . 4 )0 ~~ > a/2

INext we note that the image space is truncated by a device aperture

fun ct ion , A(x , y), given by

A (x , y) = rectM(x) rect.~ (y) (2.1.5)

where (Ma)(Nb ) is the total area of the CdS film spanned by the contacts.

Thus the final image is

I (x , y) = I1

(x , y)[rectM

(x ) rec t~~~

(y ) ] .

[comb(x/a) comb(y/b)] * [rect,2

(x)rectb,2(y)].

(2 .1 .6)

Now we are in a position to calculate the detected signal current

for the image of (2.1.6). The surface current density at the difference

frequency is given by (C.11). The detected curren t is

isig

( t ) = f dx K51g

(X I ib , t ) ( 2 . 1 . 7 )

since the cur ren t is samp led at speci f ic po in t s a long the y—direc t ion

by the con tac ts , ra ther than being measurea ~t all poin ts be tween the

contacts. We use infinite limits here since we include the sensor geo-

metry in the expression for the I(x, y). Now since the principal value

of the two—dimensional sampling is to put the original image I~ on a

spatial carrier whose periods are a and b in the x and y directions re—

spec tively , it is reasonable to assume that I~ only varies appreciably

over many squares. In this case the sum in I can be replaced by an

integration ,

I

$4

Isig

(t) = ~ f dy f dx K

sig(X~

y , t ) (2 .1 .8 )

I which we can write as a comp lex phasor curren t

I I a ~~

- f dy f dx I(x , y) exp—j (k x + k~,y) (2 .1 .9 )

a~~-FfI (x , y)}. (2.1.10)

Let us now examine FfI (x , y ) } , from (2.1.1)

F~ I(x, y ) } =

~~~~~~~~ fy

) * S2

(f , f ) * A ( f , f ) (2.1.11)

where = kx/21T

~ f~ = ky/2~

T~ spa tial freq uencies normall y used in Fourier

I op tics [20], and the functions 1, S, and A are the respective Fourier

transforms of the func tions 1, 5, and A. We see at once that

I (f f ) * ‘~ (f , f )

i x y 2 x yI sin uf a/2 sin rf b/2

= ‘i * [ S

1 . __________ . 1 (2.1.12)

I x

yielding,

sinrf a/2 sin nf b/2x

_ _ _ _ _ _

L ~fx y

* (ab) comb(af~) comb (bf

y)1 (2.1.13)

I sin r f a/2 sin r f b/2-

‘ = (

~~~ iT ~ [I (f , ~~~~

(2.1.14)

I x

where

I ~~~~~ f~) - 11(f - ~~~, f~

- ~~

) . (2. 1.15)m — n~ —=

I Thus the effect of sampling is to reproduce the image transform over and

i over again . With b a, these reproductions are centered at (0, 0),

(1/a , 1/a), (—1/a, —1/a), e tc.

I.

1 12

We chose to operate with transducers whose center frequencies cor—

~ respond to spatial frequency (I/a , 1/a) and which have a bandwidth of ap-

proximately 20%. It is important that the image transform I . have a

I bandwidth limited to approximatel y the ’ same 20% otherwise the rep lica-

tions will overlap and cause distortion . This statement is equivalent

I to our previous assumption of slow ly spatially vary ing I1

(x , y) .

I Note also the e f f e c t of the rectangular nature of the actual sampl-

ing grid. It serves to envelope the transform with a sinc function along

I each axis whose local maximum coincides with the centers of the rep lica ted

spaces as shown in (2 .1.14) .

• I It is convenient to define the space centerdd at (1/a , 1/a) as the

reduced Fourier space by introducing g , g , where

f = -~~ + g (2.1.16)x a X

and

f = ~- + g . (2.1.17)y a y

The e f f e c t of samp ling can now he w r i t t e n as

I I ( f , f ) = I1(g, g) . (2.1.18)

I Now we can include the last f a c t o r in ~~~~~~~~~~

fy) from (2.1.1), the e f f e c t

of the finite device aperture. From (2.1.11) ,

sin liN af sin liMaf

~‘x ’

F ) = 11(g, g~) * [ lTf

X (2.1.19)

I x

where we assume a square sensor contact region , Ma on a side . Thus the

I effect of the finite device size is to smooth the reduced Fourier space

_________ _ _ _ _ _ _ _ _

a

13

with a sine function which is reduced to 1% of its center value (for

$ M = 100) at the edge of the space. The main lobe has a bandwidth of

1% of the center frequency which imp lies that data will be smoothed out

over one per cent of the center frequency . Indeed , this is observed in

the devices. It is interesting, in this regard , to note the Shannon—

Whi t t ake r Sampl ing Theorem~2 0 ’ , which s t a t e s t h a t , for a space l imi ted

f u n c t i o n , evenly spaced samp le s in f req uen cy space , separa ted by (1/Ma),

wi l l s u f f i c e to comp le te l y c h a r a c t e r i z e the f u n c t i o n . This is the smooth—

ing width ! Therefore , it is theoretically possible to reconstruct the

f u n ct t o n 11

(x , y ) . In practice , however , the f a c t tha t the current de-

tected at each square of CdS is an averaged version of the optical in—

tensity over the square , the fact that the sampling in real space pro-

duces possibl y overl app ing reduced spaces , and finite bandwidth all pre—

vent precise reconstruction .

Finally , then , we have that the signal current phasor at the dif-

ference temperal frequency (w1 ~2) is

I = ~ (c~~ + o~~ )E1

E2

E0 ~~~~~~~~~~~~ g ) . (2.1.20)

sin irf /2 sin ITf /2/2 h f /2 (2.1.20)

x y

where we Introduce standard elasticity notation from Appendix C , and def ine

T as the smoothed version of the transform of the image intensity (2.1.19).f From (C.l0) we calculate the bias , or d.c., val ue of the curren t

to be

1d.c.

= b

~~: dy dx K

0(x , y , t) (2.1.21)

— b

odEO

f dy f I(x, y)dx (2.1.22)

Fwhere

- -

- °L +

~~~ E~ + f (o~~ + o~~ ) E~ + o~~ E~

[ + ~~(ch~~~ + ~ )(E~ + ~~~

(2 .1 ,23)

____________________________________________________

14

Subs tituting (2.1.22) into (2.1.20) eliminates E0

to yield

LE LE

~°13 + 066) —

= 1d . c . °d E 1 E 2 11

( g , g ) >

sin ,if /2 sin irf /2

iTfy~ 2

( 2 . 1. 2 4 )

where

I > 11 /Jdx fdv I(x, y). (2.1.25)

Thus we have comp leted our model showing how the geometry of the sensor

when properly coup led to the acoustics yields a two—dimensional Fourier

transform of the image . It is interesting to note that a choice of S(x , y )

which uses the same samp ling rate in both directions results in the Situ-

ation tha t, for =

~2’ no output is obtained unless very special cir-

cui ts are emp l oyed to measure this d.c. signal. Thus the transform com-

ponents along the line g = g will not be obtained. This problem can

be avoided by usin g a/h equa l to an irra t ional number su f f i c i en tly large

to make this line pass outside of the reduced space . Fig. (2.1.1) and

Fig. (2.1.2) show the ~~~

f~ and g , g spaces schematically. Next we

discuss the experimental confirmation of this theory .

IIII ‘-

-—- U

I /~~

/~I _ _ _

_ I ,_______—___ I! I I ~~~~~~~~~1

I a

II IM A X

-

_ _______

2 N A

I //~~~~

~~M A X

IFig. 2.1.2 Schematic representatation o . the original Fourier space (f , f~)

and the reduced Fourier space (g , g ) . The value of is

I the number of Fourier components available in the reduc:d space

and is determined by transducer design .

I

16

II. 2. Layered CdS Film Deposition and Curing

The most difficult task encountered was t i l e p r o d u c t i o n of good pho to—

conductive thin films of CdS . Since we had been making such films or.

I glass for several years , we had hoped that the techni que for making the

I f i lms could be used rela t ively intact . (Glass cannot he used as a sub-

strate since there is no travelling electric field accompanying the SAW.)

I U n f o r tuna tel y , this did not prove to be the case.

On the glass substrates CdS was evaporated out of a fused qu a r t z

I b o t t l e onto s u i t a b ly pos i t ioned s u b s t r a t e s in a vacuum of 10 Torr .

These evaporated films had very high conductivity but a poor light—to --

I dark conductivity ratio. To increase their sensitivity they were cured

at 650° C under a N2

f low which had traces of 02 , HC1, and Cu , to optim-

ize the photoconducting properties of the films . We finally succeeded

I in ob tain ing L/D ratios of about l0~ at 275 m.cd . At the same time , the

conductivity of the films was quite high , about 0.1 nanomho/ 1—m .cd . This

I is eseential to obtain reasonable signals with sensor contact lines hav-

ing ab out 20 ,000 squares in parallel.

We hoped tha t by suitab ly adjusting the doping levels of 02 , HC 1 and

I Cu , one could get good photoconducting films of CdS on LiNbO 3. However ,

after numerous attempts , we came to the conclusion that this is not pos—

I sible due to the degrading effects of material diffusing out of the LiNb O3

during curing.

Next , we decided to overcome the problem of the substrate entirely,

I by laying down a thin layer of Si02

be tween the sub stra te and the CdS

film. This layer would insulate the CdS from the LiNbO3. At the same

I time , Sb 2 being stable thermally, would not itself diffuse into the CdS .

I This idea proved successful in that we improved the L/D ratios by a fac-

tor of 100 in our first attempts .

I

17

Unfortunately, the Si02

lave~ had it s own problems . It worked fairly

well with v—cut LiNb O3, whi ch has a low co e f f i cien t o f ther mal exp ans ion

l ike the Si02. But on z—cut LINb O 3, which has a coefficient of therma l

expansion 10 time s that of v—cut , the films peeled off during curing.

At th i s poin t , a number of a t t e m p t s to prevent the CdS f rom peel ing

off the Si02 were made , such as g r a d u a t i n g the Si0

2 — CdS boundary dur—

I ing evapora t ion , and roughen ing the surface of the LiNb O

3. All of these

were unsuccess fu l . We a lso t r ied t o o u t — d i f f u s e the LIN b 03

b e f o r e eva—

I poration . This would obviate the need for the Sb2

film . This was also

unsuccessful . The films were still degraded during curing.

I We also tried Al203

as an i n termediary diffussion barrier . Unfor-

tunately the CdS f i lm peeled o f f the Al203

jus t as it did f ro m S102. At-

tempts to prevent peeling by making very thick thin films of Si02

to

allow for different expansion rates on the opposite sides werealso un-

successful.

Eventually we struck upon the ideal solution . We knew that CdS it-

self would stick to LiNhO3

Therefore wh y no t insula te the photoconduct—

ing CdS film from the LiNbO3 by a second CdS film. If this film were

thick enough it would not permit out—diffused elements from LiNbO3

to

reach the second film of CdS. Moreover , there would be no sticking prob—

I lem at either surface . Our very first experiment with this technique

yielded very good LID ratios . It soon became clear that one could eas-

ily adjus t the dop in g levels of HCI , 02 and Cu for the second CdS f i lm

to have L/D ratios of l0~ at 500 m .cd.

Since then we had made a number of CdS films on LINb O3

by this

I “double—deposition ”, or layered , technique . Reproducibility and film

I uniformity reiiain less than ideal essentially because our dop ing techniques

are still rather crude .

I - - - — - - - — ~~~~~~•- - ———--— ,-- - - - ____________________ a

18

Appendix 0 reveal s the “recipe” for evaporating and curing the

I films .

It should be noted that these films represent the first reliable

pho tocond ucti ve f i lms on LiNb O3 or , as far as we kn ow , on any other piezo—

I electric substrate. Luukala[11] has made CdSe films on v— cut LiNbO

3 but

has reported great difficulty in reproducing these films because of

I cracking and peeiingH~9]

.

IIIII

-

I ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~

• ‘ - ---

19

11.3 Measurements of CdS Film Properties

Once the CdS films have been laid down and cured , it is of consid-

erable i n t e r e s t to s tud y a numbe r of parameters that can he correlated

wi th known and p redic ted properties . These include surface un i formity,

absorption of ligh t as a function of optical wavelength , conductivity

change as a function of ligh t level , change of conductivit y with strain ,

and chang e of c o n d u c t i v i ty w i t h t e m p e r a t u r e . In t h i s sect ion we descr ibe

a number of such me ’a ’-;ur emen t s and co r r e l a t e the r e s u l t s , whenever pos—

sible , with our expectations.

11.3.1. Film Thickness

Only r -cently we established an optics facility including a pneumat-

ically—isola ted tab le (4 ft. x 8 ft.), associated harduare and lasers ,

and a high sensitivity spectroradiometer.

Using the table , ~~ were able to set up a simple Michelson interfero—

meter in order to measure the thickness of the CdS films . We had for

years assumed that the ( d S was approxi ma tely lOp . ‘l’o determine the true

thickness , we depos i t ed a f i l m on glass th roug h a mask t h a t produced an

abrupt edge . We then deposited aluminum over the f i l m so t h a t t h i s metal

f i l m u n i f o r m l y covered the region on both s’U d~ s of the abr up t ed ge. By

us ing t h i s a l u m i n u m f i l m as one mi r ro r in the Michelson i n t e r f e r o m e t e r ,

an int F-rferene’& ’ pattern was formed on a screen . With care , the differ-

ence in t h e l e n g th if t h e p a t h s f r o m t i l e ‘~test” mirror and a standard

first surface mirror could be adjusted to be a few wavelengths of light.

We thus oh ervt ’d a patt ern containing between three and six fringes .

Th 4- I lte rn was sp l it in the middle because of the abrupt junction . A

shift of 0.~ fringes was observed across the ‘-attern due to the CdS film.

_____ a

l 21)

IU s i n g the result

I distance moved = (/ 1 of f r i n g es d i s 1U o e d ) ( ~~)

I = (cdS f i l m t h i c k n e s s ) (2. 1.1.)

0

fo r a wave leng th of 6328A , ‘.~e e s tim a t e t h e ’ f i l m t h i c k n e s s to he 1582A.

An alternate method hase l on t h e g e o m e t r y of t h e e va p o r a tio n p r o —

I cess was a l so u s e d . Assuming uniform evaporation over a hemisp here and

knowing t h e total j r - t i n t of CdS evapora ted y ie lds a t h i ckness of approx—

imate lv 2300A . U s i n g the T n f i c o n Deposition R a t e M o n i t o r i n d i c a t e s a film

t h i c k n e s s of 4 500 A h u t i t s sensor is o n l y 2 / 3 as f a r from the evapora-

t ion boat as is the s u b s t r a t e which implies i thickness of 4500A (4/9) =

0

2000A .

in any case , t h e s e t i In s art much t h i n n e r than e x p e c t e d . ‘I’h us i t

is poss ft I t t h a t s t i l l lar t ’’r conduct i vi t i e s cmi be obtained by lont’er

evap oration s . Of - cu r se , on LiJ’thO ,1 the otal t lj i ckness of th e ’ two layers

is t w i c e the t i i icknes ; measured in these e x p e r i m e n t s .

1 1 . 3 . 2 . ~~~~~~c~j ’r e ’~~’ r t ! es

The f i l m s were e x a m i n e d under an e lectron mic roscop e at var ious

s tages of fah r i - a t ion . The s u r f a c e of t h e b o t t o m f i l m is smooth w i t h

no obvious ~~ rii ct iir ” . On a lu set le it looks s o m e t h i n g l i k e roug h con-

crete. Once cured , a definit e crystallization becomes evident. The

film looks lik e a t ile floor with irregularl y shaped h~it smoothl y fit —

I ting tiles. On top of the film is what appear s t o t ’e excess CdS dust.

The crystall itt’s have between four and six sides and are between one—

half to one m i - m u j i n d i a m e t e r .

II

- - -— :‘-- - - - _ - — ___________ — a

21

Wi t h u t h e ~ eu -o nd I a v e r deposited , the surf o -e looks v er y hilly and

4 i r r e g u l a r . But a f t e r c u i r i n s ’ , we a re ag a i n left with a ‘ m o n t h f i l m w i t h

c lea r c r y s t a l l i t e b o u n d a r i e s , a l t h o u g h n o t as smoo th Is t he b o t t o m f i l m .

Fig. (2.3.1.) shows the surface of this final fu r ’ . ~otice that again

there is dus t s p r i n k l e d on t o p of the f i l m . We t ar e g iv ing some thoug h t

to see w h e t h e r t h i s curing artifact can he p r e v e n t e d or r e m o v e d . I t is

q u i t e s i g n i f i c a n t t h a t t h e r e ’ are no ho les or t e a r s in t h e f i l m even

though i t is q u i t e t h i n .

11.3. 3 . F i l m C o n d u c t i v i t y

We have me as t i r e e l t h e i — V c h a ra c t e r is t i c s of CdS f i lms on LiNb0 3.

The vo l t age was v a r i e d over 4 decades and t he i—v curve was f und

he l i n e a r . The c o n t a c t s e p a r a t i on was ahou t 75 - . Assuming the crystal—

li tes to be l~; w i d e , t he re w e r e ’ 75 c ry s t a l l i t e’- b e tween any p a i r of con-

t a c t s . Let us assicie ’ t h a t t u e h a r r i e r model of c o n d u c t i v i ty ho lds f o r

our f i l m s , so t h a t we can ; l r - t su lme t h e c u r r e n t to he f t h e form

I = TØ

( ex p ( .~ : / k ’ !) ( e x p ( q V~~/k T) — 1] (2.3.2.1)

I where ~ is h a r r i e r v o l tA g e b e t w e e n e r v st a l l i te s , and V 1

i s t u e d r i f t vo l tage

• across each ha rr i er~~81 . N ow i f qV 1 < kT , the above exp re s s ion can be

I suitably expanded to give a l inear i—V characteristic. However , at the

h i g hest applied voltage we used (20 V), Vd is about 20/75 ~ .‘67 V.

I Thus qV~ l~ about 10 times kT and one would expect highl y non—linear i—V

curves. Sinc e this does not happen , we think that either the barrier

model does not app ly to our films or (2.3.3.1), quoted in may papers ,

I is an over simp lif ication . Appendix E describes an alternative theory

which does agree with our data.

II

22

I,’ .

.r

Isw .

• -:

I .

-

.

~~ ~~

-

. ~~~~

I

’,4 ~~~~

I . .

IFi g. 2.3.1. ScannIng electron microscope stud y of the finished film . Note

t i u c ’ ljj sc-tie i n d i c a t e d j u s t under the p ict ure.

F

-r - -—- - - — -- — - - - -

23

We ob ta ined a l i n e a r curve for or rent ‘‘ i ’ rsls v o l t a ce ’ r ‘n; V = 0.002 V

to V = 20 v o l t s , i . e . , ove r 4 decades , the d r it t — , ’ I i’ - t r i c f i e l d changes

from E = 0.2 V/cm to E = 2 x 10 V / e m .

I We next measured the variation of p~iotocurrent wi th lie ’b t wavelength.

The normal ized curve , given in Fig. 2.3.2, shows the hand edge to be at

505 nm which corresponds to a hand gap of 2.46 V . The photocurrent

I drops sharp ly at hig her wavelengths. However , there is a secondary

maxima at 560nm , wh ich corresponds to a level 0.25 V from the valence

I band. A similar level was detected in our CdS films on glass. We be-

lieve this to be a sensitizing center with low capture cross—section for

electrons . These measurements were made using the new E.G. and C, Hi gh

I Sensitivity Spectroradiometer .

The next measurement was the variation of photocurrent with white

light intensity , showing clearl y the power—law behavior of the conduct—

i’;lty. There is a supralinear variation with an exponent 1.4. However ,

our films on glass had an exponent l ying between 0.8 and 1, probab ly

due to sensitizing centre lying closer to the valence band (0.15 ev).

We also measured the rise and decay times of the photocurrent for dif—

I feren t wh it e l ight intensities from 40 to 1O 3 m.cd . The rise , as ex-

pected is an order of magnitude large r than the decay time . The de—

I cay time changes from 28 msec at low intensities to 3 msec at ~~~ m.cd .

I From the decay time one can calculate the trap density at the position

of the Fermi level. N~kT seems to he about 5 x 1O~~ cm 3 at a distance

I of about 0.45 e.v. from the conduction band . To investigate

trap densities closer to the conduction band one would have to perform

I cryogenic measurements.

II

a

24

IC ~~~~r _~~ -, - _ __________________

9 . -

7 -

6 -

I 5

II

I.6 - :~~~

I I I II 350 400 450 500 550 600 650 700X (nm )

I Fig. 2.3.2. VarIation of Photocurrent with I,ight Wavelength

II

- - a

- 25

I 411.4. Exper i m e ’ n t a I ~~ r m a t i o n of Pseudo Beam Steering and Fourier

I Imag ing

IThe successful fabrication of the CdS films is merely one crucial

I step in building a sensor. The next step is the metallization . By

this we mean the process of pr oducin g con tact s wh i ch serve as b ias and

I signal detection ports as well as transducers. In addi tion , the sub—

I strate may require metallization for shielding purposes. These steps

complete the sensor. Next comes a challenging prob l em in circuit design

I to enable the extraction and amp lification of the signal.

Work on all three aspe cts (Cd~~, metallization , circuits) has pro—

I ceeded in parallel. During the Summer of 1976 , we designed a self—

contained system using voltage—controlled oscillators for scanning Four-

ier space. We tested several sensors with this system but the results

were ambiguous -since the sensors were very poor. At that time our films

all suffered from either peeling or poor light sensitivity. We did ,

however , make some measurements and report them. The description of

these experiments and their results may be found in Appendix B. Among

I the conclusions was the fact that the circuits designed for driving the

I transd ucers so as to provi de cir cula r sweeps thro ugh Four ier space needed

to be significant lv improved for quantitative use. For this reason ,

I we decided to concentrate on improving the sensor and to use test equip-

ment to evaluate performance rather than concern ourselves with circuit

improvemen ts.

Once good films were available , our first problem was tnetallization .

We observed years ago that indium makes good ohmic contact to CdS while

aluminum , the material normall y used in SAW t e c h n o l o g y , does n o t . Also ,

I_ _ _ _ - -_ _ - - - -

I 26

I we c a ruuir r t use t h e ‘ . 1 c h ~~n t ’ t e ’ e i i n i q c c - ’ I n i ; c ~ F i inn t h e ’ m e t a l f i n g e r s f rom

I t he va cu c um ~~~ le’po~ ted me’ t ;I I t i I n , Any c t chant that w I 11 remove m e t a l

will remove . Th u s we had to l e a r n a mor e ’ sop h u i s t l e n t e d t erh n iq u i e

kno~~ es t hu “ l i f t — o f t me t h o d .

in order t o p roduce t h e m e ’t ,- u l l i z , ’ ut i o n by l i t i — o f f , one p repares

the CdS film as d i s c us s ’cl i n ‘ ec t ion 11. 2 . After c leaning, photoresLst

I is spun on us i n c u - - n t r i fio’ , ul platfnrn . This photor e ’-J st is then exposed

in a mask aligru’ r o un p t h e a l patt ern p r o v i d e d i n t h e shap e of the

wanted metal 1 i zu t ion . ‘l ’hi e r e ’ n i st i s then tb ’!’- loped and washed .

leaves heh inch a pat tern of r e s i s t . Then the i n d i u m m e t a l is vacuum de—

I posited and wn shie ’~ wi thu acetone . If the depos it inn is p rope r lv done ,

I some i n d i u m w i l l s i t on top of t i le r e s i s t l i n es , and son ’ will sit in

t he v a i l e ’v s . As I on; ’ as t h e photoresist is thicker than the indium and

I t h e i n d i u m ‘‘beam ’ i s hi n h i l v ‘ c o l l i m a t e d ’’ d u r i n g e v a p o r a t i o n , the i n d i u m

f i l m w i l l he sc m ’: ’ r ’t ed. liii ’ a c e t o n e can then b u r r o w i n t o t i c e re ’ s i St

I l ines from the e d t - a n d ” l i l t ” ;uwav the re ’si st and th e ’ metal r e st ing on

t o p . T h i s l eaves lie ’ low—l y ing m e t a l line’s int ;ict

U n f o r t u n a t e l y , i n d i u m f i l m s thin enough for li i t—o f f turned out

I to have a very poor -ouu, huuc ’ t l v i t y . Tb i s is a ser i o u t s prob len because

of the l e ngt h of our I I nyc I p a t t eros . We solved this pr ob lem

I by a t hu in f I I m ( c i t I c i m in u im on t c i p u ) i t he ind ium f i l m . Such sa n d w ich

I f i l m s p e r m i t t h e I n d i u m t o make o h m i c c o n t a c t t o t i r e ’ ( hIS , wh ile the al-

u m i n u m f i l m s p e r m i t t Ia ’ long su i r f a c e ‘ o u u d u o - t ion r o u t e ’ . The d e t a i l s

I of t h e s e s t e ps ar e ’ s till undergoin g improvement it t h i n t i m e .

One prob l em u I l l remains he - acuse ’ of the need f o r t i t e Image to be

samp led in ho t Ic d i re c t ions . The m e t a l sandw i cli samp Ii’ s a I ony one d i r —

oct ion . I’ E t h e r an e x t e r n a l grid r u u n n ing, perpendicularl y t o t he meta l

I — — — —— — - -- - ———- -- ~~~~~~~~~~~~~~~ r’ —

~~~~ ~—_

_ •———..-_

1 27

lines must he suspended lust above t he sensor , or e lse the CdS i t s e l f

I must he etc’hed into lines before using the lift—off techni que .

Both m e t h o d s h ive advan t ages . E t c h i n g t h e CdS film produces some

degradation and in t r o d u c e s a s ma l l n o n — u n i f o r m a c o u s t i c mass l o a d i n g .

It also encodes the dark current on the same carrier as the optical

signal. On the other hand , the pro jected grid is somewhat inconven—

I ierut .

The most signiI leant factor until the present , however , is the fact

I that with the etched films , the lift—off is much -u more diffi c ult due to

the non—uniform surface for the photoresist to spread over. Fig. (2.4.1)

I shows the typical result when etched CdS is used . The CdS (black ver--

tical l ines) is not comp letel y covered by the metal lines (white hori-

zontal lines). At tire upper left and lower right , small sections of

the transdu°er contacts can be seen . At the l ower left , the contact

pattern and the CdS fingers are evident. The m e t a l c o n t a c t fingers are

broken at the stee p edges of the CdS film. We are working to prevent

this by adlust ing the various thicknesses. Thus all successful devices

flre of the prolected grid variety .

The photograp h shows clearly that the sampling of the image cor-

respond s to one samp le per acoustic wavelength in each direction . The

I distance between n et lye CdS “snuares ” equals the period of the trans—

I ducer pattern . l }c e’ se ’ sensors have 100 x 100 CdS squares covering an

area of 12 nmu x 12 mm. ‘l’he whole sensor fits on one half of a two—inch

I diameter LINbO,3 wafer. The transducers contain 15 finger—pairs and are

apodized wi th a sinc function overlap .

III

‘ ‘_ __ — - — _~~ - —--——-—- -

~. ——-— —

_

28

II

.~I ~~~~

‘ -‘~

I I .~~ ~~~~ -

-t-- “-—- - - -

I Li i ’~~~~ in T1~

_ _ _ _

-

F i r . 2 .4 . 1 . h m n , c i I ‘ -~ ‘ m e ’ ; c t c i E t H u e d t c l ’ - F i l m ( e v i c t ’ .

III

I

F

II

d-J

29

II

II. 4.1. Unidirect Iona l Sensor

The first t r u l y s u c c e s s f u l d e v i c e was a unidirectional sensor based

I on the princip le ’s and fabrication techniques described. This device was

built since it was deemed easier to evaluate than a full two—dimensional

( system. The crystal was subjected to x - r ay ana l y s i s . We were able to

iden tify the x and ‘i axe’s of the wafer by comparing our p ictures to pub—

I l ish ed da ta [241. The wafer was cult in half along a line 13.5 ° from the

true x—axis. ‘rhis cut became the “x—ax is” referred to in all of the

previous equations. This axis was choosen since it yields the most ident—

ical acoustic behavior to its orthogonal axis , wh ich we have referred

to as our “v—axis. ”

In this first sensor four transducers were fabricated , two on each

side of the CdS film/contact region. These transducers were made from

the same mask as those mentioned above . They had center frequencies of

[ 28.2 Ifliz and a useable bandwidth of 4.0MHz . Acoustic insertion loss

measurements were made by exciting one transducer and detecting the de—4

layed signal at the f arthest transducer. Since we observed a 40 dB loss ,

we decided to tune out the transducer capacitance using small ferrite—

- filled adjustable inductors . Using an r.f . impedance meter , we deter—

F mined the required inductance to be approximately ltjh . With match-

ing, the insertion loss was reduced to 18 dB , a very respectable figure .

I In a unidirectional device , one has to measure either the sum or

differen ce frequency. The capacitance of the contact pattern made mea-

surement of the sum ( 60 MHz) an unappealing choice . Thus the device

was operated without samp ling along the x—direction , and excited by two

transducers on the same side of the CdS/contact region . In this case ,

I in place of (2.1.24) , we obtain

I

30

1 (t) exp j (-~1 - w 2

) t 1i~~xl

-

~~~ > (2.4.1)

where ~~~ ~x2 ’ a re the spatial frequencies of the acoustic waves at

radian temporal frequencies 1 ’ ‘ 2 ’

respectivel y. Thus we obtain base—

band spatial components without sampling.

Our first attempts to measure the signals were not successful. An

enormous feedthrough si- n d presented itself. We were quite disappointed

as we had naivel y expected that feedthrough at 30 MHz would be n ice ly

removed by a l~~ —pass filter in the detector , leaving us with a low dif-

ference f r e q u e n c y . U n f o r t u n a t e l y , the 30 MH z s igna l s were so large t h a t

mixipg was occurring in the detection electronics. This was largel y

solved by depositing an aluminium film on the bottom surface of the sub-

strate , grounding it effec tivel y, and by grounding one side of each

— transducer to it. This reduced the r.f . fields straying into the con-

tact area.

Even with atte ’nt ion to shielding, we still observed image independ-

ent difference f r - ~ uency signals. There appear to be two methods that

could reduce this furt r e’r. one is to operate the device in a gated mode ,

using the surface’ wave delay property to detect the signal during the

t ime when the t ransducers are not bein g driven. This requires a sepa-

ration between closest transducer and the CdS/contact region of at least

two contact pattern lengths. At the t ime , we were unable to fabr ica te

a device with that much “open space ”.

The other alternative was to build a very selective low—pass filter

to be inser ted d i rec t ly at t he contact terminal port , rather than at

the detector output. Such a filter , with 60 dB separation between 30 MHz

and the desired signals (between zero and 4 MHz), did effec tively put an

end to the feedthroug h problem.

-~~~-— --—- -. r -

—

I 31

With these improvements , we were able to det e ct the long hoped for

I difference t requenc~’ si gnal which was image ’ dependent , and d isappeared

in t he d a r k . The d e v i c e was c c p e r a t c i w i t h - u t h e h i - c s c u r r e n t e ’l ec t r o n i c a l l v

I m a i n t a i ne d at a c onstant level of 0 .2~ mA. .1,1-ui ’ sensor , w i t h a res i s tance

I of 1K, , , l e t e r ! l i k e a cu rr e nt source s~ nce’ it was ter :’cinated in I Sd

In a t y p i c a l e x p c r i n c - n t , nic e I r a u c s d u c e r was tuned t o 2 8 . 2 MHz ,

w h i l e t h e o t h e r wa s swep t ~‘e ’t w e e n 2 6 . 1 3 MHz and 31 .33 M H z . G r i d p a t t e r n s

were illuminated b y un 8 milliwatt !c’~ie laser and focused on the CdS

[ film. Periodic yrids were chosen whose spatial frequency corresponded

to the ava i l ab le va lu es o f f1

—

~~2’ A typ ical output is shown in Fig.

2.4.2. The upper trace is for a grid with period 2.08 mm and the lower

trace is for a grid with period 3.47 mm. The center portion of both

traces represents the average value of the light intensit y , zero spatial

frequency, it is a sine function whose width (0.3 Mlix) is the’ same as

the high frequ ency p c ’ .-iks due t i c the aperture’ smooth log effect. The cen-

ter null in the zero component is an artifact of t h e d e t e c t i o n e l e c t r o n —

- ics having a low fre’quencv cut—off.

Since we have a wave ve c to r k — k and a r a d i a n f r e q u e n cy ,. —

the e f f e c t i v e ’ “v e l o c i t y ” of t h i : pse~~~o wave is

1

v(k - k )v =

8 1 2 — (“ 4 2)P k 1

— k , k1

— k2

V R

I where VR is the Raylei gh-u wave velocity of each acoustic wave . For z—cut

LiNb O3 w ith “x—d lr ected” waves , 3820 rn/s. This tells us that tin

I object with a spatial period of 2.08 mm should he imaged when

II1

— -.- . ~~~~~~~~~~~~~~~~~~~~~ - = - -— . ‘ . — - . *

32

1

III

II -

“I

I I

I ‘~~ :i

‘

I - - -- -)

I Fig. 2.4.2. Typical Results Using the Unidirectional Electrophotocon—

I ductive Sensor. The traces represent amp litude as a func-

tion of spatial frequency .

IIIII

________________ ____________ _______________ a

33

1 1 = V }~( f1

-

~~~~ = y

R t f

~ =

.00208 = 1.84 MHz (2 .4 .3)

I where ‘f is the measured difference temporal frequency. Indeed , the peaks

in the upper trace we ’ re c en t e r ed a t ± 1 .84 MHz.

I f an imaged g r id was expanded and c o n t r a c t e d icy the use of a tele-

scope , the twin peaks moved in and out just as expected.

Thus we were convinc (’ul that the electrop hotoconductive effect was

I significant and yielding the expected Fourier behavior. -‘u ’ studied the

magnitude of the measured modulation of the photoconductivity and found

( . . . 1,F 2 1, -that our acoustically induced signals correspond to -~ F I- ’ - 0 . 2 , i . e . ,

I a 20% modulation of the ph o t o c o n d u c t i vi t y . T h i s is a r emarkab ly la rge

n o n l i n e a r i ty and r e p r e s e n t s a s i g n i f i c a n t p h y s i c a l measuremen t q u i t e

I apart f rom Its information processing aspects.

ITt is also worth) noting that these measurements correspond to acous-

tic p~~ er densities of 4 milliwatt s per mm of aperture and is achieved

I with driving v o l t ag e s of a few volts. The modu lati .rc was linear as a

func t ion of v~~’P’ wh e re ’ P 1 , P

2 are t i le t i cn c i s t h e ’ powers g e n e r a t e d at the

I two t r a n s d u c e r s .

IIIIII

34

I

$ 11 . -~ .2 . ftc i’w o — l ) im e n s i o n n l Sensor

R , i s c , ’d on the Succe ss 0 t h e u n i d i r e c t i o n a l s e l d ’ ,o r , we’ F a b r i c a t e d a

twu -d imens iion al sensor such as shown i n F ig . 2 . 1 . 1 . As p o i n t e d out in

Section 11 .4 t h c e . c t u a l sensor is at present fahrh - ;c t ed using an tcnetched

CdS film. A contact pattern is met ti l l izec i on a thin glas s p late which

is s u p p o r t e d ~c us t abov e ’ ti re ’ substrate by tape’ . T rc ’se in’- al l i nes are

perpendicul ar t o t h e cdlb cont .e’ts l i n e s and t h u s p r o v i d e t h e r e q u i r e d

image samp ling .

This sensor r e - q u i r e ’ ’ . a l l of t h e i m p r o v e m e n t s desc r ibed in t h e pre-

vious section . It is mounted in an aluminum box with an opening fo r

the l i g h t . No lens i s i - -t i ’ ! on t h e box at present s i n c e ’ we can p r o j e c t

good q u a l i t y images u s i n g a beam o~ c o h e r e n t l igh t (6 32 8A) .

F ig . 1 ,4~~3 shows th e s c h e m a t i c r e p r e sen t a t ion of the test set—up .

The o r t ho g o n a l t r a n s d u c e rs g e n e r a t e the acous t i P wave ’s because of t lce

p iezoe lec t r i c nature of the LiNhh3. As in the u n i d i r e c t ional sensor ,

the m e t a l f i n g e r pa tt erns p r o d u c e e l e c t r i c f i e l d s which then coup le to

the substrate exciting Ray leigh waves.

The experiments performed ‘,~,e’re meant to give convincing proof that

the strong ru e usticallv induced modulation of t he p h ot o c o n d u c t iv i tv

did produce the pseudo beam steering effect required for two—dimensional

transform imaging. Basicall y this required us to do experiments similar

to those performed on the’ ei n ldi rec tl ona l device , and then show that as

the image is physicall y rotated , the device can elect ron l ca l lv track

the image spatial components ang le ’ for angle.

II

35

I V H F S W E E P E RLOSC ILLATOR GENERATOR

I

I ~o~~~LLo~~~~j

p + v cc

- —

~~

-- -. --

~~~

P A S S IV ELOW PASS IL T ER S

1;~~ ]-

C

- I_i,—C 1I1-~FEr, T 13~ /\~~INGA N D ~3ENS I NG C I R C U IT

Fig. 2.4.3. lest ~ e ’t — U p fo r t ic e ~~o—flimenslonal Sensor. N t e’ the corn—

• pensAt ing Induct e c r o for each t r,inuduce ’r.

II

_______-.~--— - ‘a ’ —. ~ - - - -—~~~~~~~~~~~ -~‘- -~~~~ - - - - — - ----

36

We also were able to make ’ p i c r u s e ’ measurements ye’r I f”ing that the

sensor satisfies ti re F o u r i e r t r a n s l a t i o n theorem along any arbitrary

axis.

Again we used oh~~e’cts with oni y thuree components w i t h i n the band-

width of the system. ‘l’he projected gratings , wi th pe -iod i cit i es of

between two and four millimeters , have their zero spatial frequency corn—

p ont- n t s in the midd le ’ of thee ’ r educed space w h i l e t i n . i r f u n d a m e n t a l fre-

q u e n c i es were w i t h i n t i c e 4 Mi-hz x 4 MHz reduced space . Harmonics mos t l y

f e l l o u t s i d e the ’ r e d u c e d space scanned b y t h e e t r a n s d u c e r s .

C ur f i r s t t ask was to map out the reduced space in te rms of tempora l

frequencie’s. That is . we had to f i n d out e x a c t l y what temporal f r e q u e n c y

combina t ion ~~ i ’ ~r ,, ) c o r r e sp o n d e d to g = 0 , the c e n t e r of the re-

duced space. ~e also used to identif y the temporal frequencies corres-

ponding to the axes , g = 0 and g = 0. This we did by pr o~ ecttng a

grid so that its spatial frequencies w.ire along the g axis. By search-

ing we found a line i n t empora l f r e q u e n c y space (f 2 = 30.15 Mi-hz, 26 .63 MHz

— 30.93 MHz) along which we observed the two symmetric fundamenta l

lobe s, plus a large ’ ceaetr, -el peak. Other lines would at most have one

peak . Similarly , by rotating l ic e ’ grid 90°, we were able to find the

usef u l g axis corre ..~u c ii d ing to (f1

= 28.72 MN3, 2 7 . 6 MHz < f 2 < 31.5 MH z) .

Thus the cc ’ui te’ r of freq u e n c y space corresponds to (28.72 MHz , 30.15 M14z)

From this point on , we can e x t r a p o l a t e to f i n d a i r y p o i n t in the reduced

space .

By using t h e e ’ grid in the two d i f fe ’ i c n t o r t h o g o n a l orientations ,

we can calculate the ve e l e c u ’ i ties of thee ’ two s u r f a c e a c o us t i c waves. Kn~~ —

ing the ‘;alue of g1, the spatial frequency , u I ’ I . e h t e e ’ .i i v r u ler , we can

I.

I_________ a

37

de t e r m i n e the e x p e r i m e n t a l v a l u e ’s ‘ 1 and a t w i e i c h tire ’ fundamental

appears fo r the two o r i e n t a t i o n s . Tieen

‘ 1= = ( 2 . 4 . 4 )

where v and V ar e ’ t h e ’ p inise ve ’ l o c ’ i t i e s of t i l e two a c o ust i c waves .x V

Thus we obtain

f ‘ 1 v ~6 e 3 NH~= = ~~~~

:-~~~~

--~~~~~

- = 0 .988 ( 2 . 4 . 5 )2 y

for the rat i c r of t i r e v e l o c i ie s , u s i n g t he’ a c t u a l f r e q u e n c i e s .

The next strege’ in these expe’r iments was to e ’ l e c t r u r n i c . - i l l v r o t a t e

the spatial frequency space. Starting out a long t ire v - a x i s , we r o t a t e d

the grid in 10° steps , wh ich was s u f f i c i e n t to comp l e t e ly e l i m i n a t e the

si gnal. While continuing to scan in one d i r e - u t . ion , t he’ constant fre-

quency source Is r e a dj u e - c t e c l to r e c a p t u r e ’ t. iee signal as shown in F i g .

2 . 4 . 4 . Each t r a c e , r o r r e s p o n d i n g to one 10° s t e m w i t h t h e a p p r o p r i a t e

r e a d j u s t m e n t , r e p r e s e n t s a s t r a ig h t lin e ’ in frequency space. As can

read i l y he see n , t i r e pc - u k mov e ’s “ in ’ t r e~’; i rd t he a x i s o r t h o g o n a l to i t s

original orienta l j~~ - . l i r r e t I s the p r o j e c t ion of t he s p a t i a l f r e qu e n cy

vector starts out parallel to the scanning dimension and is then rotated

away. Note that the zero frequency peak disappears after several scans

since it remains c - c-n t e’ r e ’ e i on the original scan line . Tt will reappear

as the rotat ion near- 180° . Each trace represents a horizontal line in

frequency space ’ , each with a different w2

intercept.

Fi g . 2 . 4.5 shows thee ’ resul ts of an Identical experiment using a

— d i f f e r e n t g r i d . The se’ peaks are the same width as in the previous fig-

ures as predicted by (2.1.19).

I__________________

____________________________ a- - -~~~‘~~~ ~~~~~~~~~~~~ -- - - - - - — - - - . - -

38

IIII

II

r4~~LI _ _ _ _

I _ _ _ _ _ _I ~~~~~~~

I - i _

~~

_

~~~~~~~t_

~~_. . ‘

• Fig. 2.4.4. The Magnitudes of Spa tial Frequency Compo nen ts Along Hori—

I zontal Axes in Fourier Space . Each trace shows the move—

ment of a frequency component under successive 100 ro ta t ion

of the image .

H‘ I

II

— — ~~~ ‘“ ‘ “ ~~~~~~~~ — - — ,.. - - - ——— - —- —-____________ — a

39

-1

I

I

I

IIIII

— II

Fig. 2.4.5. The Magnitudes of Spat ial Freq uen cy Componen ts f or a

I Different Grid Object.

IIIII

— -_ 1 ~~~~~ — ~~~~~~~~~~~~~~~~~~~~~~~ — ~~— - -

~— .. - — —

41)

Experiments of t h i s type were carried out over an entire 360° rot e—

t ion . It was possible to track a spatial frequency component through—

out . Furthermore , it was usuall y possible to find both non—zero funda—

mental frequency e’omponents. Tab l e 2 .1 shows typ ical data from such an

experiment . Here , after each rotation , the princi pal three components

were reacquired by changing one frequency, and scanning along the other.

In ea ch c-rise’ , the zero frequency peak was checked and the other funda-

mental lobe was found also. As the data shows, a rotation of the image

away from the y—axis is compensated for by decreasing F1

and increasing

f2

for the third quadrant fundamental peak (A) , and by increasing

and decreasing f2 in the first quadrant peak (B). The zero frequency

peak remains virtuall y unchanged as expected.

__________ _______ A _~~~~__ B

- Z ero Spatial Frequency

8 F2

ampi. f1

f2 anrpl.~ f

1 f2 ampl.

degrees MHz MHz mV. MI-hz MHz mV. ‘4Hz MHz my .

2.5 28.72 26.96 150 28.72 30.86 50 28.7 29.26 200

12.5 28.27 27.11 250 29.31 31.36 150 28.76 29.26 235

22.5 27.87 27 .16 200 u

29.62 31.16 -

150 28.77 29.26 230

32.5 27.68 27.26 200 29.87 31.16 150 28.76 29.26 250

52.5 27 .03 2 7 . 9 6 390 c 30.42 30.66 50 28.75 29.26 250

62.5 —

26.89 28.26 390 30.64 , 28.26 e 160 28.77 29.26 • 200

Table 2.1 Pseudo Beam Steering Experiment. For each rotation , the spatial

frequencies of all three prominent components are given and

their amp lit udes noted . 0 is measured from the y—axis . Amp-

litudes are at the detector output.

II

., - • . ~~i.._ - - - - —~~~~~- r~~~~ . — - — -

41

t h e s e ’ re s u l t ’ , ,- c r ~- s t r o n g ev idence ’ t e r r t i r e P s e ’ c 1 e l ~ he ’ ; e r r st eerinp c- f—

f e c t . I c u t e - c - u i , t h e e ’ V e r y f , e c t ot o b t a i n i n g t he se s i g n a l s a t all Is st rc rn g

e v i d e nce’ sinc e ’ t h e e e X I s t ‘ n u t ’ o t s i - ’n ; e l s depends on tire ’ vector addition

I ect t i r e ’ w , : c e -v ’ ’c t o rs i n oc i e ’ r f o r the ’ image transform S p a r e ’ and the a c o u s t i c —

S t ’ - c ’ t c c i r e ’ s uper I ri

I t w i l l I r e ’ n o t I e n t h a t . t he e d a t e p o i n t s have’ some’ quant It r e t ive dis—

I C repan e ’ I c-u . u r u m p ‘ t i c - n t - - - A and II do not have’ t h e- sace’ amp l i t rul e’ s , and

t h e - f r c - r p i e ’n c i e ’s ho n o t v a r y u n h f o r m l v f r o m d u n e p i n t t e n t h e n ex t . t’-’e he—

I 1 ieve that this i s due t - u ‘,‘ e ’ rv s t r o n g ri r e r u n ) I u ’ r m i t h’s in t he a c o u s t i c

f ields. In t h ei s sen -ro r . apodized t t r i n s e i u c e - r s -ir e u sed . h o se t r a n s d u c e r s

a h i i’h lv n o i r — c i n i t erm ,inirl j t u r n l e ’ uve’ r i re wave f r en t , s i n c e some f i n —

I u ’e ’ r pai r- - on Iv ‘e’ tce ’r .e t e’ i o u S t I c w a v e ’ — , Tie - e r t h e ’ c e n t e r l ine , w h i l e o t h e r s

ge n e r a t e i n ross t i c , ’ c - n t i r e ’ - : . e v c - I r en t

I The’ r~- er .- a e’rer at i ye’ designs , however . ‘~e’ c o u l d e m p l o y m u l t i s t r ip

coup 1 o r - r i t - - e e l I - r m w a v e ’ f r o n t s , or we cou l ci use’ a simp ler t r a n s —

I d uc e r w i t h nc r- ~‘e ed eii l t a l l o f f i t b a n d edge’ , but einapo dize’ cl . We have

chosen the Se’ , r e t r u ! app e - u - h f o r t he t i m e b e i n g .

To p e r f e e r n phase -c . r ’u r e ’m e n t s , t h e se t up of F i g . 2 . 4.3 needs to he

I m o d i f i e d . St n e t - t h e e ’ i i I t e ’ r e ’ n r -e f r e q u e n cy is gene ra t ed in the ( ‘dS f ilm ,

t he r e is ne d r e ’ f e ’ r c n r e ’ a g a i n st w h i c h the s i g n a l phase can he measured ,

I T h e r e f o r e ’ , a r e f e r e n c e must he c rea ted e l e c t r o n i c a l ly by m i x i n g s i gna l s

I from the same gener ri t cers that drive t in.’ transducers. Fig. 2.4.6 shows

the test set—up devised fo r this purpose . Naturall y , absolute phase is

I not particular ly u s e f u l at t h i s s tage , b u t relative phase is of great

interest. Specificall y , we wan t to de termine if t h e device operates con—

I sis ten t ly with the Fourier translation theorem. Thus we want to measure

FI

—~~~-

42

II

H P 3200B [H P 860 1 A 1I VHF : S W E E P E R , I

OSCILLATOR I GENERATOR IL ‘ ‘ w 2 j

I ,~ J~~INPUT

I --

OUTPUT

- L~f 1 -

~~ C A M E R A- 1iö~o~1

—- -- - - - -- - --- ---— - --~~~~~~~~~~ I TO

I50 uIMAT CH[~~MPL

M I X E R‘~- C O S (~~2 t + 4 )

_ _ _

+ 4)M I X E R

(1)2 .

1 --

~~~~ REI- H.P840(A Iw _ _ _ _ _

NEIW ORI( I -- IA ,I - .-f ~~1

~~~G . A N A L Y Z E R L~. LI — - - -

Fig. 2.4.6. Phase Detection System . The mixer together with the quadra—

tive hybrids create the reference signals which the HP Network

Anal yser can compare to the DEFT sensor output .

Ia

43

the change of phase’ ot a given spatial frequency component as a function

I ot image dlsp iace ’mt’n t . These measurements were made s u c c e s s f u l l y fo r

the g l ees sce 1nstrat e sensors using t h e e ’ l i n ea r chang e ’ of p h o t o c o n d u c tiv i t y

I with strain and formed t h e b a s i s fo r the m o t i o n — d e t e c t i o n a~~~l l c at i o n . 16 1

I Oua litative me ,’ ise ir emen t s r e s u l t i n g f rom our set up were most encour-

aging. Fig. 2.4 .7 reveals the linear variation of phase with frequency

I for a stationary grid i m a g e t pattern . As can he readil y seen , the pha se

is linear in the reg ion of the peak , and d e t e r i o r a t e s o f f the peak since

I there is really no signal being detected . As the image is disp laced in

I any direction , the s t r a i g h t line sweeps across the peak region , wh ile

the peaks remains stat ieu n re rv , save for some minor changes in detail.

This is quite consistent with the Fourier translation theorem ,

I .(r - r0) 1 (r) ~Xp -j2i~[ f ~o 1 (2.4.6)

where f = f x + f v~

I Mncr e g:neraliy, f r om (2.1 .19 )

I IJr ~~) S(~ ) A (~~) • - . T( ~) exp-j2~~~ . (2 .4 . 7 )

F where ~‘ = ~ + g t / , the reduced vector spatial frequency, and 1 is the

smoothed version of I.

I Thus we expect a linear change in phase ,

I = —2ci . (2.4.8)

I for a disp lacement of the image r0. ‘fhi .s imp lies that in order to tell

the direction and distance involved In the displacement , two measuremen ts

I of must be made , al tho ugh onl y one sp at ial f req uency is needed .

II

44

Fig. 2.4.7. Phase and Amplitude versus Frequency . With one frequency

f i x e d , wi see ’ t i re ’ amp li tirde and please as a function of the

s e r u m l i r e ’ I requencv neac a peak . Phase is sho’~n in the up—

p e r t r e ‘ , ;enip l I t ra t e’ In the 1 ower

_______ a

45

We Set out t i c check ( 2 . 4 .8) q u a n t i t a t i v ely as w e l l . A f t e r adjusting

the image so th at it t - u fundamental spatial frequency vector Is along a

p a r t i c e m l a r d i r e -- c t ion , measured by p r o t r ;~~’ tor f rom the o b j e c t i t s e l f , we

t r a n s l a t e d t h e ima g e- , u s i n g p r e - c l ’ s i e u n m i c r o m e t e r t r a n s l a t i o n s t ag t s , in

the x — d i r e ’ r t I e u r r u l e 1 ti r e v—direction unt il we had measured some specific

phase change .

From ( 2 . 4 .8)

= —2 - ’ g. .’ X ( ‘05 1) (2.4.9)

for motion along t h e x—a xis , ‘y ,, w h e r e 8 is the smallest angle between

and the x—axis.

himi lar ly for translat ion along the y—axis ,

= —2~ g•~ ‘, v sin ~ , (2.4.10)

As one additional check , suppose that we choose =

~~~~, then

cotan 0 (2.4.11)

Table 2 .2 shows data for this case , ~

= = 2~~, a par t icularly

easy phase to measure of the HP Network Analyser . The results f r om

(2.4.9), (2.4.10) , and (2.4.11) yield the correct orientation for the

pattern to within 10%. This is a very strong quantitative test of

pseudo beam steering, and therefore , of Fourier imaging.

tnX 0 0 0 8

measured measured (2 .4 . 10) ( 2 . 4 . 9 ) (2 .4 .11 ) measured

.0659” .0761” 4 5 . 8° 5 1.6° 49 0 47

0

TABLE 2.2. Quantitative measurement of angle from phase measurements. Here

J g 1,~

— 833.3 m1, a period of 1.2 mm.

I______________

__________ a

46

11.4.3. Conclusions

We have succe’ssful lv fahr I cat i’d a monolithic , two—dimensional acous—

toopt I cal proces sor , v i e l d inc rea l—ti me’ Four i e r t r a n s f o r m magn i tude and

phase data. This is a s i g n i f i c a n t b r e a k t h r o u g h . As we will discuss in

Section V , this technology n r , r v r a v e ’ benefits considerably beyond the con—

fines of thee app lications already demonstrated , and even e’nvisioned , f o r

Fourie r irmig e’ pr essing, systems . The magnitude of thee acoustical ly—

induced r - n n d t m l a t i ’ r r is so large that con .rolvers and correlatc’rs , includ—

ing memory , ‘tray I- c fashioned using CdS on LiNb O3, using c i t l e e r a one— or

two—dimensional geerne ’trv.

The f a c t that t h e ’ eft c - c t is so p r o f o u n d (20 7 m o c l u l a t ion of t he photo—

conductivit y ), coup led with our success in making stable’ , highly photo—

conductive films of C1S on z— cuit LiMb O3, imp lie’s that we can concentrate

our efforts on improvements. We are striving now to improve film uni-

formity , since ’ the’ current films have photoconduct lvitv variations of

50% over the surfa ce ’ . Evidence leads us to believe this problem is cen—

a- tered in the curing st ,Igc , caused by non—uniform gas flow in the dif-

f usion f urnace.

Improvement in t r a n s d u ce ’r d e s i g n is a l so a high priorit y aimed at

devel op ing a more uniform ,col h i ma t e’d wavefront.

Electronic design to incorporate drivers and d e ’ t e ’ c t ion electronics

is also of great m t c rest in order to make tire “sensor” into a “camera ”.

Fig. 2.4.8 sleows the e xtent to which this has been accomp l ished .

The sensor is r uue ep, irl y t h e s i z e ~~ a penny , while the’ entire system (sen-

sor , detection electronics) is contained in a hex 25 cm. x 6 cm. x 2.5 cm.,

containing five transistors and about thirt y passive elements.

II

47

I ‘-

F i g . 2 .4 . h’i a . h u e ‘ c - H e i r ( r 1 p - ; , - omi t a c t s t r a n s d u ce r s , on 1, i ~-ThO3

) . ‘ote

I I t c - s i z e r e ’ l ; e t ion to -i I’ . S. on e cent p 1 ece ’

I Fig. 3.4.8 .h . l i c e ’ Came r a . On t l e e ’ left is the cavi tv c e c er t el i n in g the sen—

sur rind the Inductors used for tuning the transducers.

I ‘l ice central f cccer e’avfties hold thee’ d e t e c t ion f i l ter. The

ri g ht cavity holds t h e ’ bias ing r i n d amp l i f y i n g c i r c u i t s .

- - - ~~~~~~~~~~~~~~~~~~~~~ - a

48

- I I I . DEF’l’ Llc;IIT \ ‘ - \ i V !

-‘c. cons i d e r a b Ic I I or t was e ’xp ‘itde ’eb t o fabri c’ e two—dimensional

DEF ’r 1 ig ,h t v a l v e . h i e c h a syt-t t c -fl u is shown in \ju i ue ’n d ix I whic hr nd udes

I a u i e s c r i p t i o n c u f i t s ba sic princi ple’ s ot op e r a t i o n . Th i s d e v i c e r e l i e s

— on st rn i n — jnducc -ui h i r e f r i rr :~e ’nce ’ in a 11 giu va lve s; enu hw i ch . The image

is f o c u s s e d on t h e ’ s e i r f u r c e of a Li~Th0 3 c ry s t a l . T h i s d e v i c e is very

- sImilar otce r a tiu ime l lv rend rem -a t ically t c t i r e - e’lectrop hotu eccnduct ive

de!’.’ice ex~-e ’p t th at t h e ’ interaction region iras no CdS and no contacts.

This is due to t h e ’ f e e t t h u ct t he e’ intensit y moclulat ion has a component

p r o p u r t I o n n u l to th e sI r e in squar ed . ‘l ire modulated ight passes through

thee’ polarize ’r— valve’— cm r e l v z e ’ r sandw i cie and is d e t e c t c - u i by ou r E .G. + C . high

I sensi t l v i tv spectr oradiome ’te ’r w i t h c i r c u i t ry tuned te c t e e ’ difference

f r e q u e n cy . This struct ure requires more l i g h t b u t is e a s i e r to build ,

I at Ic-rust t e ur use as an optical instrument , si n - c ’ i t is made from exist—

I in~ hardware , It will also not suff c ’r from re h ax ,e t i o n time effects ,

t h u s p r o v i d i n g much a s t e r t ime response t i rn: e e ’e changes. It should

I also p r o v i d e me re sp a t c l i v uniform process ing .

Such a sy s t e m was hem i It ar1d assemble’ d on our c d u t i e ’r il table. At

I present , no use f u l dat a h e n s been obi ,i ined h e r - a r i s e th e’ LiNbO3

crystal

appears to i e , c y e ’ a very n o n — u n i t - r n i n t e r n a l h i r e i r in g e n c e . ‘l ’heore t i ca l

work m d I a t c’s r u r c i e nat u r a I b I re ’ f r e - m e g e - t i c e - s ho u i 1( 1 he ’ v e ry small , h u t

I would not h u e - t e d u- ,e r i ous eve- n i f i t e ’x i s t s . U n f o r t u n a t e - i ’ ,’ , t h i s e f f e c t

is quite’ non—uniform over the active Ire’;! of tire device . Thus the poi—

arizers are unabl e t i C cancel c ci tt this natural bire -fringence , causing a

comp lex pattern of light to e ’xit the sandwich . This amount of light Is

I too great t ic allow the detection of the signal si mm - c ’ it induces a photo—

I current in the high ly sensi tive det e ’ctecr oh approximately 10 amperes.

Although this effect i c-i essentiall y d.c., It c reate’s a large noise signal

I — - - - ..— --- - — - _ _ _ _ _ _ _ _ _ _ _ _ _ _ a

$,c 4

In the’ d e t e c t o r h e ’ e , i c e s e ’ i t i s d r i v e n ( ‘ l u ’ s e ’ t o - , , - e t u r a t i u r i . l i c e ’ s i l ’n n l i s

I we ,ure ’ searcieing f i r sTre i - c i c u l n i t e d t i e h u e in t h e e ’ r a n g e - of ii ) amperes

and thus are c ur ue h eie t e- lv l o s t in tire ’ no i se .

I Our current el t~- ‘r t is te e build c h u l k t r a y ‘11 ing wave c h e ’ v i ce con—

I sist ing of a i r i g i r lv pol i - - b e e -1 , opt i e ;u l l y f l ;u t , fu sed qu;rrt7 slab with

tr ;em r u-nu h i -err -u C C c t es sion nel ly bonded . ‘th is sve-r t c - rn w i l l york - el

I center t reqcienc I c ’ - c t -~~ ~l h i ; ‘ w i tic 157 b andw id tic . Fused r i u l a r t z I s not

natural lv hire r rin r’ e ’nt , thc i s oh im inat in g our main di fficult~’ . A l s o ,

by gc e in r ’. to u bul l: s - ev e- devie c. , a much l a r g e r i r r t e - r ; c c t j on l e n g t h b e ’ —

t r,s- -n tire’ opti cnu l iflt e’nsi t \‘ r ind the c r i e s - - r e d ,-r c o u s t i c fields is provided.

Thee bull ’ w a v e s w i l l h u e ’ ~ ffe ’ct ively absorbe d by care’ful sandhlasting of

the edges ecf ti l e ’ quartz on p 1ec c s it es e ’u l g e s f r u u m u ; I h e ’ t ransducers . Thus

we’ w i l l ha ’j c a t r ave l l i n g m~,-uvc a co cm st ic system . St - e ndin g waves are noc

deslrab he- since’ thee’ r ’ c c ’ r u e t rv of tire’ rr vs tnel heas a h u nire a u rt e f f e c t on

p- -ous t ic h e e ’ i r n i ” i o r , ra t i re r t h a n t hee t r uu u u , ei ue -e ’ r - -r .

I— I

I

_ _ _ _ _ - - - -~~~~ --— ---~~- - _ _ _ _ _ mu

Sn

IV. A DISCUSSION ( I F l i i l - ~ A P P I , i CA l ’ I ON o}- h)F,FT SPATIAL SPECTRAl, ANAlYZERS

I\’. 1. j~~~j ’e’ Rec v e s t r u c t i on f r o m lWF T ~~~~~~ct r a

A DEF’T u - r N - c t r u m a n a l y z e r p e r t o r m s the f u n c t i u - u of Fourier analysis

by i n t e r r e e - e t i n g t h u ’ i n p ut image w i t h - I c - o c r s t i c y- ; ’v e-s of various - -~p a t ial

frequencies , in particular directions. An acoustic wave , l aunched in a

given direction , with a perticul a r spatial freq uenc y , tests the’ image

i t one - c o r r e s p o n d i n g “ a l u m e u in tire tso—d imens i c u i ra ] k—space. By measur-

ing the spe c t r u m in t l r i s mano r n e t a numbe r of p o i n t s , the e n t i r e ’ image

s p e c t r um can h u e - s amp l e d .

To re -co n s t r t e u t the ima ge f ro i ir t i c i s i n f o r m a t i o n , all the measured

F o u r i e r c o m p o n e n t - n h a v e to he r e a s s e mb l e d , w i t h clue re-g ard to t h e i r re-

l a t i ve phase valu e - s . In ne t w o— d i m e n s tona l spat i n t l -ni t i c a t ion such as

t h i s , each f r e q u e e ’n c v componen t can be viewed w i t h p irase- e x h i b i t e d as

the proper transin etional position at the bars of t h e g r a t i n g . The co-

h e r e n t a d d i t i o n of — i l l these gr a t i n g s r e c o n s t r u c t s the e ’ original image .

The require d g e cu rri , ’r process is provided i u v an optical lens between

its fron t and hack local p lane’s . If an image’ t ransfeirm is offered to

such a lens w i t h t u e spe ctral components properly arranged in k—space

in the front focal p lane , with coherent illumination , an image re ’cor—

structi c’n will h e ’ f o u n d i n the ’ hack focal p lane . It is important to no-

t i c e - t h a t n u l l t h e spectral c omponents ace require rcl t i e he av - u i l a h l e s imu—

I t nineo ti s l y a t a g i ven t i m e f o r t i le c o h e r e n t o p t i c a l t r , i n s f c e r m ; u t ion t o

t ake p lace’ .

The DEFT device’ generat c’s t hese components In ser m u f r r s hi o n , and

thus some st c )r ig e’ ruue ’di um hetween th e e ‘ EFT forward transformer , and the

optical inverse transformer is needed to handle I)EFT data. This storage

I_________________________ a

~

- ‘ -

51

bu .- u i uisr’ u - - n e ! he ’ - c r c u e p t h e - n i l m i e d c e i c ! e r dr t i e ’ f ron t t i c c i p l ane — tira t su re’-:

i in put lot ’ I- rue , e t I on 1- r e g c ‘I t C~ e ’b e f u r a I I a t t i c e - s i c ’ r r i f l u e n t I O u r I’ ’ r (-ompon—

ent t-i t o he’ t c- - t e d In ’,’ t ho - h )b’ l”l dev i ce ’ n O d 1 eun u c h -n h u r t - - I i ce ’ m odme 1,- c t r

Fi r c - ’ r ; i f l t e ’ S t l i e g e ’ r c ’q u i r u - u - c c - r e t p m .’ - a n t s it s e - I t if ci se c ond tel - fl di’—

v i c e ’ i s t i c he u t - n e - u t o p r ’ ,’ r b1 ’ t i r e ’ t- i u u r r i e ’ r i n v e r s i o n . ‘l ’ice sp a t i a l s p e c —

-n t r i m ” of l i e u - it ’u n c n ” - ‘ r u r a t h u e I c e ’ i n c i d e n t i n ‘ b u t h a l form , n i r r n i n e ’ c - u h in 1~—

spac e - , on t i r e ’ ‘ - n c - c c e - ~ i l l - VT , n e l l s i m c e l t n e n e ’ e u e r s l y in i n ’ . Thus an u~ I c t i c — e l

m o d u l a t u ’ t - ‘1 sc e n e ’ k i n d i s s t i l l nee de d to s o r t and s t o r e I ce c i n c h in

t h i n w ay , d i d h e - n c - i m u b)[I- b c k ’ v f u ’r’ i s c - s c - c en t ia i l v a t- e ’p l e c e - r c u c ’ r r t for

t ir e Ic -ms i n an c u b i t i c — r i r ec — u c c e s t r u c t ion .

II:: . 2 . l u l u c r e - r e t ~ ‘ - c c u r u - r t r r e c t i o n

$ The m e - -a ’ o f nor i r e c - c u b e c - r e ’ n t op t j c n e l m o d u l a t o r , s t i r - h i as a C R 1 doe ’s c c ’ ’

I e p I c - n c r t ‘ C p r u c c b u c u e ’ - e v iable’ op t ion . Such a ligh t sour - c ’ doe- s u i - I p r c o .’ l d t ’

the ‘p- c t i re ! b u h e o n e - coherence from m u m m t t n p o int that i s r e t u i r e - b wh er r a

( lens iu n to b e - c u - n e d as a c - l e e - r e - n t F o u r i e r t r r i n s f i - r r - u e ’ r . i - n u c h r a d i a ’ ing

point in t i m e - p iios p i r e e r a c r e - e tc u t a CR T i s an i n d e p e n d e n t ‘ t u r t l e - of l i g i c ’

J and h e - n c r ’ no r c - l a t i i c r c - r h i p to t i m - p lea s e of t u e l i g h t c - n i l t e c h u tm any ‘ t h i n r

p o int . The c n e ~~c i w u l t h c f I b : ’ m c c i i n u t 1c m is 1 a l- g e ’ c-no uc ’bc I h a t b ee ’ t i i u , i c e

r c ’ l r r t i ’ r r - n h i p c - ; i omp l ’ - u e l y u i c , e ~t~’e ’ i n a t l i c e ’ much r-niecr rt -r h , eur t h e ’ i ! u i n u d r ;

t I se - r e ’ r-n u c I c i t I (in o t c ay 1< c r u i s e r c h i t u - c - t ccr . Fiec is t h e ’ ( ‘R T , ic r I t e q c c i ya l e n t

l e e - - r u e t s at i s fy b e e ’ ue q uu ir om e ’nt t i c i t t h e e v n i r i c ’ es’ - t i e - c t c a l v a l u e s in ~~~—

I c - , t c a c e he - r e - n i - - - i ’ d - l i - c l w i t be tire ’ r i ce -I - t e l cnc u n e- re I i t I on siri ;c i~ i m u u e’ c ’ r e ’con—

I- _ I rue t i on .

‘ l i c e - c e d e - - I h e n rrmuv n u r t u r e - i f I t i s jrer ssihlc ’ t e e 1 u e r f e c r t ’ c C I I I iCnill\ ’ In—

u - c c i p - r e ’ r i t r e c u r c c - t t r o i l fccrr fitt i ng , a CR 1 , -i nc h an Ir r te- e rn ut In c . mccl i c c u ’ u , S uch us

phot c u ( r n e p h i i - f i l m . ‘l’he’ t e - e - i c n i d b e e e ’ whi c -hi Is see ~‘g.e ’u:t ed t c writ e a t~rat inc em tiue

CR 1 f e c r e ’ n c !c m e ’ ; c c c e r e ’ ct ‘ r j u e r t r a l ccn ~p e n c e ’ I r t f r o m t i n e ’ DEfl’ d e v i ce , i n tb u e f orm

I- - - --~ - —-- - _ s~~_

52

of a black and white- grating pattern , and exposing the photograp hi c f i l m

to this pattern . The process would he repea ted f o r each measured spec-

t r a l component hv the DEFT d e v i c e ’ . In this rn-inner, each of the proper

I spatial frequenc Ic ’ : p i e - ,e’r rt in t i e ’ o r i g i n a l image w o u l d be b u i l t cu p

I in t o a r e c o n s t r u c t i o n.

Thee d i f f i c u l ty w i t i r t h i s approach is t i re r e q u i r e d b i a s leve l i n

I c-ache contributory grating. In coherent methods , each spatial frequency

can be introducer! w i t t e u c n r t b i n s , i.e- . the grating can he written with ()

I b i a s , s w i n g i n g p o s i t i v e ’ and negative in value . In the i n c o h e r e n t m e t —

t h ods , t i r e ’ g r a t i n g c ’ninno t have a value less than 0 (black). Thus a

- large number of bias terms are added to the final reconstruction , which

greatly reduces contrast , si gnal— tn— ncuise ratio , and image qualit y .

I IV . 3. Coherent ~~p L ic a 1 Reconstructors

There are ci number of o p t i c a l cohe ren t area m o d u l a t o r s in e x i s t e n c e .

The o p e r a t i o n of these are b n e s e ’ i t on 1) a mechanical deformation , 2) an

e l e c t r o — o p t i c a l e f t e - e I ( u s e i a l l v the P o c k e l ’ s e f f e c t ) , (Cr 3) interaction

I between an acoustic wave and n u n o p t i c a l be am . Com r c ’t iti ve ’ two—dimensional

devices can aci r ieve n e t least sever .el hundred resolution el em ents along

a side , up to 1000 elements. C l c e ~~t of them fall wi ti ei n a narrow size

range , 1 to 3 cm. on ci side.

Mechanical deformation can h e ’ i nduced by t u e e l e c t r i c a l fo r c e assoc—

tated with charge’ depos i t e m ! on nu deformahie film; this i s t h e case in

the General h-i e ’ e t r i c coherent li ght valve , where’ the active medium is

a thin film of fluid that deforms according to the electron beam pattern

i produced on the surface of the fluid . The charge Is deposited by an elec-

tron beam in a sealed tube , so that the device resembles a CRT with the

oil film t arget as a repiRcement for the phosphor screen . A 2 cm . square

I— , . - _ _ _ _ _ _ _ _ _ _ -a

53

ape’rture is used he re-se -nt lv f o r t h e e n e c t i y e ’ r a s t c r r — f o ~~~at ted dat n e . An in-

crease in s i z e w o u l d e n t a i l i major expense for tim e mechanical reehe’si gn

— of the sea led v a c r u u m t r i b e .

Al so , th e r e ’ has been de ’ve loped at P c r k i n — 1 : l m e r a ’aemhrane modulator

in which mechanical deflection is provided by a thin membrane overcoated

I on a s u b s t r a t e , c c e n t a i n i n g a matrix t ef holes , each one delineating a

I r e s o l u t i o n ce l l of thu. - neshulator. The e l e c t r i c t o r c e can he provided

directly by charge , or by combinin g the device with ap hotoconductor which

responds to an input u j cticai image ’ .

E l e c t r o — o p t In ccl i f f e e l s in c r y s t a l s are t i re ba s i s of a n o t h e r class

I ( C f m o d u l a t o r s . A r u r i n i ’ c ’ r of f c - r r o e l e - c tr i c e -r v s t a l s e x h i b i t a change in

I optical birefringence whi c - mr sub j e c t e d to an electric field. Birefring—

ence refers to that qualit y of an optical medium in which there i s a d i f —

ferent amount of optical phase s h i f t , or phase delay , through the medium

for light polarized in different directions . Thus a crystal eesed as a

f Pockel ’s cell , phase—modulates a ligh t beam passing tirrough i t as a func-

tion of the e l e c t r ic a l c x c i t a t i o n and , moreover , the amount of t h i s phase

modulation will he different for light polarized along the t w o orthogonal—

I ly oriented princi pal axes of t h e c : r v s t n e l . I t can he sirown t h a t if light

polarized at an angle of 45 0 to ticese axes is modulated in this way , the

I linear polarization is converted to elliptical polarization by the hi—

refringence of the crystal. If the r e s u l t i n g l i g h t is now analyzed by a

polarizer oriented at right angle- s t e u the original polarization axis of

the light , the final light beam obtained from the second polarizer will

be amp l i t u d e — m o d u l a t e d b y the action of the Pockel’s cell. Thus both

I phase and amplitude opt ica l m o d u l a t i o n can he ohtalned . h 1~~

17i

II

a- - - - - - -—- r — - — - — —

54

‘lIm e’ h t t ’k ‘‘ h ’ k ’ c ’~ ’ ’ ( Pcec ’l : e ’ I ‘s In e , i ! _ c u c r t c p t i c n i l e l u u c i r m l e t c e r ) is an examp le

of t h i s t e ’ c in i d u m e - . In t i r e - c - i - - i ’ 1) 1 ti r u~ h’b ,a u~-1 , the f i e l d p a t t e r n is c r e a t e d

by an inc i d e n t I i ~‘ b n t i d _ i t t . - rn ie n ui .c 1 u h c c t c u c c u m r c i e e l l i v e e f f e c t in tire c r\’nnt a l

A similar ( l e ’V l c O i t , u s b e e - n d i ’ ’ j e - l i c ; c e - u h ct Carnegi e--Mell on br ei ve ’ r sitv ,

w h i c h l i k e ’ t he G . E . l i g i r t v a l v e ’ , i s w r i t t e n on w ith an eul e ’ctron beam .

In tie is ecu- n e ’ t i e e ’ t n c ~’ , ’ i - t i~ ci l ocke l ‘s effect thii n crystal ,

. \ n u c t h e r i r n p o r t n r m i r - h e Ce’ i c d p n e ’ n t is the use of .c l i qu id - r v ’n t a l to

r e a l i 7 e - ,‘ i c ’ u c h l e ’ r c - n t ‘ : , n , I - .i l a t o r , such as has been f c r i c r i c c e t e d at Hughes Re—

search La b s . , ~‘l - e lihu , Ccclii . The device nnrk e’s use ee l ci very thin

layer cf ci liquid crvst ne l t b c c t exhibit s a pn ert ic -re tar mol e cular orienta—

t I (en depend i n n ’ en t h e 1 m e n d volt a ge ’ e p J m I l e d across tire f i l m , w h i c h , in

turn , is made tc u repre sent an input l ie ’ b e t pat te ’rnhv t h e a c t i o n of a p h o t o —

c o n d u c t i v e l a”er ‘-c . endw ie lee-e l c e ’t wi ’ i ’mi t bce Ii q u i d c r y s t e l l c iv e -r . The photo—

c o n d u c t i ’ ,’e ’ l i ve r causes t i e - excitat i -mu ‘,‘ol t c r c - t ed be r u c u c l i f j e ’ c l in areas

whe re ’ l i c irt strike’ s it. In t h i n re”-p ue - c t , t i e ’ nb , - v e c e- is cu p o rn u ted much

I ike the’ I t - . - k ‘‘ PR O M ’’

— The f o r e - c u i n g d i s e n n - c i orm has touched d i n t i e . - n . e j c ’ r t v j e e ’ s of coherent

area m o d u l . c t o r s a v a i l a b l e - , t h a t ir c v e ’ s u f f i c i e n t t i n e - s t o r a g e to be a

poss ib le c a n d i d a t e ’ b r a F o u r i e r ln v e r t e ’r of I)EFT Information . The G.E.

light valve , and t i r e KI)P l i ght v a l v e - have t h e e advan tage of using input

electron i c- data directl y , which is t I c ’ f o r m of the DEFT O u t p u t . The

PROM and t h e Hughes ’ L iqu id C r v s t c e l r e q u i r e an o p t i c a l input , so tha t

they must he p n u ir e c l with an opt ical protector , such as a CRT .

( At the p re sen t t im e , al l thee-u - ne ’ m o du l a t o r s are in deve lopmenta l s tages ,

and single units cost in excess of $40 ,000 . A complete Fourier inverter

system e u s i n g any of them Is estimated to cos t at least $100 ,000.

II

55

I V . 4 . Cone! usions Regarding DEll ‘t 1 c n tt ia l S1c ,’(- t r al Anal y z e r s

In the ab’o’,’e’ d i r - n c u s s i ’ - r , it is e ’ v j u i e ’ n t t b e , u t t h e e r e - c u r e ’ comp lexities

to he f a c e’ c h f t i t i s d i ’s i r e - h t e e F e c u r i e - r — I n ’ ,’ e ’ r t e - l e ’ c t m u n i c data f r o m a

DEFT d e v i c e ’ t i c r e d u C e r cm o r i g i n a l i m a g e ’ . Fr u e ru e this i n t c r e -s si ie n , i t Is

su s p e c t e d t b e e t t hee mos t ‘ b I c i en t m e t i i i z a t i on ~c t a DEFT d e v i c e i s t o

use ’ the Four h e r ce n e i u d e n e n t s me a s ur e d by t i m e ’ DE F ’l d c - v t c - c ’ from an image-

directl y . Tirus tire r e c egn ft inn n c r c ]a ss iflc cct ion ot crh ‘- c ts contained

within an i m n d g , e ’ on t h u t u n i s is of oh-ne rved Fc u r e r i c - u c n n r ’ n j u m c m r c ’ n t s w o c e l d be a

natural u t i l i z a t i o n ot the DEFT with reg, crd to the e - f c r - u’ -. o t da ta which

it provides.

I t is thus felt that tire most fruit f u r l app i I c n u t i c u n u— tud i e ’-n for the

DEFT would he- in the t i r e - a of i m , - uge ’ ana l\ ’ s i s m s im r g me’,eu n u rr e ’ ,t F ouri e r data.

In many r c r s e - - ~, t he s l c c c t i , a l shap e ’ of t h e Fourier pr e - s e-m u tc i t ion is a good

i n d i c a t i o n of t he r e a l dou ’e ai n shape of the important f e - n i t eeres in a scene ,

and the F o u r i e r i n f o r m ’ r n c t ion can a lso p r o v i d e an m d i i -a t or of the presence

of some type of ob b e’ct of interest. The advanta ge ’ of using t i ce Fourier

domain for tin s type of d e t e c t i o n is t h a t t he Fourier intensit y pattern

does not t r a n s i c i t e ’ i t s p o s i t i o n as the object translate ’s . Thus is may

be considerably easier to - n e - a r c - i t t i c e Fourier p lane t c r a u t o m a t i c f e - c u t u r e

recogni t I (In t h an t i r e ’ r e- n il p 1 arm e- i t s e l f . i- xc i ncp lc- u - m cdv he pointed out

p a t t e r n s w i t h s t r o n g re -pc- I i t ion p r c e c h c e c e ’ s t r o n g i m p c m l s & ’ s e r r b r i g h t spc e t s

in the F o u r i e r p lane ’ ; t r i a n g u l a r s t ru c tu re s produ c e - a s t c r — l i k e p a t t e r n

w i t h a h e x a g c u r e c i l c i e c e r n u c t e - r i s t I c in t h e e F’o u r i e r p l ane ; c i r c r i l n u r oh j e - e t s

p roduce c i r c u l a r t r a n s f o r m s .

This type of Fourier rc’c ’ ec gr nl t ion anal ysis should he tire subject of

a separate stud y, in wh i ch the concepts are experimentall y tested .

II

- - -

-

56

V . F1cTI’R!-’ l,~’(r}~-

V .1 . Nea r T~~r mcm F f t u r L ~

Current lv , we cere ’ p er ru - e me I n n ’ t ine- work mandated by thee- contract. We

have n o t e n c o u n t e - r e - c i any de ’ -’neh c- rid s , and i ndeed are very encouraged b y

importan t ‘-n u e-ce’sses eccompi I s i c e ’ u i t i l l s y e a r . R a v i n g f a b r i c a t e d good

f i lms of CdS on l , i~~bO , n c u rci found that t i u ev possess a truly remarkable

acoustic modulat i o n - t in e’ p h c o t c e c e u n d u c t l v i t y (20% ) , we feel confident

tirat no si g n i f i c a n t p r o b l e m b a r s the way to u s e f u l app l i c a t i ons for

the DEFT te cb en -logy .

As d is cre u cse- d in m e ’ c t l n n II , current efforts are to improve the

curing prec ce-s s in order t o obtain greater uniformity, larger light—to—

dark conductivit y ratio s , and cleaner surfaces. Extensive studies of

ultimate sensitivit ’.- and carrier lifetimes are c u r r e n t ly u n d e r way .

We a r e a l s r u ev a l u a t i n g two methods of p roducing etched CdS films .

One me tin c cu l is to e t c u i t c e r e t squares of CdS r a t h e r than lines. Thus the

indium/aluminum e :ontects will not have to go up and down over the CdS ,

but mere ly t o u ch t i e e - ‘ n i e c e ’ s . The other method is to make the metal

thicker so t i e c c t i t won ’ t b r e a k .

Since ’ the CdS f i l m s t u r n e d out to he so the m , we- w i l l endeavor to

increase the e-vap ierat I cen t I m i n e ’ t o increase thee-fr t im I c l’mte ’u- n s by c i t l e a s t

a factor of five . ‘b id s will Incre ase the sheet conductivity by the same

f a c t o r , b i t t w i l l make - e t c h i n g ‘n i e m e wbe ci t h a rde r cu rt w e l l

One pri d e len c u m rr c-c mt lv being addressed is t I m e nonun i f o rm i t y of the

a c o u s t i c w a v e f r o n t s . I t 1,s q u i t e evi ( l en t t h a t apod ize d t ransducers are

not well suft e -d for a two-dimensional sensor , although they may be ade-

qua te for a unidir e c tional version . The smoother , mo re symme t ri c behavior

II

---- ---—--——-—- -~~--—-—--—----- ---=--~ - — - --—- - - a

57

of the unid i r e -ct i c e n n e l ‘censor can he observed in tiu- u h n e t ci of ~e ’ct ion II ,

and upp e - i r s dire ct I v related t o t i r e a c u d u s t I Cs

We are’ p r e p a r i n g t o t es t new sen suur s with an unapodized design .

These tc-sts s h o u l d c l e c r r l v e s t a i m I I sir t i r e d e g r e e ’ to wi i cbe f u r t h e r inprove—

ment of ci c c e t i s t I c S i s m C e - e ’ u h e ’ e b

By i n t r o d u c i n g f u r t h e r s p a c i n g s be tween t r a n s d u c e r s~ ,~nd t he CdS f i l m

contact reg ion , we hope- tee ope rate devices in t h e gated mode , making

possible spatial r e nter c ,- in n imr g sensors in additio n to Fourier imaging

sensors with \‘er\’ le)w spurious signal levels.

During tire coming months , we will he interested to try some new

F materials to evaluate their promise in DEFT sensors. u. J ~ expect to study

the a c o u s t i c m o d u l a t i o n in in f r c e r e ’ d s e n s i tiv e f i l m s p rov ided by N V I . . I t

F may be poss ible’ to f c d b n r i c a t e e i t h e r a dev i ce d e p e n d i n g on the linear

chan ge of phot oconehcectivItv~~ 7]

, or tine quadratic effect in electric

field dlsc”issenh in th is report. Tire quadratic effect i n-n , of course , es—

F sential for full t w c m — m i i r u c ’n s i r c n a l F o c i r i e r i m a g i n g .

CdSe is another r, - e t e -’rial win ch migh t offer some a d v a n t ag e s over

CdS , including faster opt ical behavior . Single crystal films of CdS ,

now ava i lab le, ‘cciv u1 so i c ~~ use ful , providing tinat we can Ic-am how to

cure them , icr dope them by othe r means.

The work on i i n ’ i r va 1 ye’ processors continues as well with the fab-

rication of a hulk travelling acoustic wave device of fused quartz des—

F cr ibed in Sec t ion l I T .

Based on the’ next month of work on i m p r o v i n g the 30 111hz center fre-

quency semsors , we will design and build cc sensor In the 100 MHz range .

Such a sensor w i l l ver ge on a prac t ical pr ocessor capable of y ielding

I

5cc’

- ‘ y e m 1500 Iour Ic r componemn t —

I Art the nnd’ rm n - - c ’r e - n hec e Cme p r a c t i u n c b , we must continue to develop fields

of app l l c u tie c n . An on~ r~1ng effrrt is to study pattern recognition and

I reconstr cect ion uspects of their use as an incentive to the more funda-

mental physical re ’ - n e- nc re-bc . We- intend to continue to investigate the area

I of signal des1gn~~~ , aimed at obtaining other transform representations ,

in order t o bronn den tine app li cat ion s .

59

~~~~~~ N \jep lic

New app i i c i t i er r s are enem .c cp c-n to l)EF ’I ’ t e c in n o l o g v beyond those en-

v i s ion e d when the s’- -r~ render this con t r c u c t was m i t n e t e ’ d . I t is q u i t e

cl ear t i e , ut ~~e a r - n c ~ ’w in .e posit inn tcc uh e ’si gn and fahricate convolvers ,

corre-lator s i t c h c u’cp l it i rs tom e’ l e c t r u u r I c m l c r c u w c u v e s i g n a l s . l i t e ~se

three sig n - il -ron e - S s i f l g applications can b e - a c h i e ved t i r r o u g h coup l i n g

betwee’n drib t In g c : e r r ie’ rs cinc h c i c i e r i s t Ic w a v e s .

Exte n sive- m e n - n e- c rchi u r n t h e e - s e n e pp l i c a t i o n s has led to a number of

useful devices , pr inc ip cu llv convolvers for radar processing.

Most convolv e-ms and correlators presentl y being developed are

based either on a nonlinear p iozoelectric effect in a crystal such as

[26j , . [25 27 281L iNhO3 , or on the ’ separated meulum structure . The

separated medium s t r e e c t e e r e cons i s t s of a 1, 1 ~b0 3 scc h n - n t r a t e ’ , with a Si

b a r suppor ted over ci piezoelectric substrate with an air gap in between .

The nonlinear p Iezcee- Ic’ctric devices are’ simple- , mo n olit ini c structures

hut have very small convolution si gnals ih tie ’ to tine sm ce ll nonlinearity.

The separated medium c e-vi ce s irav€± had tire most success s i n c e t hey r ise

the ’ usua l p i e z o e - l e c t r i t’ b c ’heav irr r of t he su h s t r c e t e ’ and a nonlinear coup l-

ing te n t he e l e c t r o n s in t h e e - Si b ur. U nfe e r t im n a t ol v , practi cal vers i o n s0

of t h i s d e v i c e c u r e - ve ry d i f f i c u e l t t e e c o n s t r u c t b e c a u s e ’ cc u n i f o r m I000A

air gap m u s t he m a i n r a i n e - d i u e ’ t w e ’ e m t t i n e l , i N b 03 s u b s t ra t e nind the Si

[ 2 8 ]bar .

A y e ’ r v p r e e u - u l s i n g s t r u u c t d i r e ’ i s t i r e monol i t i c i i ’ conducting th in film

on a p i e z o e ’ l e ’ c t r l c - - , u r l c u n t r a t e . L u t xk k a l a ~ 111 h as e h e ’ s c r i h e d ci C d S e/ l i~~~0

3

Ima g i n g e ’O f l V c ) lV er ci nch Sol ~~ [ 2 7 ] i c ce s d e s c r i b e d ,i C d S c ’ / 1 , i N b O

3 amp l i f i n g

I convolver. WI t i f l e e r t e c i e n o l og y , we c o u l d make a C c h S/ l ,j N b O1

dev ice for

II

_____________________________________________________ __________ ‘a

60

either purpose , dependin g on t i le c r t r l n n e , j c r o c e - c f e e r - e ’ c i n c h on the pick—u p

contact pattern d e s i g n . W i t h additiona l support , these app lications

could be’ e ’ v , u i r e t c h r a t h e r r e a d i l y . We l eave e v e r’-,- reason t o expect

t h a t or er f i l m s c u r e ’ as s n e t i s f a c t c u r v n d m r u l r e p r o d u c i b l e - m s those presently

used in convolveru .

A l l of t i r e ~‘c u rk discressed in this se ct ion so far is concerned with

I unidirect ic en e’ ,h , o n e — m i ‘ c e u c s i nn a l , si gnal processin g .. The’ newest front—

[~~9 3D]ier in SAW pr oce ’ssinc’ is t i re m u m e ’ m t e r v c o n v oly er . ‘ Here the Si bar

I !s made into a l i r ee ’,dr cerr ce v of either p—n diodes or charge coupled de—

~i~ e~s (CCD) .~~3~~

’ In such structures , tire ’ SAW can Interact with a stored

I electronic signal . In ti le- CCD , tire SAW could also serve as the “clock—

I ing e l e c t r u l e s ” i n thee- ensu:nl CCD , by storing the cirargc’ in wells created

lry the SAW electric fields.

The maximum utilit y of this approach would be for a two—dimensional

array of diodes or CCDs . At present , the onl y suggestions for accom-

p lishing this have re-He’d on u cam m y li ne’ s each wit ir its own transducers .

— Our proposal is t - - ‘-c crke CdS CCDs on tire z—cut Li Ni l)3

su rf ace , thus

producing ci m o n o l i t h ic s t r u c t u r e . F i ~ ’. . 5.2.1 shows the schee m ce t lc s t ruc -

t u r e of one l i n e ’ of s uc i n an a r m i . In t h e da rk , t h e c - h c m r g e could he

i nj e c t e d a t one end and c l o c k e d a h e n g . A l t e r n a t i v e l y , t ime s i g n a l could

he read In optical lv . This structure mi gir t have’ , u c i ” c d n t n i c e ’ s c e v e r Si dc ’—

• v i e - e r - n due tic t ine- f c i c t that in t he da rk CdS is cc mu e ’ l n be t t er i n s u l a t o r

than S i y ield ime ,’ ii i n’ i ee’ r trcuc r u - ;f e ’r e fficienc y .

Witie t h e e ’ c i e l u l i t ion of n i c c e r c u c t I c tr ansc hce cers and c i cl i ect in c electrodes

we can now mode e I c ut e ’ the- n - c t ored charge’ wI tie tire- acocest ic waves. Consider

f a SA W— CCI ) monvo I v e - r shown in Fi g . 5.2.2 . rh Is d c v i c e ’ u - cc cls l o t s of a double

II

61

SOUI-~CE E L E C T R O D E ,I N D I U M ~4 L U M I N U MSte I J )VJ IC H S T R U C T U R E

I / G 1

o / ~~~

ALUMINU M 2

I G A T E S

I ~~Jc~~~ I N S U L A T O R

-~~~~ P H O T O - C O N D U C T I r - ~G Cd S F I L M

____

I NON CON D U C T I N G CdS F IL I I - ______

L i N b O ~ S UB S T R AT E

I -~~~ — - _ _ _ _ _ _

F ig . 5 .2.1 . ~- c ’hnemat i c n t cc CdS c r r e , ’ oh b ic c er ge (‘n u n t r o l led Devices, It

i s s h c ( e w n on Li Nb () u c i t c o u l d e;ns 11 v be’ made on gi cuss or fused

quartz.

- IIIII

- - - . - -— - -- — - 4

62

I

CCD C L O C K ~~~ GATES

I ND IUM -ALUMINUM

_ _ _ _INSULATOR

/~~~~~~~~I~~

:URRENT

PHOTO-CONDUCTING CdS FILM

NON CONDUCTING CdS L A Y E R ______

L i N b O ~ S U B S T R A T E

I ________--—

~

---- --- - -— - - — --~~~~~~~~

- --— - —- - -

CROSSECTION V IEW

~H ~~~rI - I F J0

I g ~~~ G A T E S ~~ , ~~~

F- I-z ° r~~~~~~

—i L~t - (ii wI-

° I0

I F-IL (4..w w

( T(, P V I E V ~’

Fib ’ . 5 . 2 . 2 . Sr ie e ’rr-at j r c rf a SA\ ’ l— ( Cl ) ~1 e - u ’ u e n r y Co n v olv er ~ t reu ct cure . ic e ’ c u t —

ou t e ’ - e t e r - p r o v i d e ’ t i e - J , c k — u z t t e n e t n c r - c h c s c r i e u c , c t i n t i u c t e X t

II

—~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ —~~~- — — -— - -

1,3

layer CdS film I abri c - - ut - cl ‘11 a 7—cu lt L1Nb D3

subs t r a t e . T h e r e c c r e ’ two

current i ’c d 1 h e -ct lu g c - u n t n o t on e’acir s ide ot t lee d cvi cc ’ shown . A stan—

chard CCI) i s l u u c , c t e ’d i e e ’ t w e ’ e ’ rr t i r e ’s e ’ c u r e ) ,uc t s . To p re v e n t the S t c r c - n h charge

from being s l i c e r t ed w i e i I t ’ t h e e ’ ( T I ) i s h e i n g 1 e e n c c l e ’ c l I t is n e , e ’ s s , ’c rv to iso-

l a t e the CCI) f r o m the c e c h l ec t l r r g e h e - c t rc c u t ~~’y . T h i s i s i c - -or ’p l i s }r ed by

po t e n t i a l h c u r r i e ’ r s c r e ’ , c t e d by a pp ly i n g cc s u f f i c i e n t ly l a r g e n e g a t i v e

v o l t c n g e ’ to the lock out ~‘ a t e n - c .

The’ loci< — orm t ‘at e- C - i r e act I y, i te - d b’~’ tl cer appl h - i t h u n of ci nec’ative

potent i i i . Tieis generates ci pot ‘nt ial b a r r i e r i c e t n - e ’ e ’ f l the r e p and the

current coil e ’ ’t i n g e- l ect n i c h e - s . Ne c-n t , th e e (TI) is loaded in tire convent-

ional manner by c l o c k i n g i t s e t n e t e s . After thee’ l u e . e c i i r m c process is corn —

p le teeh , tine trap st n u t C’S ni ne ’ a l l u c ’,~’e ’ c h to f ill . This pr i ce c ’sS t cckes about

I ms. N e -c t the potential harriers between the ( ‘Cr ) and t h e c o l l e c t i n g

e U - c t ro i he ’ u - e c u r e - h c - v e ’r e ’d by t he ’ app l ie ’ i t ion of a onc e II positiv e - voltage

to t h e lo c i- — c u e c t g c t e ’ s . A t tine scer’ce t i n e ’ , ci b n c ~ t c - n t ia l d i f f c - r e - u r n e- is

I t o t - d - I I shed h e - t v - c - r e t bce- two c u r r e n t C o l l e c t i ng e l e c t r i u c h e ’s. ‘bin is is

I acco mp l i s h e d b y t i c e ’ i c - f t e’ I e ’e ’ t r o l e ’ i n . - ing ee rc e rmn ci e- d and app l v ing a volt-

age to t h e e r i girt e l e c t r u l e ’ . A s i r o r t h u t l a r g e c u r r e n t p e e l se l a s t i n g

1 abaut 20 nS w i l l occur across the e l e c t t i d e - s . Tills -cer rent pulse re-

sult s f rom t lie m e eb I le c i r c u r g e ’ in t h e CCI) charge ’ p a c k e t s in c lu g empt ied

I At t h i s h~ m t in t i m e , a ~AW i e c t v l ng cm e ’ r e ’ ,’e ’ 1 op e t u n c t i o n f I t — x/v )

I and an angu la r I re q cee ’n c ’v “ 1 and c I ’ c &’ , - u u ~ i cur s t tnt eemp l I t tide Sn\W ~f

angular f r e ’ q e e e l l c v ‘ 2 cu re l a u f l c ’ h r e ’ n i i -e l i c e - u t l i v , h e c t i c in t i n e same direct—

Ion. The SAWs pr opaga te ’ , i l e e t r i ’ t i r e ( ‘I ) w i t h w av e f r e m t s p e r p e n d i c u l a r

to the elect rccde s.

I

II

64

-

The trapped c -iearge will now I r i ckle cur m t . ‘t Ile c u r r e n t r e s u l t i n g

f r o m thIs trappech cinca rge’ will behave e ’ss entic il l y similar to the phot o—

c u r r e n t mie u~r m l c e t e d by t i r e e l e c t r i c ’ f i e l d a s s o c i a t e d w i t h t h e e - - AW- ~ in a

I way s i n i l a r to t h e p r e v i o u s ly desc ’n ihed . We are n~~ i nt e r e ’s t e d In

l e t ec t i ng t u e , -ee rre :’nt component oscillating cit t i n e - sum f r e q e m e n c v - +

c~~ f rom each e e l ) se c tion . The s u r f c u e -e c u r r e n t d e n s i t y c o m p o n e n t K

re- -uI t i ng f r o m t i n e ’ n e - Ic CCI) c-hc u rge packet crud c c c i l l at ing at t he sum

f r e q u e n cy w i l l he u r o p o r t i on a l te-

K (q / r ) t ( t — era/v )expj(. + -~ - )na/v (5.2.1)n n s 1 2 s

where q is the trapped c il c e n e e e , r is the r e l a x a t i o n t i m e assoc ia ted wi th

emptying of t h e t r a p s , v is t i e e SAW v e l o c i t ’ - , and a is the p e r i o d i c i t y

of the ( ‘CD . Let us choose the ’ angular I re’quencies in such a way t h a t

I ~~~

+ , 2 ) /y 2~~/a . ( 5 .2 . 2 )

The srmrfa ce current densit y component of int erest due to the trapped

charge reduces te u

K ‘- (q / T ) f( t — na /v ) . (5.2.~~)n n s

The total collected current oscillating at the sum f r e q u e n cy i- sig

I is the sum of all the individual currents of all tir e ’ CCD sections

I 1slg

n=l

K0.

( Thus

I 1sIg

n~~i (q / T ) f ( t - na/v). (5.2.4)

FI

—~~ - ______ -- —-- ~-------- ~~~~~~-~~~ ~~~~~~~~~~~~~~~~ — — — __________ 4

65

The ’ t c u t a l c c - I I ect e e J S i g r r c e i c u r ren t component cesci h ating at the

sum fre’qe ee’tr c los of t t ee ’ t w e e SA W s ~s pr op or t lonal to the discrete con—

vo hut ion of tine trapped e ’ i e n r r g e - di st rihut h c cc q cnncl tire envelop e’ cu f

e ene ’ of t he SAW signi ls . The other SAW amp litude is maint ained constant

during this t ime pe riod .

It we -euld i c e- adv i u n i l l e t e u op (’rcite a number of CCI)s in parallel in

order to incr e ’ne se - the e .s i ~n ,u 1 c u r r e n t . The signal current of such a

devi cc’ s iri u cri d he cd u ru e g c i r c i i ) le to thee’ S i g n u ccirrent from our present

m a c e’ act i v a t c - h h )EF ’h ’ d e v i c es , t y p i c a l l y 10 to 20 (I A.

Of course , much work would he required , fir st t o perfect the CdS

I CCD and then to fahr icc cte a memory convolver. Until row , no one has

I been able to make adequat e films of CdS. Once app lied this technology

is quite adaptah h e ’ to tire ne’norv convolve ’ r t w o — d i m e n s i o n a l a r r a y . The

separated medium cecnvolve ’r mciv he v - n v d i f f i c u l t to reproduce since

the air gap must he maintained e v e r cu l a r g e ’ area.

I The h i g h dark resistivity, hi gh li ght conductivity , and extremely

I large acoustic ’ modr ilation , make these new app l i c a t i n e n s very temp t ing.

- If such devices could be’ developed , they would have a profound impact

I on si gn a l pr ec e-unsing and migh t he- superior to those avenues alread y be-

ing explored.

I Using, the DEFT t e c h n o l o g y , t h e s e ’ nre r mory c o n v o l v e r s can he opera ted

I with se’v e ’rce h m u ge transducers , rel ying on pseudo beam steering, ra ther

than on a multitude of small , highl y diffracting transducers operated by

a comp lex sw i t c h i n g svsterre . The pseudo beam steering transducers can

single out a given l ine ccl CCD devices by vary ing the rela t ive delay of

the driving signals.

4

66C~~e t ion V I

rB I RU ( e n - b -~ A T ’ i i Y

1 . P. C. Kc c rnr e -tch , S. T. En-ewe ’ 1 , 1). 1. Fleming, N . T. Yang, A . C u p t a ,and 0. lev is , e J ) h .F . l : D i r e c t C ] e c t r o n i.c Fo u r i e r T r a n s f eu r n s 01 c ) p_

t i e c ; i l l r n , u n ’e’s , ” IEEE Procee-rdin gs , Vol . 62 , pp . 11)72—1087 , August ,1974 .

2. I’ . S. Per tent 11 de ,8 tfn ,7 12 ( 9 / l~~1)7A)

3. S. h. Kc e v e ’ l , P. E ’ c r rr ne -ic h , 0. l e w i s , A. Crnpta , N . T. Y a n g , andR. Zawad e , “h )F T - i i’ rogress on l)irect i-lectronic Fourier Transformsef Two—Dimens i e c n n n i h nue , -~gr’ s , “ RADC—TR—244 • C.-nt ember 1974.

4. S. T. Ke wc- 1 , P. C. Kcurnre’ich , 0. LewIs , A. G u p ta , and R . 7-awada ,e ie r c) b ,r e ~~~. i n n iwe c— i n imensiecna l Direct: Electronic Fourier Transform

(DEPI ) Devic es , ’ l 7 4 UIt rasonics Cvmposium Proceeding,~~ pp. 763—767.

5. P. Kornreicil , N. T. Yang, and S. T. Kowel , “A Direct ElectronicFourier Trclrr sfe e rr c Device for Imag ing, ” IEEE Proc . Letters , Vol., 61 ,A u g u s t 19 7 ) .

6. S. 1. K cu r’e - l , P . C. Kornreich , 0. Lewis , and F. 1). Kirschner , “Pass-ive De t e ct ion of ~‘hc tj o~ Tr cini — c v e ’ n n ; e - to t h e e Op t i c a l Viewing Axis , ”I E E E Trans. on Instrumente ution and Measurement , V o l . T M - 2 4 , p p . 248—255 , Se p ’ . 1975.

7. S. T. Kowel , P. C. Kornreich , and 0. Lewis , “Focu s Detection Using

S Direct F lect ronic Fouerier Transform Sensors ,” 1. of App i . Photo-grap h i c En g in e er i n g , V l . 2 , 113 , pp . 113—118 , -urrurrer 1976.

8. S. T. E i e w e c - l , P . C . scrrnre -ich , A . Mahapatra , I). Cl everly , B. E rnmer ,and R . Zawad ;n , “ ly e — D i m e n s i o n a l F o u r i e r Imag i n g , of L i g h t Us ing Acous-t i c - Pseu d o—Beam S t e - c - r i n g , , ” 1975 Ultrasonics Symposium Pre ceedings,

~~~ 1 36—14 0. (i’rovidech in A ppendix A.)

9. S. T. Kowel , P. C . Kornreich , A. Malcapatra , and B. Emure r , “Experi—u-ue -ntal Cr nfirmation of Tw o—I ) imens i ooa l A c o u s t i c P rocess ing of Tm —ages ,” l’~e76 Pltrc u son irs Symposium Pr (ecessing of Images ,” 1976 Ui—trasonics~~ yrnpos flim_Proceedi~~g,~~, pp. 668—67?. (Provide’) in Appendix B.)

10. H . Ca tie r , C. S. Kino , and H. J. Shaw , “ A c u c e c u c t ic b r n i n n n f o r m T e c i n n i —qcie s A p p l i e d te e Op t i c a l I m a g i n g , ” 1974 U l t r a s o n i c s C~vr c p o s iu m Pro-ceedings, pp. 9Y—103 .

11. M Luukkala , P. Merilainen , and K. Sciat i nen , “e\c - ce te ~ to—ReslstiveEft ect uc In Ibm Film CdSe/I.,itThO3 Delay Line Svste’m ,” 1974 ltltra —

4 sonlcs hvmpos i inm Proceedings , pp . 345— 148.

II

.5 - ——- — ~— —--—--—~~~‘ -----~~---—- —~~ ,— -- — —~~~~~~~~~~~~ - - S - ~~~~~~~~ 4

(c 7

12 . V . I . . N i ’wi e c ’ u r s e ’ , ( ‘ . L. ( i c e - i c , cinch K . I . T )ccv j s , ‘‘Pecssibi lity of Switch-ing and Ste erfnc ’ Surface W , c ’ ’ e - ’ b y ~, e r n I i n e a r M ixin g in Anisotrop icMed ia ,’’ . App l. Pt~y~~~, V i e I . -‘. ~~, PP . 2 6 0 ~—261)S , June 1972

13. 14 . Franz , Ze ’ i t s c h c r i f t F’r i r N a t rrforsclrung, Vol. 1 Ia , p . 484 (1 958)

14. L. V. K e ’ l d v s l l , Sb . i-xs 1,e e r i uuc . i ‘[nor. Fiz., Vol. 14 , p . 1138 (1958) ;English Transl ;nt ion Soy. Phvs . JF,TP, W e d . 7, p. 788 (1958).

15. E. S. Kohn cnnd M . i .cempe-rt , Pinys. Rev.L Vol . 134 , pp. 447q_44c)l , 1971.

16 . ~I . F. Nvi- , Plrv siccn l i’ rce p c rtie- c of C ry s t a l s , e ) x f o r e i , 1969 .

I 17. A. Guptci , P11 .1) . D i c n s e ’ r t ; e t h u n , ~ ‘;r c u c u s e ’ ln ive’rsity, 1975.

18. A. FJ axman , .‘ . e e ’n reich , F. ocirehicross , I I . P u c r h e r i n , P . Weiner ,J . App i. 1’1 cv c~~~ Vol. 3Ee , Ti . 168 ( 196 5 ) .

19. M . Luukkala , pr iv cit e communication .

$ 20 . J. W. Goodman , F ’ e c r r i e’r ( P c t i c s , McGraw—Hill ,

I 21. J. M. Ziman , Princip les ef the Theory of Solids , Cambrid ge , 1965.

2 2 . F . A. , I enk ins and H . F . Whi te- , Fundamentals of Optics , 2nd E d . ,

I McGraw—H ill , 1976. -

23. H. 1. Smith , F. J . Bachmer , and N . E f r e m o w , ” ,-\ H i g h Yield Photo—lithographic Teclrnique for Surface Wave Devices” , J. of the E l e c —

‘ trochemical Soc., Vol . 118 , pp. 821—825 , Mciv 1971.

24. A. 1. Slobodnlk , J r . , E. D. Conway , and R. T. i)elmonico , Microwave

I tu-oust ic s }landhook , Air F i er c e ’ C a m b r i d g e - Resea rch L a b o r a t o r i e s , 1973.

25. G. S. Kino , “ Ae ’e u i r ~c t c c ’ 1 e - u ’ t ri c Inte ’r :uct ions in Acorrstic—Surface—WaveDevices ”, Price . r b-:EE , Vol. 64, Mcix’ 1976 , PP. 724—748.

1 26. P. Defranould and C. Mcue’ rf eld , “A Pi ,unnur Piezoelectric Convolver ”,Proc. I E E E , Vol . 64, Mcc v l i ~~~ p P . 748—751

I 27. L. P. Sol le , “A New Mode of ()perat ion f o r t h e C u r r f a c e — W a v e Convolver ” ,Proc. IEEE , Vol. 64, May 1976 , Pp. 760—763.

1 28. S. A. Reible , J. H. Cafarella , R . 4. R c i h s t c e n , and E. Stern , “Con—vo lve r s f o r l) i’~~K I)emodulation of Spread—S pectrum Signals ” , 1976U l t r a s o n i c s Symposium Proc., PP. 451—455.

1 29. R. 3. Schwartz , S. D. Coalema , rind R. L. Cunsh or , “Sur f a c e Waveinteraction Charge Coup led D c - v I c e ’ s , 1976 U l t r a s on i c s S~~rposium Proc.,pp. 197—200 .

I. 1

—-5--—— —-— - -5 - - - — - — - - - -- ‘ ____________________ an

68

30. P. Defranould , H. Cautler , C . Ma erf eld , and P. Tournois , “P—N DiodeMemory Correlator ”, 1976 Itltraso nics Symposium Proc., PP. 336—347.

31. C. F. Amelfo , V . .1 . Bertrcum , Jr., and M. F. Tompsett , IEEE Trans.on I-:lect ron Devices, Vol. ED18 , p p . 986—992 , Nov. 1971.

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Is d h,s. ‘ 1 , c , e , , i e n i u i c t c , , , -~~‘ c ’ c e p m a r , - , d.c I c c e i , ’ , b - . i t l e ,‘ , c m ,t I 1 ‘ ‘ u en ’, ’ , , ~~~cc i l l , ’ c - n - I c e ’ 1 s - ‘ , I i ’, I t I ,il I t e l - I ’ . , c c t ,. t ‘“ “ct ’- -’ , ’ - d C I Ic e ’ i i - 1 eren e ’ ,’ I r equ en cv

,, I ) I ’ . r be e 1 c - a - ’- - - I t m . - - c r n c ,ne i ,i , n , e . ,‘ c’ ~ ’’ ul , I c I, - - c C r I b- e l u’s n u i

- -i ti je ,, V ,

‘-b 9

ie _ b ,cw . - I

licie inter cjigc te, I con ta cts (inch m n,m I c-i I ect t ve’I ‘,‘ u’-.’cipe er .’et ion , -end m - ce s, n e cc ning the su rt eu u en of t he I i NIL

combine a l l c ’ f these rn c c ta r e g les in p a r a l l e l . Thu s I ~ ‘ n e - - , e n e ,r , ’ , e r r e - n ’ , l y t m u - l og - s’ new .c l c i c - ‘ - -,e hes t o t r c v ~ n o ’ , , —- en ‘ a - c t response to u-c consta n t v-u - e l i age ‘ e l I ’ I 1 , - e l ac r oss ton i ‘-1 1 d l i p e e l i n g d e e r ‘ rc a - - c en I ng . One Is o u t — d u f I u s i m i gt he C, c n t m , n t s r e s - i t t a in ci d i f f e n m e n e r , en f r e - 1 , e e e c u - ’,’ C - u - rIte the 1, 11 111 ‘e cu l - - -, C r e c t e n C c - f , c n u - ’ e v a p e c r ~c t 1 ~~r~ . Th is would

,‘C’v C - , T , I tce- need f , r t ’ r he - n-i i c - u r I i n s , rhe other approachI ns ~ ~ ,LE

Eu-

1 (x ,v) , (8) is C -i re..u -- a 111mm, cd A ’e cc ena ~nder ‘,Cce c e lC n , Alum in a i.’d

~~~ a lom e - 12 z ‘

useul - - ‘ cc ’ ‘ c i s t I ’ , as a -- ceC , l n a t u -’ C on cered CdS in photo—f i n g e rs in each long :,-‘,ectons .

-contac t en- ‘ ang le llci- ’ ,,e,,een ~c I the problem ccl keepireg CdS on LINbO3

after - rIng, we Csu ’,’c’ aC -se ’ rried an alternate approach.

* As t C e CoCi t ed u- C I cic e - ec S C c e - ’ ’’ nct’ unenn :u - ’ e - - - s and very th in , d c l - ‘‘, I ned d - l ’~ f e i ’u ~+ u- c r z - - : c c t I ! N ’ • and dis pensedwe obtain the re au lt i~c (1;. with - - cr lr ,g, :e - IcdCi c,,e , not ete l eed ~nto lines , but

Th us , i-V ce )ntr OLll nn ’C the frequencies of the two ~~~~~ ~~~~~~~~~~ and tra nrce c l’ c - ,,rs were fabricated in t eea-

t r ansducers , arbi t r a r y Four ie r ej - c r i p e c c e c c e l n , a n c be m ou n d , c e miu a l rr eanner . ~ n _ _ c - ‘ el C~IS ‘ - mci a ~, ‘ or l I e - c t ~ t o — d a r kThe c~rl u-’c In which the - scan of Fn ’ ,ei ier space is perf ,~c’nc’- en nndu ctlvi tv ratio ‘cinec e it c,,cc - c - - t C - ce - re recn-’.’stsi lc ed ,

i’~ I ce determined by the part i cular app l lc:e c c i-crc . I’ ~ e..’en - , by p~e mt1’eg the dark e c r € - r c n - on a dlf F ,,re-ntt

s p e C i al carrier frc” : - .c ccc ’ . t han the ~ i~~i e C - — i ’ , d - j ~~e-~ cur—Exoe~~innrental Ft e ce-:Cts

t-c nt , a substantial improve -rime-nt in li ght—to—dark signa lcan re -sb -Tam ed. This is an -co ru L lIsh eci by projec ting

The mecl 3ritv c m l - , c ,r e f f o r t s w a - c e directed T o CV8~ C a - o~nt ie- ,iI lineage ll erc n c gn e a ~.a’,k W l e i u - C samples in theors t.,.s ace,1 curing CdS uilmns, Ne curing vastly di rec r ic i n per n-’-ndicular to the contact r ingers, Thusimproves the light—to--dark conductivity ratio. t I m e cl n , C ’, Ci “ c - c ” ’ iS c ’ , ,etc- al - ’cn sc one axis in k—

It ce, - c - ’ic beca me clear that C - C - u- technique used on space , wh ile the ltg i- t -urrent is trans1at~ d o f f this;lSbn) could not be the same as that used on glass- uxis inc to a a’tfjerer~t region of k—space,

- ,ci- C-~~~lcc charac teristics d epend stro ’,glv on the sub— The f i r - I c e-vice e el this type ‘-ad a li ght—to—dark

.‘etra tes used. : c c e r l c l g c u r i n g m a t e r i a l dlff ,,ses n - - c e t of Cc n c e , J C ion r a t i o u -c f 0.1-1 . Thu s tC ee’ i lue~ r c , u - - a ’n e n ’ a f m , s r d —t C ’ e ccc C ’ e n r a c , ’ i n t o n - n - ic f i l m. Icr e x a - ’ c p l m • sodium e1 Ccv — - u : ’ , , L i n g was o c t s u f l l c i e n t n - u - - obtain algolf i—

r ,~ if !uccc- -, out of ~ i i 5 s . I c e - c:, m r, c I e n i s t l c -cc ‘I t h e - can ’, l i g l c t s e n s i t i vi t y . Howev er , t Cce n -esponse to dark

ii C-ms - n e c e r e g e - -, c ulie r a m , lv d ep e -u e d i n g c - ce ‘.rlee t l e e - r t h e - a cc - - e m n - c - c - ’, was very i e d n - g u -’ , en s t a b l i s h i n g n - C a t the u n - s o r t -1 1 C c cee~ ma t e r i a l s form t r a p T b n g C e n t e r s , re - c o m b i n a t i o n C l I m s u-’~~ c i ’ c I r a l a rge - F r a n u - - c < e l g ’ -el m c u e C C e c t . Indeed ,

c enter s , or s c’ . c - i t i z l n g c cc r , t e r s , one found c i , . i c in these filmst - — 0 . 4 ’ ,0~ t i - u - - g I c e~~s su b s t r a t e s ’ CdS same ev,,p-ira te-d out of Fi g. (2) ‘ l l e - cw ’ . a t yp i c a l s le~e r’~ frequency space

a fused qua r t z b o t t l e onto5

a u l t a he ly p o si t i o n e d sub’- ‘~-s’- this - C u - s l o e . With no In - I.,, to ha- t r ansduce r s , we

s t r a tee in a ‘.‘au - uu m ~‘I 10’- t o r n . li e .’ -u- c , ’ e p c S i cc l ed ob t a i n e d ‘a-’ l - , t n c i n - c : i c s , ~ith n - i , . ’ t r a n sd uc e ’ r ’e exci ted ,

I i i uees -d cl ,e n - CcIgI’ , co n o u c t i v : t i e s b e e t a v e r y p c e e m t he - se - - - I n d ex e xf e l b l t s t rong v e r t i c a l m o d u l a t i o n . T h i s

1C5 ‘,-t o— dsr ’s ratio , T a i n c r e a s e t hei r a u - m u s i t I ’ , I t y d i n a p le ev is c c C ~ t , u l c c e c l b y s i n u so i d a l t v exc i t ing each

r n - a - were cur e d at ~ - uc~c C u - ’ t a N f l ’s which il transd ne cer - s i r h -, v o l : , e g a - controlled os cillator. These

trs c c c of D~ and Md i c c it , At th u - ‘ C u r u time the I Urn two ,sce c- l l l ators cci ’s in turn - i r I ’ ,- en nn by a sl s.ely -~- a n - - c cn g

s,c a Cu do pc c~! h ’c p l a , C r u - It in a quar mu - . bc - ‘ c , c e i c h u-,~C (30 0 I-hz ) ext ,c , r scl s in u soid a l g e e C e C a r - - r , sn - a p leasenabout 25 c c ’ , , of CdS yc r e a 1 2 . 5 gram ,’, c - i C,. Is j m

d i f C c - - c e n n e c a - o f I C C ’ l m l c r o d u c e d b e r s e e c n t i m e - c c . This p r o —Ftam v ex p e r l e m i e - a sue-re con du c t e - I by - e l t e i l n e - I n s . - en - C C - S d uces t n , ’ c i r c u l a r scans of k - a ; - ~c a - seen i c t h e f i g u r e .,ct M C I , 0 , an - ’ Ccc , ti~ optimize the pho tocone i cc e t iv i tnu - The p a r C - i c e e l . c r c i- ii- radius is controlled by the ampl i—p r c ” ; c e r t l e è of C - I- a - films, We C - b e l ly s u c c e n e , C u - C in - -S

t ude “I the -.c.c c’ep ,u- ,’i e e ’ r a t ’ n r . TC - e -outp ut of the d e v ic et a in i n - w r a t i o s c r 1 about 1000 at 25 f t . ca n d l e s . At the

Ic, shenw m’c .us t r cc -.‘ e r t I - cci — e n I g ‘ above , or below , n - n e-same t ime , the ,,ot’cdu — C- lv i ty of t I C I t i c : ,-, w e ’ q u l n - u ’ hig h

n- ’ ‘ cc - C-a c C ’ po in t on 1,,.’ circle represents a specificabou t 1 nanccmho/EJ This is e s-ce - i nc C a l to obra in

ce-op - n - u -c l I n - e q e e e n c . . ’ d i f c e n - e c n e and a s p e c i f i c spa t i a lr. asc~n 5 l I c signals ea t’ h , a e - c n e r c - c r C ’ , - t l i n e s having I r e c ~ ec e i c - creeb ocit 211 , 000 sq uares . The , e c c c i C - C device was fabricated identically. cx—

We hoped that by suitee Cc l y a , C ’ ,c,c t C n , g the , b c c i d l e T g c e p t tha t a d d i t i o na l c c i c l p h ur was i n t r o d u c e d duringles-e c a of 0~~, M CI and lu , one mc r” Id u - - c good photocon — ,‘va” ,a r a t l - ’ n , This produced a f i l m w i t h l ig h t — t o — d a r kd . - t i n e - f i l let , of ,.dc~ Ofl c ITu ’ c

i. IienWe .’en , , , m I c r num er— conductivity ratio of approximate ly h e ) . This device

o- .c. a t t e m p t s , we cam a to the i , jn c lu s i o n t I - e a t t h i s is a u f f e r e ci f i l m peeling, p i , b i b e i s ’ c,e, eseet by i n s u f f i c i e n t- t p e c c s c ~ibIe c t , , u - e i t is d i f f i n - c c ’ H I , ‘ve,’ , ee mnm e the -Ic- su hs t n at cc pr’eparam 1 , - c ’ and/or b y an ,cnse ec ’ualiv hig h level

g c - C c g e l l a - a t r I m a t e ri a l c , i f f a c celn g nu t of t h e LiMb o 3 ‘I a ir pollution in t he t .e b e r a t o r y , Th us only a smallu l u n l r c g c u r l c , , e .

I I C I ’ - , , . a p p r o x i m a t e - I c c 52 of the f i l m re tmain eu- i usable ,‘ci. x m we decid ed “ ov et come ‘ le e i c r c r n - l e m ~f the eu b— The tu- t e c u - - K e l l y -.h ccce T i c I c -n C , however , was as l a rge as

c i t ra t e e n t i e i - ] . , by l ay ing n -u , a t h I n , l aye r of St O , be— in C ’ - ’- ‘r evioces devic u -- , and i t was in , - , e n i e d in c , , -

t w e en t h , - su b s t r . e ~~c and t I - ce cTc j S f i l m , Th i s i ,cu-r su u l d - , i n n e i a and tested. Fig, (IC shows t v p t , - a l resul ts of

i ns u la t e t he (‘di from t i c e I INbO , . At the - - cm x,’ t ime , 1r a - ’~C - e . ’ c e e - ‘, ‘, c e : - - w i r I , this , i m u - Ice. I n , - the d i l f e r e n c eS i ” , beln g ‘u - c r y . m e e l , i ’- r he ~~ a 1 l ~~, u - - i d not - C t l l c c c e e between r I c a - $c-cn s w i t h and w ItIiu ,ut li e-hi . By c h a n g i n gl u c r ~~ the (dl. c l , 1 . ’ I d u - , , p r , -.’,’d u - c ry c c , - a I , , I , , l c , - cc

the c ’ x n e r n a l ‘ s” u ’~~’ C ,m n et io n ; ic I - i C - r a t , , u- ,’ dt T S can hewe I m p r o v e d t h e e I - u - n c n , ,-c i crk r a n I c ’ ‘ ‘ I , c , er f I l m , , C c ’u a , l , ’ c c b c rc , ’ l , m a k i n g t h e dcv i e m ’ random access , One reasonI c e , n - - n c c i I d l e in ‘ Ic e f i r s t I , w a t t e m p t s . U, an ” ‘ c r 1 1 1 I e , r u - ce - odd c,vimrirr etniu - ’,, e x h i b i t e d is that the voI C ,c e - ”

- . r k l ~~~ on o p t i m i x C e u - v an - c e - s d u p I Olu- l e v e l ’. - \‘c’ r’,ce ,. I r e q m c e n c v e n - u - e r a ” - C t ‘ i c r voltage contro lled osc il—l l n f u , r t e c u - e e C - e l y , t I m e - S i c ’ l a y e r l e n c . I i t ’, u,wn pt - cub Ce -m r ,’

‘ d l or I s non ] i , e i ’nc r , a s showni in F 1 g . ( 4 ) .m m .‘ , r s we l l w I t h y — u - c n~ t I . I ~~ l , c . b , t c h h a’, a I , ’ ~~ thermal I ’ b e c ’ ’ , ” r , ’ s u l t u - - . t b re ’ ee g h q e e n c l i t a t i v e and rather crude ,

- . ‘ l f i u I ” r e t ~ l enx p a n tci c i n I I c i f l e e - ‘ l u , bc ct 00 Z — c u t a re bmlg b t l v - c t id ‘rIng. The signal s a re large , even inw b I c Fe Ic ~~~ a - - - e - f f i~ l e n t , , t ‘ x I- ,ccesi ~~tI .c ’ - - i u l 10

the cen c-i, r ed I u ric ’, ; a re de t t ’ u - m ed a t the - d i f f e r e n c e fr e—- - ca’. he r -ci ‘a’ - - c m , (be e ’ I I late tend to peel c _ C I d urin g n4’eenc fes of the two C c c m ’ , e , I c m - ’e r cc ; and au Image depen ”

t i n e - Hec wcai’ r , cr .’ wo u i ’I p r e t e r C o - n . ’ C h i c C — n e a t -- :~~ d,e e a t e , e I t ‘e M s I- , - m n c r a m , , e e B t l en I t -4 ’”, i i ’ ’~ Onl y We c c , - - - , c - , l t Im ,- I , c l l . u w l c e g rh arac teria t le, c C e ’r the

C I e m a d w eqecec I i y v1 c l- I , ’ ‘, , e r I . , ’ a- wave I I cc - ‘Ice\ - I c e ’ S , Cci’ ’cu r I bed :

- c I C r e c I c - - It hi oquc a I ‘u-e’ I ce- l i t cc - Sie ri ac ,’ c.e-I’,’u’ S ~‘ c I b 35 Icr / sI ‘ ‘ C . - - , . I n t , a ‘ m , c C c e l - i ’ c ‘ 1 a t t e m p t s r c ,cv,-iul t h e - c r m r - I e i u - ’ n r c v ,‘,e- .tn,1 MH z

I ‘Ce - ‘~ I n ‘ - c - C I ) h .cv ,’ hi e n . . l i nsece e i’ s ’. fu - i l . c ,~~~ ~~~~~~~~~ s I c e - 12 . 7 mac a l,’,7- . . . - I - c a ’ i ’ u- ‘ he-’ S I ~~, - I ‘I’ b oe,ridci , ‘cc c i c c r l - ‘ - , - t u ’. c ’ i I ’ l u - n r c r , u u , ’ e , i c e - ,’ rc , 17 u~ cc 17 n~

1,70

S. W, cwe l

contact gri d petiod i ci t v l) , i . l i tma .

I I ce t r a n s d u c e ra were f i t c i ’ : , F I t e g e r — p a i m , apod icc ed I , I’ . - K - - c c e ’ c c c l e , S. I - i c - s c -I , I ) . I . i l e ’ c n l n i g , N T , Y a n g ,indium conta c ts , A , , c 1 c ’ ,, , , , m d ce . 1 c ’w c . , “ I ll - IT : D i r e c t E l e c t r o n i c

- I c e - u - C - ‘rItes ci o p t i c a l I mrrag ecc ,” lF :EE F r ” —c , c r c -, i c i s i c ,ic , c c C i r m y s , c c c l . h , p p . l072.” l ei M , Augu st , 1974 ,

- 2. 11 , : ; a u t i , ’ r , ‘ i S , b c i m e , n and H.J. S- m w , “ Aco,i -, c I - i n - ans —We have shown experim e n t a u - l y C - c m em s i r - leg s Ign - c c -

- - c e l i a - - c c c l - c c i ’s c’,a

~ l i e d I n el p C - I cccl I m a g i n g , 1-1

of the form of the integral ,c t I n-C e p r r c J r c u - t of a n elec— , , , ‘ - , ‘ -tron distribution multiplied by time product e l l W ’ sun- Ht c as , - r r e - a ~~j-s..ccCre ~r0ce- us,~~~~ ,~n- , pp. 9 - l - c ~

face acoustic wave fields can be obtained on CdS films , 3. S.T. Kowel , P C , - re , re jch , A. M a lmapatra , 0, C e’:e r 1- ~B, Errmmer , and , e w a c l - , , “Twcc -Pimen sl ona l Fourier

1 (t) I (x ,y)f1

( x - v1

n - ) f2

(y — v 5 t )dxdy. ‘n - e ,s n - :y of II,:) ’ , ‘ s i • ,~ - - : ‘ e - e C - I o Pseudo be - en c . Steer l: n ,~1975 Ultrason l - ‘i’.’Iclposl unn Proc eedinga , pp. 1 C c s — I 4 n - c ,

Clearly this sUe - u - C - cart be -nc’e e.C f or ima g ing by -‘ J R . E l l i o t t , R , i , . e , , c n sh o r , bl , F . P i e r r e - C - a , -~-~ a L .

choosi ng the acoustic fields , f , f , to Ice s c ann ing ,, -1 2 ‘ ‘ n m--L ~c , r Ce’,ra, t e n I SC i a of Ii no, Oxide—SI 1 I r - - ~ ‘-e . r—pulses , and for Fourier Ic’se .aglng if f , F are sinu— , - , cc1 2 Ia ’ a ‘u-c ’, - ’- C o n - ,- - c t - ,’c’ r s l , ’ r - l p i l c a l Imaging a~’,-C M e di c-ru - ,solda, Such a device could a l so be used as a mono- -- -

- 11 5 ’ e l m c c s c c c : i e 5 s ’ - c p r ’ s l . u r n P ro~: e e c C i : r c e s’ , pp i’d — I - c llithic :,nvac l -ce r. tie have shown t h i s ns Cf, ’ :I t o belarge even in er ,,n-e re cJ sulphur—rich e c I m s of Cdi ’ . - n , e n c l C l , S.T. ~~~~~ P. m. . “ c r r c , r e i c h , i l . , iSW Ii , a r C F.D,se n s i t i v i ty could p ia ’,’ a ro le in n -h e u - c ’ c c ’.’ o e l v e r app l i c a — ‘ r a r h i c e r , ‘ ‘ Casa’ ,’,’ en l i e ’ , a - t i o n of ‘ - i c r ion ~n -a rs -.’crset i on by p r ov i d i n g Ir c’qre ec -r m a n i p u l a t i o n , t o I - c c p t i , - a I V i , -’ ,c i e c g A x i s , ” I I ’ , , h i T r a n s , on l n s C r .

We are o - ’ n C - in u in g to t r y to i m p n - a - ’,- cs t e c e y i e l d b y and ‘c u- ’, -, ,, V ,~l . I ” — .i4 , Icc , e , pp. 2 4 8 — 2 5 5 , S en t , ‘ 7 5imp rov ing the adc -e ’c l u - cn of t ime f i l ms . W i t h t h i s accom— - - - - -

~i . M . , e e e e c e e a , P . I ’ C e r i i a c C c c c : e and ’ . S aa n i n en , ,‘c-1 c ’ : e S I . ~pu shed , the c rude prototypes descr ’L r’e l h er e cou ld b e - -- -- r’ u,SC - c r 1 - cc E f f e c t s in T h i n F i lm C -, l ’ c e / L I l i n - u e c a vimproved sufficiently to enab le de ’.’ n - es of t h i s ty p e - , , - . - - - I -- i tnc, c-o.- steen - , 1 9 - .c Ul trasonic ecn- ’,’c ,ei eu-c r - : e e n i n g a ,m ’s be used in a wide r e - c~gcr of a p p i i r ~m C i c r i , ’ , .

u - , , 345—348.

Ae k n .cie4j~~nceinccn C 7 , U . Franz , Zeitschrift Fu-e m- N atu rCu-rsu - :,ong, Vol. 13a

- ‘ p. 1 54 ( 19581 .c e a u t h o r s wish to .ao~~m c o w c e d gc- t im e suppor t of t h eM ational c cr ier .ec e t “ c e n , I a t i o n and - ‘I the U . S . Ar ms - N I g , c i i . L. V . ‘ciJysh , SIt . :‘ie~ jcerlnn i Teor. Fit . Vol. 3-, ,1 i s l o n l aborat ory , Ft. Be lvoir , VA. We are a ltec e ~ c - c c . - - - p . I i ‘I (1958); E n g l e s C e Trac e slaC -l on Scc s’. Ph ys. J ETPfu l for the i r n ere s t an d enccouragement of the I’ S. Army V o l . 7 , p . -‘ Cl’s ( 19 5 8 )I - - ’p Cg r a c -C ,l En g inee ni’ .c ,aC, c - r a C - n - cc , Ft . b e lv cc ir. - -

- . E n . -C , ee ec cc C ’I , l a m p e r t , Ph y s . R e v . , V o l . B4 , pp ., terlxd are due to Thoma s M c , r C - e C , John Wood , and - .

~~- 91 1 9 1 -________

Paul Reck I - e r c c e e l r a s s i s t a n c e In carrying out the ‘ “

measurements - C c ’ s , m I - e d in this paper.

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‘ m u -

t i~~ . ( 1 ) . S c l , e m a t l e ‘ I t w , ’ - - e I I r m c e - n s l , - c , , l IIIP’I n c e i c . e - m -

‘Fle e , l a t c i , e b e , c m a c c In I ” i g . ( I I ia , ’n , ’ I n , k ,’n mci , mc,’ .n It’

vi e’’ b c ’ t w h i e b e C C c c - 1 ,1 - , s i c m e t et c h e - I inC , . t r I p ’ -

I- / I

4

700 0 0It It 0 ) ,, It

£000 ,, 0~~~ 0 c

cc

5000 :° ;‘sa-’

~- -

-

cc ‘ ~ ‘~

- a a-

‘

4000 - c, ,c •

I~~~~~~~~~~~~~~~~~~~~~~1

3 3 ) 0 0 - - 0 cc

5 , coc a-,

2000 cc

S

- - a

FIe- - , n - n c i ‘ n - . - q c c m - n c y s -u - a n ‘ I ‘ 3 c m - i’ u r r e n t . Fig. 1 4 1 , ,-’,, tu al s p e C id f r e u -~~e cen ’ ia -s in I rm - ,- .- m s m ’ meters1t ’ C C I e . inc de j i ’ , t i c im’ , u- I r u - i ” ’- I~ l e a - C - e u trans— ‘f t ’ s p l e c c l d ry- ,’- n e p r e s e c r l ed b y c i r c l e s in F i g .

r’- cii a c i r .cc la ’s e r ’ : I - u ’ r f c c I iici c— - ‘ .c-~~~~r bC I item , (2) and C I e’ - ( I c . I, - t e C - 1 ce -’ - e ‘- ‘c c-c- n - i c - c c - - :c ces ed bythe n ,nl ine ar behavi or c’ I t h e v e ’ i t . a ,-c ’ — r ’n t r o l l e dns ci ll ,e t ‘ C - , amsel IDa- l an- i ’ 1 p m u- c r a l L - n e c , , ” n t of

sw, - ‘ cen r e.r wi Ch t ’ d e ’ , n e n t e r - ‘F t i : , I ’ : , , e , ’ eF , - ce r I , - r - -n - -n- i- , — ely 0 , C e - s e - q variables ared e f i n e d l’s’ q ,, . — C2 ” I ai , am’,,I ‘h = — ( 2 ” / a ) ,smi mere a Is m c - e c - c l c c ’ i ‘ n - h o ~ ,a - c c l U e ’.

F i g . ( Ic e ) F i g , ( ‘Ib)

Fli . ~P . i’i ‘ip c c s r ’ . 1 l I s I - ‘-i - - i t ’ ’c ’ , b e v I u ’ e , Bo r i c I g l e C and c l a r k u cc r r e n t s c m ’ u e S r c r F c l e c I en n i . ,, ‘cc the , - ‘ m e t e - I f i n—g . m -‘ , Sic no P h i l c e c I l e ’ c - i - i - n t - - C l u e - ’ - .m t m F c I e X pat t er o s ar e caused by the si c,’ . .’ ncm nmi nl fre rmit tes of the CdS

fiI ~~. Fig. e’ ’i a I it, O c r r e - d e v i c e in ( C c i ’ - l , i r k , w h i l e F i g - ~~h 3 it, I c r t i me IliumIcu. ,r c ’ ,b e lc ’~’ Ie e - , ‘1, -IC that

traces re t - rc’s en C ce-imp C i t - c pa , r a m l e t ’ r - c c c n a , ’ c c i t u c i e s o f [Icc’ Fc’c , in i e r o e u m F ~n-c me, . l c C cc -

6 12

_ _ _ _ _ _ _ _ _ 4

C—i

APPEND I X C

pHENo~4F:NoLOc I CAL CALCULATION OF THE CONDU CTANCE OF CdS

Though we realize t h a , various electric field and strain effects

may vary through the thickness of the CdS film , we shall , for simp l icity ,

assume all effects to be uniform over the thickness of the CdS film. Thus,

the conductance per square can be expanded in a Tay lor series to f irs t

order in the ligh t intensity l(x , y) and the strain tensor ~ and to

second order in the electric field E , due to the SAW in the p iezoelectric

LIN ’b 03

substrate.

D DS L LSo~~~(x , y) = 0

3 + 0

B + a~~ I(x, y) + a~~~+ i (x , y)

+ E + cJ ”~~ l (x, y ) E + -~~ E E (C . l )

-iv p ).i ct~ ~ ~.i r~ p U’) II “.1

LF+ 13

3 I (x, y ) E E

where summation over like Greek indices is implied.

-- I Here

Dis the dark conductance per square ,

is the change of the dark conductance p ’r square w i t h s t r a i n— C e

~ ( lv

I is the change of the conductance per square with light fluxI ( x , y~~, the ordinary photoc onduc t ivitv.

o9 -

Is the change of the conductance per square with light ands t r a i n ,

is the linear change of the conductance per square wi th elec—tr ic f i e ld ,

c c”1’ is the linear change of the conductance per square with light

U and e lec tr ic f i e ld ,

is the chan ge of the conduc tance per square to second ordera~3pv wi th e lec tri c f i e ld , and

I ‘- —- --‘ - - -‘- -‘5 - -~~~~~-.--— 4

C—2

LEa is the change of the conductance per s q u a r e to first order

I ~~\) in the- ligh t intensity and second order in the electricfield.

Our pol vcry sta lliiii ’ (~d~ films have essentially isotropic symmetry . There-

fore all 3rd rank tensors must be zero .[l6

~ This requires that 13DF and

U

he zero . This also requires that the ordin,’uerv photoconductivity

be essential~ y a scalar , . The dark conductance is small com-

pared to the photoc onduct ivitv at reasonable l igh t leve ls for good ph o t o —

conductive CdS . This allows us to neglec t the terms -~~~~~, , 0

D~ , and

I CI c 11’ ii ’ ) ‘i~~ C)

This reduces (C.l) to

j a a -~ 1 T ( x , y) +

LS i(x, v ) t +

‘i, ’ - L n-i c- -11’ j J C) ~n V

LE ( C . 2 )+

‘i~~p~ I ( x , y) E

icEv

The term o~~ is t he one t h a t has been used in a l l our prev ious DEFT

I devic es. However , here we are interested in the change of the conduct—

Ivity to first order with ligh t intensity and to second order wi th elec-

tric field , the laot term in (C.2) . I t is this term that will permit

the effective steering of the wave vector ,

A similar effect has previously been observed by M . l,uukkala et .al~~~~~.

They observed a substantial change of the photoconduct i\’ i tv of a CdSe film

deposited on a I,iNb O3

substrate when a SAW was propagated on its surface.

They observed the effect with 70 MHz ~~c\)1S . There is substantial evidence

that the effect is to second order In the electri a field , They observed

a change in the d.c . ph o tocond mir tivftv as well as observing an output at

twice the SAW frequency ; and t t m e ’ effect di sde( (ecc ’l r e ’d when the experiment

r was conducted on a no n—p i ez oele’- tric fused quartz substrate. The etfect

also known as t h e ’ Fran z—Ke ldvshi oft e e l has been observed in CdS by W.

I

C— 3

[ 1 3 1 [141 15Franz and I.. V . Keldvsh as well as by E. S. Kohn and M . A. Lampert

They also reported the eff ec t to lIe - quite large .

The electric field E due to the surface acoustic wave in the piezo—

I electric suh str -i t e’ has the form

E 1~ cos(- - : 1

t — k x)

E c\ i i~ = E2~, Cos(c , c

2t + k

2y) . (C. 3)

I E3

= EA

= E i~ cos( ’ei

1t — k1x) + E 2z co s( - c - 2 t + k 2

y)

I Note that the y directed wave is propagating in the negative y direction .

The app lied d.c . electric field E0 applied at the p ickup contact is in

the y direction . The componen t of the surface current density K in the CdS

film is given by Ohm ’s law

IK = E (C. 4)

where a is given b y ( .2 ) . However , we detect the v component of the

current density,

I K~ = 3xv

EAX

+ ~~~ (E0

4- EAY) + ~y Z EA7 .

Assuming that the CdS film has isotropic symmetry in the p lane of the

f i l m , th e xy p l ane , c d t has a un ique axis norma l t o t he f i l m in the z

‘ direction , the photoc onductivitv tensor components of interest and using

[161the matrix notation of elasti c it y

I T ( x , y ) o~~ EAX

(EAV

E~ )

I 0yy 1(x, v)01

+ 1(x , v) 1’ 1

A + l(~~,y) ‘~~~

(EA

+ F0)

I + i(x , v) c i ’j’!.~ E~,

(C.?)

I

C—4

= l ( x , v) LE (E + E )E (C.8)

vz 66 Av 0 Az

wh ere , for the moment , we neglect tite small strain - dependent photo—

conductivity terms ~~~~~~~~~~~~~~~ S u b s t i t u t i n g (C . 6 ) , (c.7), and (C.8) into

(C.5) we obt ain In-e r tie surface current density

K = I ( x , V )Ic~L

EQ

+ c~~~ E~ + ( I~~E~~y + 3o~~ E~ EAy +

+ ~~~~~

+ c 1’ 1 )E ” + 3o~~ E,~

+ (a~’~ + 066 )E

Az IEO

+ 0LE F2 E + LE E 3 + 10

LE + O LE ]E 2 E } . ( C . 9 )

44 Ax Ay 11 Ay 13 66 Az Ay

- From (C.3) and (C.9) one can nbserve current components oscillating

with the following frequencies

zer o

I -

cii2

2w1

- w2

2-- - 2- w

1I 2w 12~ 2

2 cc )i

+ w2

2 - - + w

I lie ) ,.

2

I The d .c . component e e l the surface current density is

I K~ 0 = [a , + ‘~~~~ E~ + ~~~ (a~~ + a

~’~

) E~~

+ ~~~~~~~~ ~~~~~~

+ n~- (a~~ + ~ ) ( E ~ + E~~ )]1(x , y ) E 0.

II

-J

C- 5

There is onl y one term oscillating with the differ encc frequency, w1

—

K 5jg = I ( x , y)(’~~ + ~~ ) E

1 1 F

0 cos[(~~1

— w2)t

- (k1x + k 2)I .

(C . l l )

However , note the fact t ha t for a1

> d 2i tha t is k1

k2, the ibove con—

duc tivitv wave propagates in the positive k direction . For ‘ 2 >

that is k2

> k1, the wave propagates in the negative k direction .

I

III

IIIIII1

i -- - -~~~ - - ‘— - --—---- - --- - 4

APPENDIX F) D—l

CdS FILM FABRICATION

D.1. Fabrication on Class

Here we g ive the reci pe for making films , including curing and

cleaning stages. To some extent , the steps are specific to our set-

up and equi pment . However , be cause of the significance of the develop-

ment of the layered technique , we decided to give the recipe intact.

Others knowledgeable in this field should be able to implement equi-

valent steps in t h c ’ i r laboratories .

A) Cleaning of substrates:

I (1) Clean substrate’s with soap , then with “m i c r o ”

I (2) Rinse in super1) 11 2

0 at least 15 minutes .

(3) Put in ultrasonic cleaner with acetone 15 minutes.

I (4) Rinse in superQ h120 15 minutes .

(5) Blow dr ’, with filtered

I __________________________

B) Evaporat ion of cdS

(1) Fill one of t h e e quartz bottles with about 3 grms . CdS using

dropper.

(2) Mount bottle with “wrap—ar ound molybdenum filament in alec—

I trodes 2 and 5 of vacuum system .

(3) Surround bottle with fused qm la rt z chimney.

1 (4) Cover chimney with stainless—stee l lid so that neck of bottle just

f i t s hole in the center.

(5) Mount ,substraten -; in the holder .

(6) Assemble b ase—plate , chim ney and subs t ra te ho lder in vac uum

system .

II

_ _ _ _ _ _ _ _ _ _ _ _ a

D-2

(7) Insert therm ocoup le ’ into substrate holder and connect heaters

to e le ctr o des I and 4 of suhst rate holder for maintenance of

substrate temperature .

(8) (‘lose ’ vacuum system and flush thrice with N , .

(9) Adjust substrate h ea t e r assemb ly as exp lained below and heat

substrates to about 212 °C.

(10) Grad uall y ( 10 minutes) bring power in evaporation filament to

~~~~~~~~~~~

( 11) Open shutter , evaporate for one hour .

1 (12) Close shutter and switch off all power supplies.

J (13) Let substrates cool to room temperature before removing from

vacuum system. c ave them in the vacuum system overnight.

I Note: We have not used the tungsten boat or the new thickness monitoring

system to evapora t e CdS on glass. ‘fhis , of course , wou ld be the

I logical thin g t’e2 do . To do t h i s , after step (7) above go through

steps in D.2 point B for evaporation of (‘dS on Li~~ cU1.

IC) - :urfn g vf I i l : ’ c — : (For a three inch diameter dif fusion furnaces)

1 (1) The furnance heater 3ettings should he as fo l l ows:

Lefi Center Right

I -

5801

178 580

‘li c e- c ’ s ’tt in g s , presumably, result in a temperature of

I 64 7 °C + 8°( over a sufficiently large reg ion of the f urnance

t ube . FI’RNACE IS A l WAYS KEPT ON !!

1 (2) Set gas flow—rates as follows :

I N2

= 12 ems . (steel b a l l ) (few litres/m m .)

= 3.5 ems . (b l ack h a i l )

I IICL = 4.5 cms . (steel ball)

I _-- -~~~~--~~ -----~~~~ - - -~~~~~~~ -~~~~~~~~~~ -~~~~ _ _ _— - -‘ —- —-

~~~~-—

D— 3

Thi s pc - n - ess c-in be tricky since balls tend to move up and

down for about It ) minutes before finally settling down to

steady values. So ch e ck on t h e m (every five minutes. To start

02

gl ow , open four valves — one on the 02

cy linder , two lever—

type valve’s on the dryer next to the cylinder , one on the

flow meter. The N .) cvlinth ’r valve is always open , so you

I open only t 1 c ’ t l e e w T c , m ’ ter v a l v e ’ . H CI — open only the flow

meter ~‘a lv e .

(3) ~~o boats i r e - e i s c m l in th e curi na process. The first one has

I 25 gms . o~ ( ; eL - ~ and 12.5 gms . of Cu in each of tv o small boats placed

on the CdS . Push this boat into th e furnace with its nearer

end 26” from the end of the furnace tube.

(4) P lace t h e g l , i - ~~ suhst r.m te on a fused q u a r t z substrate (with a

th in i - eve r at qu - i rt z we e d In hetw ’- ’-n ) then place both in the

2nd ho~~t . i c ’ l l c m w the t i c - i t - — t a b l e ’ g iven b e l o w :

(a) 0 mit ic- . — ( c ~~5 cin ch be e t in p l ace

(b) 10 m l n s . — start pushing h o - i t h o l d i n g su b s t r a t e ’ i n to

furnace’

(c) 15 mins. — substr ate ’ should be’ in place . Close mouth

of f u rnace w i t h pyrex wool b a l l .

(d) !e ’ mins. — shu t o f f 02

and 1IC1 and set N2

to 9 ems . [Close

all f o u r °2 ~~~~~~~~~~~~~~~~~~~~~~~~~~

[ (e) 105 m ln s . — start pullin g out substrate’ ho - ct

( f ) 115 mins . — substrate boat out of furnace . Place in N~

chamber and flush 2 mins. with N2. Hold on to

I the “stopper ” or “ci~j~~~when you do this. The

N2 B,~~

ssure may bl ow it o f f ! ! ! -

I- - — - - - — - - . - _ _ _ 4

D— 4

( g ) l e t su h st r e t n-c cool in “ 2 chamber f or 31) m i n s . Take i t o u t .

Pul 1 out CdS and Cu boat — cool at mouth of furnace for 5

mins. Place in N~ chamber , flush for 2 mins. Turn down

N2

tee 2 ems . (black ball). This i~ the usua l setting when

f u r n a c e is not in cisc .

F)) Add itional Ti ps on Curing of CdS:

(1) If the films cured come out black and have high light and dark

conductivit ies (l,/D small) they probably have too mu cli Cl 2. Re-

duce HC1 setting. Changing IICI setting from 4.5 cm. to 4.0 cm.

is a “b ig ” change .

(2) If the cured films look yellowish , have good LIP ratios hut

light conductivit y is low , there is probabl y too little Cl2 .

$ rncrease 11 ( 1 setting.

IIIII

I ,,,_ _~~r ~

- -—--‘ ‘ ‘- - ‘ - - - — - — ‘ - ----- -~~~~~

D—5

0.2. l’w e e I - e v e r F i l m F a b r i c a t i o n on L h ~~l i )3

.-\) C le an ing of Substrates

(I) use same- p r e c e ss as for gice s ’-c

(2 ) Use ii more e lab o r a t e t e c h n i que i f you deem i t necessary.

B) Evaporation ot Cd~

(1) Fil 1 the t s-c c o mp a r t m e n t s in t h e - teingst cii boat wit ) about 3 gms

of ~~

(2) M o u n t boat in e’ l e c t ro e b e s 2 and 5 of vacuum sy s t e m .

(3) Surround c cc i ) with fused quartz chimney.

(4) Moun t L i N b~).3 substrate-s in holder using the aluminium holders

I ch i c .

(5) As semble b a s e — p I , ~t e , chimney and substrate holder in “ i c cnem

sy st em.

(6) Insert tlo- r m e e c o c i ~ c l e - ’ and - e - llne ’ ct heaters ~c electrodes 3 and 4.

(7) Make sure’ tin- t lii ckn e-’;s monitoring head is “ I c c e c k i n g ” at the

evapor i t ion boa t and that it is rect’ I ving cooling water.

(8) Close v ,- e c u e i r c - - ‘,‘ - - te m and lush thrice’ with ‘~~~,.

(9) Adjust substrate hce - -ito r assemb ly and heat substrates tee about

2 12 °C.

(10) Turn on peew e’r c c h i ’ ~ n e ’ S ‘1 1) i t er . D e p e c u i t ion ra t e - h e e u id

I r i - - e l 000 e\ ° /5cc and - I’- n ’s’, should re ac h 0000 or 9999 ,-\ °

(11) Gradually brin g I ec ~~~~~~ t it i c - ’. i g c cr it ion b o a t to 170 W.

(12) o pen -,hut ti r and ad !uis t f ii cement power to g e t a st ~~,i ,ft de—

— p o s i t i o n eu’ of ,ubout l~ e A °,/sec

(13) Evapor i t e - to a thickness of 4. Ste = 45000 -\ ° ([Inca I th r a t e d )

t

0-Fe

(14 C l o s e s h u t I e r , swi t e h o f f p owe r s up p l les

(15) I c t s e t h — i t rat e- s s i t in v i c t i t i m system ccver njght hefor~- re—

m v ing

0. 3 Curing of [ilm,s

Follow St c ) c s Cl tiirot igii L4 ci yen f o r c e i r i n g of f i l m s on g la s s e-x cej~

fo r t h e f o l l o w i n g d l i

( a ) —~e - t gas I I e s ~’ r i t e ’ s a- -i f o l l o w s :

12 ems . ( s t & - e - I b a l l )

03

= 1.7 ems . (black ball)

nci = 1.3 ems . (steel h e l l )

(b) Hold 1~i ~‘hO 3 sti l es t r e t es in a ye rt ical posit ion using ‘lie of the

c cc c e - c! fw ’ ec1~~~~ i ci rtz hol d ers . Place hc~ t h of them in t l i e - same

boat as was u s e - c! l e e r t h e g lass s u b s t r a t e - s .

(c) When mak ing ad j e i s t m e ’f l t s in t h e HI ’ I s e t I Inc * a change’ of , -v en

0 .2 c -m u , is - e s i g n i t i r e i i t change’ . It is c c ’’- i s i b l ~~ to c i t g on e !

films by suit ,shlv adjusting t lii- F ICI . i , ey e nev er had t e d re- —

adj ust the N~ (Cr th e 02 .

I(d) The s irne ’ procedure and c-o t t ines - i l e e r c t i n - i’ )~jj~,t and 2nd

(dS t i l i ~.~- .

II

II

fl~~AQl4le 021 SYRACUSE IMTV N Y DEPT oc ELECYRTCA L AND COI UTER (—ETC F/S VSTwo—ouqcNSzopIAL DIRECT ELECTRONIC FOURIER TRANSFORM (flEFT) ocvI——rTc (u)jUN 7? S T KOWEL ’ P 6 KORNREICH. K W LOll DAAG 53—?6 —C—0162

UNCLASSIFIEF) tR—77—5 P4E_

Io~~77

-

~ ‘ .0 8

~) 0L I~~I 8

~~~~~ 6

I

APPENDIX E

MECHANiSM OF THE ELECTROPHOTOCONDUCTIVITY

I A search of the literature has not been successful in yielding a

I theory of elec trophotoconductivitv which seems consistent with our ex-

perimental evidence . This appendix will serve to sketch a band—structure

$ theory based on prev ious work~~3’ 14 , 15]

but with appropriate modifi-

cations .

We assume that the photoelectrons together with the electrons from

I ionized impurity states come to a quasi thermal equilibrium in the pre-

sence of steady illumination . This requires that the relaxation time

I associated with a redistribution of electrons due to light be much lar-

ger than the relaxation time Te of the elec tron redis t ribu t ion due to a

I small energy shift of the conduction band and impurity states. Indeed

i is of the order of ins while T is of the order of io 1~3

I L e

In the following we calculate the effect of a small shift of the

I energy levels upon the number of conduction electrons . We assume that

the conduction hand edge is shifted by an amount equal to the Franz—Keldysh

effect and that the impurity states in the energy gap are shifted by an

I amo un t of energy proportional to the shift of the conduction band edge .

We show that a small shift in energy has a large effect upon the num—

I ber of conduction electrons due to the large impurity concentration in

these photoconducting films .

II

— _______ - - - — —- — , - _ ..__ - - r -

j .

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

E-2

E.1. ThE N UMBER OF PHOT O ELECTRONS

I Let us assume that in the presence of ligh t the photo electrons

• come to a quasi—thermal equilhrium with an electron Fermi energy CF •

The total number of free and trapped electrons , n and NtO, including

I both electrons from ionized impurity states and photo electrons is

I N = n +

::° )l/2

dc gI = c f exp(c - CF

)/k T + 1 + ~~ exp(c - C;e) /kT +

(E .l.l)

• where is the density of the trapped electrons , and C is the density

I of states constant for a parabolic band)~~ Assuming that the conduct-

ion band is many kT above the quasi Fermi, energy , than one can approximate

I the Fermi distribution function by

exp—(c — £F)/kT (E.l.2)

On the other hand , in the energy gap the Fermi distribution can be ap—

I proximated by a unit step at c CFe~

With these assumptions the equa-

tion for the total number of electrons , (E.l.l) becomes

1

( N — c [exp_ (c~

— LF)/kT)]f (exp—(c — C

C)/kT)(C — c )

2dc

I CFe

f + f ~~~~~~ (E.l.3)

I ~~tting

1 ~

[ u kT C du dc (P .1.4)

I --.- -

~- - -

--

~~

_ _-_ _-

• •

E—3

and substituting (E.1.4) into (E.l.3) and integrating over u we obtain for

the total number of electrons :

CN N [exp _ ( C

c - c~~ )/kT +

Fe R~(c)dc (E.1.5)

Fig. E.l.l shows the typical density of states versus energy distribution .

U E.2 Electric Field Effect

In the presence of an electric field with components E the conduct-

ion band is shifted in energy by a small amount Ac

E~c = y E F . (E.2.l)u v u v

where is the tensor Franz—Keldysh coefficient. Assuming that the

density of states in energy gap is altered by an amount Ag t~

= — Ac . (E.2.2)

The Fermi energy is altered by the electric field by art amount a.

Substituting these changes into (E.1.5) where we assume that the total

number of electrons remains constant

C +a

N — N exp_ ( C_+ Ac

_kT

- Fe

~~~ + AC —~-.~]dc

(E.2.3)

Expanding (E.2.3) to first orderin t i and a

CN — N exp_ ( c

kT~~~ E l —

A~T

_

~~ J + 1Fe

g~ dc

E FC ~g+ ~~~(c~~~)a + ttc J ~~~~~~~~~~~~~~~ dt (E.2.4)

O c

E—4

ENERGY E

CONDUCT ION BAND

II -

g~ (E) , I M P U R I T Y DE N S I T Y

• OF STAT ES

I _ _ _~~~~r

II EF~~ — — ____ — ____

I £ O DENSITY OF STATES

~~ g(e)

I VALENCE BAND

III

Fig. E.l.l. Typical density of states distribution of a CdS film showin g

f impurity state distribution and quasi Fermi energies for elect-

rons and holes.

II

____________________________________ ______________ ~~-

E—5

Equating (E.l.l) and (E.2.4) and integrating the last term of (E.2.4) by parts

yields,

CA c — a Fe A c t~cO kT = g~(s~~ )a + Ac [i— ~~ — i— j g~Ic

I C

I = g~~~FO

)[a + Ac] — N~0 .

I Solving (E.2.5) for a and substituting the result into the first term of

I fractional change in the number of conduction

electrons

I ~~c = — v CFe)kc

— cF

] + NtO E E .

n0

c n + g ( c~~ ) k’r p V

where summation over like Greek indices is implied ,

I The numerator is of the order of the total number NT of trap states

per unit volume in the energy gap. The denominator is of the order of

I the number r~~o of conduction electrons per uni t volume . Thus the frac-

tional change of the number of conduction electrons is

An N y-. .....S — ...L... ~~~ E E (E .2,7)

n it c p v ~~cO cO c

-- The Franz—Keldysh coefficient y is —1.62 . l016

evm2/V2 t15 1,

The electric fields in our experiment are small, typically ~~~ V/rn. The

—

percentage change in the number of conduction electrons in our experi—

ments is typically 20%! The energy gap of CdS is 2.42 eV. Substituting

this data into (E.2.6) we obtain for the ratio of the total number NT

— of trap states to the number of conduction electrons

N

I ...L_ ~~~

(E.2.8)

II • - • - --- -~

E—6

Since there are approximately 1018

conduction electrons per

these films contain approximately 3 10~~ impurity states per rn3 which

is auite reasonable.

IIIIIII

- j III

P rIII,

~

- - -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~_