Gires-Tournois interferometer type negative dispersion mirrors for deep ultraviolet pulse...

Gires-Tournois interferometer type negative

dispersion mirrors for deep ultraviolet pulse

compression

Christopher A. Rivera,1 Stephen E. Bradforth,

1,* and Gabriel Tempea

2

1Department of Chemistry, University of Southern California, Los Angeles 90089-0482, California, USA 2Femtolasers Produktions GmbH, Fernkorngasse 10, 1100 Vienna, Austria

*[email protected]

Abstract: Typical femtosecond pulse compression of deep ultraviolet

radiation consists of prism or diffraction grating pair chirp compensation

but, both techniques introduce higher-order dispersion, spatial-spectral

beam distortion and poor transmission. While negatively chirped dielectric

mirrors have been used to compress near infrared and visible pulses to <10

fs, there has been no extension of this technique below 300 nm. We

demonstrate the use of Gires-Tournois interferometer (GTI) negative

dispersion multilayer dielectric mirrors designed for pulse compression in

the deep ultraviolet region. GTI mirror designs are more robust than chirped

mirrors and, can provide sufficient bandwidth for the compression of sub-

30-fs pulses in the UV wavelength range. Compression of a 5 nm (FWHM)

pulse centered between 266 and 271 nm to 30 fs has been achieved with less

pulse broadening due to high-order dispersion and no noticeable spatial

deformation, thereby improving the resolution of ultrafast techniques used

to study problems such as fast photochemical reaction dynamics.

©2010 Optical Society of America

OCIS codes: (320.0320) Ultrafast optics; (320.5520) Pulse compression; (230.2035) Dispersion

compensation devices; (260.7190) Ultraviolet.

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Introduction

Dispersive mirrors (DMs) are becoming increasingly popular for any application where

dispersion control, especially group delay dispersion (GDD) compensation for compression of

ultrashort pulses is required. The use of DMs as intracavity mirrors in ultrafast laser systems

[1], and for pulse compression of high energy Ti:S amplified systems [2–4] remain the most

common applications due to the high damage thresholds, high reflectivity, and good spatial

mode preservation of DM-coatings [5]. Furthermore, DM-based dispersion compensators are

compact, robust and user-friendly. Manipulation of the linear chirp rate can also be used for

quantum coherent control resulting in the observation of, for example, chirp-dependent

fluorescence [6]. Recently, advances in engineering and the introduction of various high and

low index materials, such as HfO2 and SiO2, have extended DM applications and commercial

products to the UV spectral range down to 350 nm [7,8]. Additionally, designs for Mo/Si

chirped mirrors (CMs) have been published for attosecond pulses in the XUV [9].

For chemists and physicist working in the deep ultraviolet (200-300 nm), there has been

significant progress made in the efficient generation of broadband pulses by four wave mixing

in fibers or in gas cells which have allowed for the observation of dynamics on a timescale

hitherto unobtainable [10–16]. Unfortunately, delivery of laser light to an experimental

apparatus generally requires transmissive optics such as lenses, windows, waveplates, etc.

which introduce significant temporal broadening [17]. There are several ways to control phase

dispersion of optical pulses each with advantages and drawbacks. Prism compression

generally results in good efficiency (~75%) but substantial third-order dispersion (TOD) is

observed [18] and precise prism matching is needed in order to avoid significant spatial-

spectral and mode distortion. Grating compression generally results in less TOD but increased

fourth order dispersion (FOD) and poor transmission in the UV (< 50% even in a single pass

configuration) and similar problems from spatial chirp are also unavoidable [11].

Theoretically, combinations of prisms and gratings could be used to compensate both second

and third order dispersion [19] but will improve neither spatial dispersion, transmission losses


nor FOD-compensation. Pulse shapers use long (50 - 75 mm) KDP crystals to precisely

control the phase of pulses throughout the UV range (250 - 400 nm). These can be used to

achieve near perfect Fourier-limited pulses (< 20 fs) or can be used to generate multi pulse

schemes in the time domain for 2D spectroscopies [12,20,21]. Unfortunately, very low

transmission efficiency (~20%), significant spatial chirp, parallel displacement of diffracted

sub-pulses and high cost all complicate the implementation and usefulness of pulse shapers

for typical pump-probe experiments where the main objective is maximum time resolution.

In this paper, we present to our knowledge the first set of negative dispersion mirrors

designed to compress pulses in the DUV. We show that we can produce 30 fs pulses while

taking advantage of the high reflectivity and minimal spatial dispersion which typically make

dispersive mirrors attractive for applications in the visible and IR [22,23], and the added

control over higher order dispersion improves the compression efficiency over standard prism

and grating compensating methods. We show that this DM compressor is effective used in

combination with a hollow core fiber DUV frequency source pumped by 35 - 100 fs Ti:Sa

amplifier systems and that the compressor can be tuned over at least a 6 nm range from 266 to

272 nm for pulses that have ~5 nm of bandwidth.

Experimental setup

Two amplified laser systems were employed to produce deep UV pulses and fully test the

capabilities of the DM compressor. (i) A portion of the output of a 800 µJ, 110 fs, 1 kHz

Ti:sapphire regenerative amplifier (Spectra Physics Hurricane) shown in Fig. 1 was doubled

in a long (500 µm thick) BBO which was combined with the residual 800 nm to drive a

hollow core fiber four wave mixing (FWM) apparatus to generate pulses having broad

bandwidth in the deep UV. This difference-frequency mixing of second-harmonic light (65

µJ) with residual fundamental (65 µJ) was used to generate 4 µJ of 266 nm third-harmonic (3ω

= 2ω + 2ω – ω) as demonstrated previously [18]. A typical pulse centered at 266.5 nm with

4.9 nm FWHM, assuming Gaussian pulse shape, was measured with an EPP2000 UV2 200-

400 nm spectrometer (StellarNet Inc.) and is shown in Fig. 2. (ii) Similarly, a portion of

output of a 3.5 mJ, 35 fs, 1 kHz Ti:sapphire regenerative amplifier (Coherent Legend USP-

HE) was used to pump an identical hollow core fiber system. In this case the same 500 µm

BBO was used to produce 70 µJ of 400 nm and this was combined with 115 µJ of residual

fundamental to produce 5 µJ of 271.5 nm. The spectrum of a pulse with 5.2 nm of bandwidth

from this latter system is also shown in Fig. 2. The difference in center frequency of the

generated third-harmonic light is a result of the larger fundamental bandwidth of system (ii),

resulting in a small amount of tunability when overlapping with the second-harmonic in the

hollow core fiber. The output of the hollow waveguide DUV source was collimated by a

custom low-dispersion curved dielectric mirror with a radius of curvature of −70 cm and

steered into a DM pair before being sent into an autocorrelator. The DUV beam was split,

then characterized in an interferometer by measuring the simultaneous two-photon absorption

generated by overlapping the two beams spatially and temporally in a 100 µm jet of flowing

liquid water [18,24]. The autocorrelation traces were collected by scanning a delay stage in

one arm of the interferometer and measuring the change in absorption as a function of delay

time. Identical material and number of coated surfaces are present in each arm.


Fig. 1. Experimental setup. Third harmonic light is generated in a hollow core fiber and auto-

correlated in a thin film water jet as described in 17. DMA and DMB correspond to the

dispersive mirror pair and M4 to the 0° low dispersion dielectric mirror that makes up the

compressor. DMA and M4 can be translated to control the number of bounces per dispersive

mirror and the angle of the DM setup can also be varied. W is a pair of Suprasil optical wedges.

CHR is a curved low dispersion dielectric high reflector (f = 35 cm). M1, M2 and M3 are low

dispersion 0° and 45° dielectric high reflectors.

The DM mirrors are 20x40x10 mm and were designed to compensate for ~50 fs2 of GDD

and ~12.5 fs3 of TOD per bounce, as well as to have 99% reflectivity at 268 nm at 7° angle of

incidence (AOI) from perpendicular to the mirror substrate. The GTI-like design consisted of

a high-reflecting 42-layer quarter-wave stack, a half-wave high-index spacing layer and a

partially-reflecting two-layer quarter-wave section. HfO2 and SiO2 were employed as coating

materials. In chirped mirror designs the frequency dependence of the group delay imparted

upon reflection is controlled by means of the penetration of the different wavepackets into the

multilayer; consequently, all layer thicknesses will sensitively affect the GDD of the mirror.

Layer thickness accuracies in the range of 0.5 nm are required for the manufacturing of CMs

for the visible and infrared spectral range. Since the average layer thickness of CMs (and

consequently the acceptable layer thickness tolerance) scales roughly linearly with the central

wavelength, an absolute layer thickness accuracy in the range of 1 Angstrom would be

required in order to manufacture CMs for the sub-300-nm wavelength range. This is hardly

achievable with any state of the art deposition technique for dielectric optical layers. In

contrast to CMs, GTI-like dispersive mirrors are much more robust; deviations in the layers

thicknesses of the two quarter-wave stacks hardly affect the GDD of the mirror at all, while

deviations of the spacer layer thickness from the theoretical design value merely result in a

spectral shift of the mirror characteristics. This spectral shift can be simply determined from a

transmittance measurement where the position of side bands of the highly-reflective region of

the mirror can be used to estimate the error in the thickness of the resonator layer.

The designed GDD curve and actual GDD curve derived from the theoretical and

measured transmittance of the GTI-DMs are shown in Fig. 2. The GTI mirrors were designed

to compress a DUV pulse with 5 nm FWHM to ~1.22x the transform limit. By tuning the

angle of incidence it is possible to shift the optimal wavelength of GDD compensation

although this will result in a change of the compression capabilities of the DMs. An increase

in the AOI results in a blue shift of this curve, while decreasing the AOI causes a red shift

(Fig. 3). The amount of GDD compensation at the optimal wavelength also varies depending

on the polarization of the incoming pulse. For p-polarized light the amount of GDD decreases


with increasing AOI while for s-polarized light the GDD compensation increases. In our

setup, the third harmonic out of the waveguide is s-polarized with respect to the DM. In the

case of both s- and p-polarization, an increase in AOI results in a decrease in the reflectivity.

The coating process was expected to have a ± 1% error with respect to the optimal

compensation wavelength which is well substantiated by Fig. 2 indicating a 2.5 nm shift

between the theoretical and the reverse-engineered dispersion curves. Since a spectral

interferogram from a broadband Michelson interferometer would be required to measure the

actual GDD curves, a capability we do not currently have, the actual GDD properties of the

DMs still have to be thoroughly tested by performing pulse compression on a DUV ultrafast

pulse.

Fig. 2. Dispersive mirror GDD curves designed (thick black) and manufactured (red) for

optimal reflectivity at 7° AOI. Typical 266.5 and 271.5 nm spectra (thin black) with 4.9 and 5.2

nm FWHM respectively produced from FWM in an argon-filled hollow core fiber.

To minimize the total amount of group delay dispersion requiring compensation from the

addition of the necessary optical components in the beam path, all reflective surfaces are

custom low-dispersion 268 nm dielectric mirrors and all focusing was done with custom low-

dispersion curved 268 nm dielectric mirrors supplied by Femtolasers. Only two transmissive

optics were used in the DUV optical path: the beam splitter in the interferometer consisted of

a 1.6 mm thick CaF2 window (CVI) with a UV anti-reflection coating on the second surface,

and the back window of the fiber cell was a 1 mm piece of uncoated CaF2. Using the

estimated dispersion characteristics of air, and all optical components in the DUV optical line

(except the DM compensating components) the total estimated GDD and TOD at 266.5 nm

are 865 ± 50 fs2 and 260 ± 12 fs

3, and at 271.5 nm, 914 ± 50 fs

2 and 280 ± 10 fs

3 respectively.

The DUV path length was approximately 0.84 m longer in the latter setup accounting for the

higher dispersion. The dispersion characteristics of the hollow core fiber were not included in

this estimated value and these should depend on the fiber diameter and phase matching

pressure [25] (GDD from the argon filled hollow core is ~30 fs2). Previous work by Durfee et

al. estimated that their DUV pulses emerge positively chirped from the end of the fiber; pulses

with a transform limit of ~8 fs are stretched to 41 fs in a 70 cm long fiber with a 140 µm inner

diameter [11]. Although there are differences in our fiber setup, we can still expect that

significant accumulated phase arises from propagation inside the fiber. Starting with the GDD

and TOD of the optical components while ignoring that incurred in the fiber therefore sets a

lower limit on the expected number of sets of DM bounces needed to compensate the GDD:


that value is approximately 5 bounces per mirror (20 total reflections) based on operation at

the designed geometry (7° AOI). Since the layout of the DM compressor allows for variation

of the number of bounces by multiples of 4 or ~200 fs2 at 7° AOI, a set of Suprasil wedges are

used to fine tune the amount of dispersion by varying the thickness of the inserted substrate.

The thickness of each wedge varies from 0.2 to 1 mm allowing for 74-388 fs2 of variable

dispersion at 268 nm. Overcompensation by one or more sets of DM bounces and adding

dispersion from the wedges, results in optimum compression of the pulse.

255 260 265 270 275 280

-80

-60

-40

-20

0

20

40

60

80

7o AOI

26o AOI

GD

D, fs

2

Wavelength, nm

Fig. 3. Dispersive mirror GDD curves for optimal 7° AOI based on the measured transmission

spectrum (black) and simulated curve at 26° AOI (red) for s-polarized light.

Results

Figure 4(a) shows the optimally compressed pulse centered at 266.5 nm after a total of 24 DM

reflections and 1.03 mm of inserted wedge. The spatial/spectral homogeneity was excellent in

the far field, verified by scanning a 50 µm pinhole through the 3 mm 1/e2 vertical and

horizontal beam axis several meters downstream of the compressor. The FWHM of the

autocorrelation is 42 fs corresponding to a deconvoluted pulse FWHM of 30 fs (1.4 times

transform limited) assuming Gaussian temporal pulses. A transform limited pulse width

centered at 266.5 nm with 4.9 nm of bandwidth would be 21 fs (again assuming Gaussian

spectral/temporal shape). Figure 4(b) shows that prism compression of a 267 nm pulse with

similar spectral shape and bandwidth, leads to an increase in higher order dispersion (mostly

TOD) due to the long transmission lengths through the CaF2 prism substrate and the residual

dispersion from the double pass configuration resulting in greater departure from the

transform limit [14,18]. It should be noted that prisms made from MgF2 would result in less

(~40% less compared to CaF2) but still significant amount of accumulated TOD. The prism-

compressed pulse significantly deviates from a perfect Gaussian at the 50% level with respect

to the peak, and this wing structure could potentially overlap experimentally with fast

transient signals from photoproducts in photochemical pump-probe experiments [26]. In

contrast, the DM autocorrelation does not deviate significantly from Gaussian until below

10%. Optimal performance at 266.5 nm required rotation of the DM AOI to 37° where the

reflectivity per bounce was measured to be 96% (compare to reflectivity > 99% verified at 7°

AOI). The large departure from the design AOI is responsible for the loss in reflectivity and


also explains why the compression achieved is not closer to the transform limit. This

performance will now be analyzed and discussed.

Fig. 4. (a) Autocorrelation of 266.5 nm pulse after DM compression (black) and prism

compression (red). The transform limit is calculated to be 21 fs from the corresponding spectral

bandwidth assuming a Gaussian shape. (b) Detailed comparison on a log scale showing the

deviation of the DM and prism compressed pulses from Gaussian (black dashed).

To estimate the actual GDD in our set up and determine the effectiveness of each DM

reflection, a pulse autocorrelation measurement was made without any bounces off of the DM

pair to measure the true accumulated phase in an uncompressed pulse. For pulses centered at

266.5 nm, the uncompressed pulse FWHM was measured to be 199 fs after Gaussian

deconvolution. The GDD is estimated by the following relationship,

22

1,4 ln 2

oTL

TL

GDDτττ

= −

(1)

whereTLτ corresponds to the transform limited pulse FWHM and

oτ is the measured pulse

FWHM. Although this formula assumes zero TOD and higher order dispersion, the amount of

TOD needed to introduce significant error at this stage would need to exceed 1000 fs3. Based

on the measured width, the estimated GDD of the system for the 266.5 nm pulse is ~1490 fs2

which is nearly 650 fs2 larger than that estimated for the optical layout of the system. The

same measurement was made for the 271.5 nm pulses generated from the 35 fs amplifier

system, resulting similarly in ~1390 fs2 of estimated GDD. These values indicate that there is

indeed significant GDD in the light emerging at the output of the hollow core fiber [11]; this

additional dispersion would require an additional 3-4 sets of bounces according to the original

DM design.

The conditions that lead to the optimal pulse compression shown in Fig. 2 therefore

correspond to ~68 fs2 of GDD compensation per DM reflection. As highlighted in Fig. 3, the

increase in the compensation per bounce with respect to the designed curve and decrease in

reflectivity are as expected for s-polarized light as the AOI is increased. This is a result of the

fact that the reflectance of the partial reflector placed on the top of the GTI-like dispersive

mirror increases with the angle of incidence for s-polarized light, leading to an increased

storage time of the incident radiation at the resonant wavelength which results in a more

negative GDD minimum. At the same time scattering losses grow significantly as a

consequence of the extended storage time in the resonant spacer layer.

With the reverse-engineered GDD curves in hand, a simple model based on Gaussian

pulse propagation can be used to describe the dispersion properties of our optical apparatus as


well as the compression characteristics of the DM pair as used. The 266.5 nm laser pulse

generated by the hollow core fiber is represented by a transform limited Gaussian function

2

2 ln 2

( ) ,o

t

i ttE t e eω

− −∆ = (2)

whereo

ω , is the center frequency of the pulse. Complex fast-Fourier transformation to the

frequency domain is followed by multiplication with the measured GDD of 1490 fs2 and

estimated material TOD of 280 fs3 representing all accumulated phase between generation and

the autocorrelating medium:

( ) ( ) ( ) ( )3

322

42 3( ) .

TODo

GDD o

i

i

inE cfft E t e e

ϕ πω ωϕ π ω ωω−−= (3)

This results in our best estimate of the uncompressed DUV pulse, where GDD

ϕ

corresponds to the material GDD and TODϕ corresponds to the TOD.

The DM GDD curve shown in Fig. 3 (26° AOI) which displays a maximum negative GDD

at 266.5 nm was used to calculate the phase compensation of the DMs at the optimal AOI for

compression found experimentally (37°). This AOI discrepancy between the curves reverse-

engineered from the transmission measurement and our experimental result highlights the

challenges involved in the fabrication process of the mirrors, but a negative GDD per bounce

near our measured value of −68 fs2 indicates that this a reasonable starting point for our DM

simulation. The DM dispersion curve can be simply fit to two opposite sign Lorentzians

which result in an R2 value of 0.9999; the resulting function was integrated twice to reproduce

the appropriate expression for the phase compensation of each reflection. Finally, the DM

spectral phase is multiplied by the total number of reflections and this expression is combined

with the starting input pulse, resulting in the compressed pulse,

( ) ( ) ( ),DM oi N

out inE E e

ϕ ω ωω ω − ⋅= (4)

where DM

ϕ is the phase compensated by an individual DM reflection and N is the total

number of reflections. The pulse autocorrelation is computed in the frequency domain and,

after inverse Fourier-transformation, compared with experiment.

Figure 5(a) shows this comparison of the simulated autocorrelation after 26 bounces along

with the experimental measurement. Although both pulses show very little intensity in the

wings of the pulse, in the experimentally measured pulse they are somewhat more significant

and the FWHM is not as short as for the simulated pulse. The higher order dispersion

generated by the DMs, which is responsible for both of these features, is due to the fact that

the GDD curve (Fig. 3) is not flat over the entire spectrum of the DUV pulse. This curvature,

specifically the quadratic shape of the curve gives rise to significant fourth-order dispersion

(FOD). Since this is less evident in the simulated pulse, it can be expected that the real DM

GDD curve is somewhat steeper in this spectral region than the simulated curves predict.

Furthermore, the minimum of the compression curve (Fig. 5(b)) occurs at 26 bounces which is

slightly more than expected accounting only for the measured GDD. This is due to the fact

that some of the accumulated FOD can be approximately nulled with additional negative

GDD, thus requiring a few extra reflections to achieve the best measured pulse compression.

Experimentally, we must also be suppressing the broadening from FOD with extra reflections,

meaning that, the minimum value in the actual GDD curve must be slightly more negative

than our estimate of −68 fs2.


Fig. 5. (a) Comparison of measured 266.5 nm pulse autocorrelation (black curve) after 24

bounces and 1.0 mm of Suprasil (corresponding to ~22 bounces) and 271.5 nm pulse

autocorrelation (blue curve) after 24 bounces and 0.9 mm of wedge. Also shown is the

simulated 266.5 nm pulse (red curve) compressed by 26 bounces off of the 26° AOI DM

dispersion curve of Fig. 3. (b) Comparison of the measured deconvoluted pulse width as a

function of the number of DM reflections for 266.5 nm pulses (experimental, black circles),

271.5 nm pulses (experimental, blue squares) and 266.5 nm pulses (simulated, red curve).

Shifting the center wavelength to 271.5 nm should alleviate the need to reflect off of the

DMs with such a high AOI and bring us closer to the minimum in the manufactured GDD

curves. Figure 5(a) shows a comparison of the compressed 266.5 and 271.5 nm pulses. The

optimized deconvoluted pulse width at 271.5 nm (24 bounces and 0.9 mm of inserted wedge)

was found to be 34 fs (1.6 times transform limited) with an optimal AOI of 24.5°

corresponding to 97% reflectivity per bounce. While the AOI is substantially decreased and

reflectivity increased as expected, it is still significantly larger than the predicted AOI from

the transmission measurement of the manufactured DMs. It can also be seen from Fig. 5(b)

that maximum pulse compression also occurs between 20 and 24 total reflections indicating

that the DMs are compensating for approximately 63 fs2 of GDD per bounce which is also

larger than the designed compensation of 50 fs2 per reflection as expected for the high AOI.

The 271.5 nm pulse appears to have more high order dispersion than the 266.5 nm pulse,

indicating that the GDD minimum is slightly shifted away from the center frequency of the

light leading to greater fourth order dispersion. In this case, a more thorough scan of the AOI

space would probably alleviate some residual dispersion. Although the precise phase structure

in the pulse is sensitive to AOI and central wavelength, experimentally, once the optimum

AOI is reached, as long as the center frequency and bandwidth of the pulse remain stable, the

DM compressor system requires very little day to day adjustment. It should be noted that

although we have concentrated here on performance of the DMs under carefully controlled

conditions to test the compressor properties, sub-30 fs pulses have been obtained for pulses at

271.5 nm when 7 nm of DUV bandwidth is employed, but, as expected, they have greater

high-order dispersion.

In conclusion, we have demonstrated that despite the challenges of working with

dispersive multilayer coatings at deep UV wavelengths, a GTI mirror compressor designed to

support ~5nm of bandwidth can successfully compress DUV pulses to 30 fs. At ~1.5x the

transform limit, these pulses have less higher order dispersion when the DMs are properly

optimized compared to similarly generated pulses compressed by a prism pair. The

performance of these DMs, while encouraging, highlights the difficulties in achieving

compression and pulse characteristics in the DUV comparable to those routinely

accomplished in the visible and NIR. This is a consequence of the fact that the sensitivity of

dispersive mirror designs to layer thickness manufacturing errors increases by a factor of


approximately 3 in the DUV range as compared to the NIR. Although difficulties in the

manufacturing tolerances lead to a significant deviation away from the optimal working AOI,

even considering the increased losses with this prototype set of DUV DMs the ~45% overall

transmission efficiency puts this system on par with grating compressors. It is reasonable to

expect that revision in design based on the current characterization should allow for 80%

overall transmission with a broader GDD minimum. Furthermore, improved manufacturing

accuracy along with more complex designs based for instance on two-cavity GTI structures

might enable increasing the effective GDD compensation bandwidth by a factor of two. This

bandwidth would correspond to bandwidth-limited pulse durations closely approaching 10 fs.

Along with this, the absence of spatial/spectral dispersion problems associated with typical

compression methods and excellent transmitted wavefront, makes this an attractive new

method for DUV pulse compression. Although not thoroughly tested in this case, we see no

damage of the DM coating with the pulse powers used. The potential for high damage

thresholds offered by dielectric mirrors should allow for compression of the several µJs

needed for pump-probe and other applications without sustaining optical damage over time, as

is typical for prism methods in the DUV. With less residual dispersion, the improved temporal

characteristics of broad bandwidth DUV pulses will effectively widen the window of

observation available for time resolved studies, as well as provide suitable pulses for two-

dimensional and other non-linear spectroscopies [27,28].

Acknowledgements

The work at USC is supported by the National Science Foundation (NSF) under grant CHE-

0617060. C. Rivera was supported by a Ford Foundation Graduate Fellowship. We thank A.

Isemann for his help in starting the development of the DM system and C. Elles for assistance

in implementation as well as comments on this manuscript.


Gires-Tournois interferometer type negative dispersion mirrors for deep ultraviolet pulse...

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