Poly(p-phenylenevinylene) derivatives: new promising materials for nonlinear all-optical waveguide...

2250 J. Opt. Soc. Am. B/Vol. 19, No. 9 /September 2002 Bader et al.

Poly(p-phenylenevinylene) derivatives: newpromising materials for

nonlinear all-optical waveguide switching

Mark Andreas Bader and Gerd Marowsky

Laser-Laboratorium Gottingen e.V., Hans-Adolf-Krebs-Weg 1, 37077 Gottingen, Germany

Ayi Bahtiar, Kaloian Koynov, and Christoph Bubeck

Max-Planck-Institut fur Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany

Hartwig Tillmann and Hans-Heinrich Horhold

Institut fur Organische und Makromolekulare Chemie, Universitat Jena, Humboldtstr. 10, 07743 Jena, Germany

Suresh Pereira

Department of Physics, University of Toronto, Toronto, M5S 1A7, Canada

Received January 8, 2002; revised manuscript received March 15, 2002; accepted March 25, 2002

Several new derivatives of poly(p-phenylenevinylene) (PPV) are investigated regarding their linear and non-linear optical material and waveguide properties, including their nonlinear photonic bandgap properties thatare induced by photoablated periodic Bragg gratings. The new materials were prepared by means of the poly-condensation route, which yields polymers with excellent solubilities and film-forming properties. Compara-tive data suggest that the new polycondensation-type MEH-PPV (completely soluble, strictly linear and fullyconjugated), in particular, is the most promising polymer under investigation to fulfill the requirements forall-optical switching in planar waveguide photonic bandgap structures. UV-photobleaching techniques andphotoablation in the UV, VIS, and near-infrared ranges at different pulse durations are investigated. Homo-geneous submicrometer gratings that serve as Bragg reflectors have been fabricated in MEH-PPV thin films byapplication of these methods. The great potential of this type of materials for nonlinear all-optical switchingapplications that arises from their unique optical properties and their patterning behavior is discussed in de-tail. Numerical simulations of a switching device based on gap-soliton formation in a nonlinear periodicwaveguide structure with the newly obtained material data have been carried out. We show that one canexpect photonic bandgap all-optical switching in MEH-PPV planar waveguides. Device performance consid-ering different grating parameters is discussed. © 2002 Optical Society of America

OCIS codes: 190.4710, 190.4390.

1. INTRODUCTIONThe control and the processing of fast optical signals is ofincreasing importance in integrated optics. As opticaldata transmission becomes more and more significant,there is a growing need for integrated all-optical devices.Various concepts for integrated devices based on materi-als with high third-order nonlinearities and fast responsetimes have been suggested.1,2 In recent years much at-tention has been given to photonic crystals,3,4 the periodicmodulation of their optical properties, and the photonicbandgaps (PBGs) that they show.5

In this paper, we focus on integrated switching devicesbased on nonlinear Bragg gratings that induce a PBG inplanar waveguides.6 In these structures the Bragg grat-ing acts as a perfect reflector for light pulses whose car-rier frequency is at or near the well-known Bragg fre-quency l0 5 2neff g, with neff the effective index of thewaveguide mode and g the grating period. However, ifthe material of the Bragg grating is Kerr nonlinear, lightof sufficient intensity can switch out of the gap.7–9 Heu-

0740-3224/2002/092250-13$15.00 ©

ristically speaking, this switching occurs because the self-phase modulation induced by the Kerr nonlinearity canshift the spectral position of the gap so that frequenciesthat were originally within the gap can switch out of it.9

The nonlinear switching can exhibit bistable behavior.7,8

The nonlinear dynamics of optical pulses whose frequencycontent is near, or slightly within, the band edge is welldescribed by a nonlinear Schrodinger equation; for opticalpulses whose frequency content is well within the gap, thenonlinear coupled-mode equations (CMEs) provide theappropriate description of pulse dynamics.9,10 It is wellknown that both of these equations admit propagating so-lutions even when the frequency of the optical pulse iswithin the PBG. In fact, both equations also admit solu-tions whose pulse shape does not change on propagation.The nonlinear Schrodinger equation admits solitonsolutions,8,9 which are robust with respect to minor per-turbations; the CME admits solitary-wave solutions,9,11

but these solutions are not robust with respect to minorperturbations. Regardless of where in the gap the fre-

2002 Optical Society of America

Bader et al. Vol. 19, No. 9 /September 2002 /J. Opt. Soc. Am. B 2251

quency lies, a gap-soliton switching scheme for opticalpulses can be used: A low-intensity pulse whose fre-quency content is within the PBG will be reflected by theBragg grating, but at high intensities the Kerr nonlinear-ity can lead to a propagating solution, or even a soliton orsolitary wave, that will propagate through the grating.In principle, these wave packets can propagate with lowgroup velocities, which enhances the effect of the materialnonlinearity.9

PBG switching devices have been demonstrated insemiconductor planar waveguides12–14 and fibers.15,16

Bistability has been observed in polydiacetylene.17 How-ever, it remains unclear whether its origin is electronic orthermal. Switching that is due to the thermo-optic effecthas been shown in a distributed-feedback grating devicethat uses a polymer waveguide structure.18 The demon-stration of high-speed all-optical switching based on gap-soliton formation in planar polymer waveguides that usepurely electronic Kerr-type nonlinearity would present abreakthrough in integrated nonlinear optics. This re-quires the identification of suitable PBG materials.

The novel PBG materials for nonlinear optics clearlyneed to be multifunctional: They must be suitable forthin-film processing and should form waveguides withpropagation losses of the guided waves agw , 1 dB/cm.Fabrication of periodic structures requires photoreactiv-ity of the material in the UV spectral range to achieve sig-nificant changes of either the refractive index by pho-tobleaching or the thickness of the film by laser-inducedablation. We show below that laser ablation can be usedto pattern one-dimensional gratings, which then can actin the same way as one-dimensional photonic crystals.Furthermore, the technique is easily generalizable to thepatterning of two-dimensional photonic crystals in poly-mer waveguides by use of holographic means.19 On theother hand, outstanding photostability of the material isnecessary at the nonresonant working frequency v (orwavelength l) that is typically in the near-infrared (NIR)range for the applications discussed here. A large andoff-resonance third-order nonlinear optical susceptibilityx (3)(2v; v, 2v, v) is also required to achieve fully re-versible and ultrafast changes of the refractive index.The intensity dependence of the refractive index is givenby n 5 n0 1 n2I, where I denotes light intensity, n0 isthe linear refractive index, and n2 is the nonlinear indexof refraction, which is proportional to the real part of thecomplex20 x (3)(2v; v, 2v, v). Furthermore, nonlinearpropagation losses due to two-photon absorption (TPA)must be negligibly small. The intensity-dependent ab-sorption coefficient can be written as a 5 a0 1 a2I,where a0 denotes linear absorption and a2 is proportionalto the imaginary part of x (3)(2v; v, 2v, v).

Conjugated polymers have been considered to be themost promising organic-material candidates for high-speed photonic-switching and signal-processing conceptsbecause of their high third-order nonlinearity, fast re-sponse times, and relative ease of waveguidepreparation.21–23 Materials such as polydiacetyleneshave been investigated in detail with respect to their ap-plication in integrated optics.24–26 In particular, poly( p-phenylenevinylene) (PPV) and its derivatives have in-curred considerable interest recently because of their

large third-order nonlinearities and superior waveguideproperties.27–39 Additionally, PPV’s are very attractivefor electroluminescence40 and plastic lasers.41–43 An-other significant advantage of organic materials com-pared with inorganic materials is the relatively easystructural variation of polymers to tailored materials pa-rameters for the envisioned applications.

Recently, high-quality polycondensation-type MEH-PPV [poly(2-methoxy-5-(28-ethylhexyloxy)-PPV)] AA (Ref.44) and a number of related alternating copolymers ABand partly ordered terpolymers (AB)(CB) (Refs. 39 and42) have been synthesized by the employment of carbony-lolefination through the Horner reaction as the step-growth polymerization process. These polymers consti-tute a novel generation of solution-processable photonicmaterials with a well-defined defect-free backbone struc-ture.

The aim of this paper is to study the relevant materialparameters of such newly available polymers of the PPVfamily and show their suitability for application in a non-linear all-optical switching device based on a PBG struc-ture. A detailed materials comparison of several newPPV derivatives is performed, and on the basis of the re-sults of this comparison the best-suited material is se-lected. Submicrometer Bragg grating fabrication by la-ser ablation in thin films of the selected material, MEH-PPV, on quartz substrates is described. Numericalsimulations of nonlinear pulse-propagation effects basedon the obtained material data and the fabricated PBGstructures in planar waveguides of the new MEH-PPVmaterial are presented. It is shown that nonlinear opti-cal pulse compression and all-optical gap-soliton forma-tion resulting in all-optical switching can be expected inplanar waveguides of certain PPV derivatives if realisticexperimental parameters are assumed. We demonstratethat PPV derivatives have highly favorable multifunc-tional properties that allow one to use them as novel PBGmaterials for nonlinear optics.

2. EXPERIMENTALThe synthesis of the polymers whose chemical structuresare shown in Fig. 1 has been referred to in earlier reports(see figure caption for full chemical names): PPV,28

DPOP-PPV,32 MEH-PPV,39,44,45 MEH-DOO-PPV,39,42

TPD2-PPV, and TPD4-PPV.38,42 The dialkoxy-sub-stituted PPV derivatives were synthesized through thepolycondensation route by use of the Horner–carbonylole-fination as the step-growth polymerization process thatyields well-defined conjugated polymers with excellentsolubility in organic solvents. This chemical route is ad-vantageous over the frequently used dehydrohalogenationprocess as it avoids both branching (cross-linking or gelformation) and incomplete double-bond formation, result-ing in strictly linear fully conjugated defect-free phe-nylene vinylene backbone structures. With appropriateside chains (e.g., long or branched dialkoxy groups at-tached to p-phenylene) the materials are completelysoluble in toluene or chlorobenzene and by use of solutionprocessing (e.g., spin coating) waveguiding films are easyto prepare. The molecular weights Mw and Mn , deter-mined with gel-permeation chromatography, and glass-


Fig. 1. Chemical structures of poly ( p-phenylenevinylene) and its derivatives. Systematic names: PPV, poly(1,4-phenylene vinylene);DPOP-PPV, poly[1,4-phenylene-1,2-bis(4-phenoxyphenyl)-vinylene]; MEH-PPV, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene]; MEH-DOO-PPV, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene-2,5-dioctyloxy-1,4-phenylene-vinylene];TPD2-PPV (TPD(4M)-MEH-PPV), poly[1,4-phenylene-(4-methylphenyl)imino-4,48-diphenylene-(4-methylphenyl)imino-1,4-phenylene-vinylene-2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene]; TPD4-PPV (TPD(4M)-MEH-M3EH-PPV), poly[1,4-phenylene-(4-methylphenyl ) imino-4,48 -diphenylene- (4-methylphenyl ) imino-1,4-phenylene-vinylene-2-methoxy-5- (2-ethylhexyloxy) -1,4-phenylene-vinylene]CO[2,5-dimethoxy-1,4-phenylene-vinylene-2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene] (50:50).

Table 1. Coefficients and Parameters of the PPV Derivatives

DPOP-PPV MEH-PPV MEH-DOO-PPV TPD2-PPV TPD4-PPV

Mna 25.000 14.100 13.000 14.600 13.100

Mwa 43.000 40.300 34.300 53.100 35.700

Tga (°C) 163 65 –b 188 165

amax(UV) (104 cm21) 14.3 12.2 30 27amax (104 cm21) 5.4 16.6 16 18.4 16.9lmax (nm) 359 489 486 426 432l0 (nm) 447 570 590 500 520n (1064 nm) 1.65 1.65 1.64 1.71 1.71a1 1.433 1.416 1.357 1.571 1.587l1 (nm) 86.9 200 180 102.5 185.4a2 0.255 0.208 0.242 0.305 0.224l2 (nm) 365.3 528.9 518.7 445.4 486.4

a Mn and Mw were determined by gel-permeation chromatography; Tg was determined by differential scanning calorimetry.b In the temperature range up to 200 °C no glass-transition or melting peak is observed.


Fig. 2. Schematic diagrams of the experimental setups for writing submicrometer gratings in polymer films: (a) Direct illumination ofa phase mask on top of the sample surface. The amplitude profile of the grating can be varied, and fabrication of the gratings occurs bymeans of UV photoablation and UV photobleaching with areas of as much as centimeters squared. (b) Imaging of a chromium mask bya Schwarzschild objective showing UV photoablation of homogeneous small-area profile gratings.

transition temperatures Tg , as measured with differen-tial scanning calorimetry, are displayed in Table 1. Thinfilms were prepared by use of spin coating from filteredtoluene solutions under a laminar flow hood to minimizedust particles. Subsequently, the films were placed in avacuum oven at elevated temperatures to remove residualsolvent. The film thickness d and the average surfaceroughness Ra were measured with a Tencor Model P10step profiler. The required film thickness was adjustedby appropriate choice of concentration and spinning speed(see Ref. 37 for details). Typical thicknesses were 50–70nm for spectroscopic and photodamage studies and 500–700 nm for waveguides.

Transmission and reflection spectra were obtained witha spectrophotometer (Perkin Elmer, Model Lambda 900).Absorption coefficients a were evaluated from transmis-sion spectra of ultrathin films (d ' 50 nm) after correc-tion of reflection losses at film–air and film–substrate in-terfaces, as in an earlier study.28 The spectra of therefractive index n(l) of the films were evaluated from re-flection spectra measured at nearly perpendicular inci-dence. Prism-coupling and waveguide-loss experimentswere performed with the setup described in Ref. 46 com-bined with He–Ne and cw Nd:YAG lasers (lL5 633, 1064 nm). Refractive indices were evaluated bymeans of the formalism of Ulrich and Torge.47

Attenuation-loss coefficients of guided waves agw were de-termined from scattered light that was imaged by a lensonto a diode array.

Investigations of thin films on fused-silica substratesby third-harmonic generation were performed by use ofthe Maker fringe technique, as described in our previousreport.28 We used a Nd:YAG laser (EKSPLA, Model PL2143B) whose second-harmonic output pumped an opticalparametric generator (OPG) (EKSPLA, Model PG 501),which gave laser pulses with a duration of tFWHM5 16 ps, a repetition rate of 10 Hz, and a wavelength-tuning range between 680 and 2000 nm. The laser beamwas focused on the sample, which was mounted on a ro-tation stage and placed in an evacuated chamber. Wemeasured the Maker fringes of the samples (films on sub-strate and substrate alone after wiping off the film) andevaluated them by using thickness, refractive indices, andabsorption coefficients at the fundamental and harmonicwavelengths, as described previously.28 The only fittingparameters were the modulus ux (3)u and the phase angle fof the complex third-order susceptibility x (3)

5 ux (3)uexp if. The x (3) data were related to the refer-ence value for fused silica (ux (3)u 5 3.11 3 10214 esu).

The pulses from the OPG were used also for intensity-dependent prism coupling, as described recently.36 Theabsolute pulse energy in front of the prism was measuredwith a pyroelectric detector (Laser Probe, Model RjP-735).The beam profile was measured with a beam profiler (Ex-itech). The intensity of the laser pulses was varied bymeans of a polarization attenuator consisting of a Fresnelrhombus and a polarizer. A spatial filter was used to im-prove the spatial profile of the pulses and to focus them to


the prism base. Typical beam diameters at the prismbase were approximately 0.3–0.5 mm. We used prisms ofLsF18 and SF59 glass, which had a prism angle ofg 5 60°. The prisms were mounted on the u arm of aprecision 2u-rotation stage. The reflected beam was de-tected by a photodiode mounted on the 2u arm of the ro-tation stage.

Bragg gratings serving as PBG structures in thin poly-mer films were fabricated by use of two different opticalprinciples—UV photobleaching and UV photoablation—in two different setups. The first setup, which offers themost flexibility in generating the grating structure, uses agrating phase mask with twice the period of the grating tobe generated. The phase mask is brought as close to thepolymer film as possible; typical spacings are approxi-mately 300 mm, including a protective quartz plate of 200mm. This setup is directly illuminated by UV laser light.Because the phase mask is typically designed to diffractmost of the intensity into the two first diffraction ordersso that less than 4% remains in the zeroth diffraction or-der, the grating period generated in the polymer film cor-responds to half of the grating period of the phase mask.The advantage of this setup, which is shown schemati-cally in Fig. 2(a), is twofold: One can generate a nonho-mogeneous amplitude profile of the grating, e.g., anapodized grating profile, by applying a laser beam with anappropriate beam profile, and one can easily achieve grat-ing areas in the range of squared millimeters or evensquared centimeters. Apodized grating amplitude pro-files were generated with a nearly Gaussian laser beamprofile (Nd:YAG at 355 nm, third harmonic), and the ho-mogeneous beam profiles of excimer lasers (KrF at 248nm, ArF at 193 nm) were used to generate homogeneousgrating amplitude profiles in the sample. The samesetup can be used for photoablation as well as pho-tobleaching of gratings in conjugated polymers, depend-ing on the applied laser intensities and the resulting flu-ences on the polymer surface.

The second setup for submicrometer grating fabricationin polymer films applies a Schwarzschild objective to im-aging a chromium mask onto the polymer surface. Theobjective blocks out the zeroth diffraction order of thegrating, the two first diffraction-order signals interfere onthe grating surface, resulting in a grating period that isreduced by a factor of 15 to 50 compared with the chro-mium mask, depending on the objective in use. A sche-matic of the setup which is used for excimer laser ablationof gratings in thin polymer films is shown in Fig. 2(b).The fabricated grating areas are of the order of severalhundreds of squared micrometers; the amplitude profilesof the gratings are homogeneous, reflecting the beam pro-file of the excimer lasers in use. This result means thatthere is some restriction on the flexibility of the gratingfabrication; however, the method is used successfully togenerate high-quality gratings in various materials.48–50

3. MATERIALS PROPERTIES OFPOLY(p-PHENYLVINYLENE) DERIVATIVESA. Materials ComparisonTransmission and reflection spectroscopy were used to ob-tain spectra of the absorption coefficient a and the refrac-

tive index n of the thin polymer films and are shown inFig. 3. The strong UV absorption peak at 210 nm is notshown in the figure for all polymers, but its valuesamax(UV) are presented in Table 1. The maximum of ab-sorption in the visible range amax , their wavelengthslmax , and their onsets of absorption at l0 are also given inTable 1. The assignment of the main absorption band atlmax of PPV and its derivatives like MEH-PPV has beendiscussed with some controversy in the past. A transi-tion from the valence to the conduction band or to an ex-citon state were considered. Newer experiments favorthe exciton model, as was summarized recently.28

In addition to reflection spectroscopy, we used theprism-coupling technique37,46,47 to measure n at severalwavelengths between 633 and 1064 nm. The resultsfrom both measurements are in good agreement, as canbe seen from the example of MEH-PPV shown in Fig. 3(b).It is interesting to note that the stronger UV maxima ofTPD2-PPV and TPD4-PPV lead to larger n but reduceddn/dl in the NIR as compared with MEH-PPV and MEH-DOO-PPV (see Fig. 3). This relation indicates a promis-ing way to tailor the dispersion of the refractive index byappropriate chemical synthesis of the polymers. Forquantitative specification of the refractive-index disper-sions, we fitted the spectra in the range 550–1650 nm bythe Sellmeier equation51

n2~l! 5 1 1 (i51

2 ail2

l2 2 l i2 (1)

Fig. 3. Spectra of (a) the absorption coefficient a after correctionof reflection losses and (b) the refractive index of ultrathin filmsof PPV derivatives. The circles represent data from prism cou-pling in the case of MEH-PPV.


by using two poles. The fit parameters a1 , l1 , a2 , andl2 are given in Table 1. All reported values of the refrac-tive index were measured with TE-polarized light. Westudied TM polarization as well and found significant bi-refringence, in agreement with a recent report.43

We measured the total propagation losses of the guidedwaves agw by means of the detection of stray light as afunction of the distance from the coupling prism, as de-scribed in Ref. 46. The data for agw , which are presentedin Table 2, contain the contributions of both the intrinsicabsorption of the material and the scattering losses,which depend primarily on the surface roughness of thewaveguide. We succeeded in fabricating the requiredwaveguides with losses of ,1 dB/cm by using DPOP-PPV,MEH-PPV, MEH-DOO-PPV. However, this was not yetfeasible with TPD2-PPV and TPD4-PPV.

Large cubic nonlinearity of the materials is most im-portant for all-optical device applications. We usedthird-harmonic generation as a screening test to elucidatethe purely electronic contribution to x (3) of the PPV de-rivatives. Clearly, a more elaborate analysis with othertechniques like the z-scan,52 degenerate four-wavemixing,53 or intensity-dependent prism coupling35,36

would be required to determine n2 and a2 , which are therelevant data for the performance of nonlinear waveguidedevices. But we restrict the application of the intensity-dependent prism-coupling method to the best material ofthis screening test only as described below in Subsection3.B. Using the third-harmonic-generation method intro-duced in the experimental section on samples with a typi-cal film thickness of approximately 50 nm, we determinedthe modulus of ux (3)u in a broad spectral range. The ux (3)uvalues exhibit a strong spectral dependence on the laserwavelength lL because of three-photon resonances withelectronic states of the polymers; the dependence occurs,in particular, if lL is close to 3lmax .28 For all PPV de-rivatives studied here, we observed that the maximum ofux (3)u, denoted xmax

(3) , occurs at lL@xmax(3) # . 3lmax (see Table

2). The reason for this shift is not yet clear and deservesfurther study. The value of ux (3)u at lL 5 3l0 is denotedx0

(3) and is supposed to refer better to off-resonant condi-tions. Data of x0

(3) are also given in Table 2 for compari-son. It turns out that MEH-PPV and MEH-DOO-PPVexhibit higher nonlinearities than do the other polymersstudied. This is roughly in line with the scaling behaviorbetween lmax and x (3), as discussed in our previousreport.28,29

It is well known that PPVs can easily photo-oxidize ifexposed to UV–VIS light at ambient air.37,54 Althoughthis sensitivity to UV–VIS light is a practical problem, it

can be used favorably, on the other hand, for structuringpurposes. Therefore, we measured the sensitivity of thepolymers to photodegradation by UV light, denoted SUV .We used a high-pressure mercury lamp in combinationwith a glass filter with transmission at 365 nm (UG1 fromSchott) to irradiate thin films (typically d 5 50–70 nm) ofthe PPV derivatives at ambient air and measured UVphotobleaching by recording the transmission and the re-flection spectra after certain times of exposure. A typicalresult for MEH-PPV is shown in Fig. 4. The absorptioncoefficient a(l) decreases and its maximum shifts towardshorter wavelengths with increasing exposure time.This decrease is caused by photochemical conversion ofthe p-electron structures of the chromophores that is com-bined with shortening of the p-electron delocalizationlength.29,54 The refractive index of the bleached polymeralso decreases significantly; in the example of MEH-PPVshown in Fig. 4(b), we observe Dn 5 0.17 at l5 1064 nm. The film thickness after UV bleaching didnot change, as we verified by surface profilometry. Evenafter strong photobleaching the films remained ratherstable because we did not observe any scratches from thediamond needle of the profilometer.

We observed a linear increase of Damax /amax with thenumber of absorbed UV photons, with no induction periodor threshold. The corresponding initial slope SUV is usedas a quantitative measure for the sensitivity to UVbleaching of the material. All polymers studied here canbe bleached. However, they show different sensitivitiesSUV (see Table 2). For practical reasons it is desirable touse a more stable polymer like MEH-PPV if the filmpreparation and optical investigations are performed atambient conditions.

The wavelength dependence of the photobleaching wasstudied by illumination of the polymer films by a high-pressure Xe arc lamp combined with a water filter and avariety of edge filters. The cutoff wavelength lc was de-fined in such the way that no change of absorption couldbe observed when we irradiated the samples withl . lc . The measured values of lc are presented inTable 2. A comparison with Table 1 shows that lc agreeswell with l0 . We conclude that problems that are due tophotobleaching do not occur in the NIR at working wave-lengths l @ lc .

The data summarized in Tables 1 and 2 were used toselect the most promising polymer for nonlinear wave-guide devices. We conclude that MEH-PPV is the best ofthe polymers studied here because it shows the superiorcombination of a low-loss coefficient agw , a large x (3), andrelatively low sensitivity to photobleaching. The other

Table 2. Materials Comparison for Nonlinear Waveguide Applications

DPOP-PPV MEH-PPV MEH-DOO-PPV TPD2-PPV TPD4-PPV

agw (dB/cm)a 0.6 0.5 0.8 .20 3xmax

(3) (10212 esu) 1.8 60 86 37 34lL(xmax

(3) ) (nm) 1341 1500 1560 1350 1350x0

(3) (10212 esu) 1.5 26 37 18 24SUV (a.u.) 0.007 0.044 0.015 0.018lc (nm) 550 590 500 520

a Measured at 1064 nm.


polymers still appear to be quite useful if special materialproperties need to be optimized.

B. Poly(2-Methoxy-5-(28-Ethylhexyloxy))-Poly( p-Phenylenevinylene)We focus now on a detailed description of the spectral de-pendences of n2 , a2 , and agw of MEH-PPV, as these dataare of crucial importance for a realistic simulation of de-vice performance. Recently, we performed a detailedstudy of MEH-PPV waveguides by intensity-dependentprism coupling.36 Significant changes in the couplingcurves occur on variation of the laser pulse energies at theentrance face of the prism in the range 1 nJ–1 mJ. Wedeveloped an improved evaluation technique to obtain theabsolute values of n2(l), a2(l), and agw(l) at 680 nm, l , 1180 nm.36 These spectra are shown in Fig. 5.

The linear waveguide losses agw were determined byevaluation of the coupling curves of m-line modes36 andagree very well with results from stray-light detection37

[see Fig. 5(a)]. Although agw is quite large at shorterwavelengths because of the tail of the electronic absorp-tion band, it decreases toward the NIR and becomes aslow as 0.5 dB/cm for l . 1050 nm. This remarkably lowloss of MEH-PPV waveguides enables practical applica-tions.

TPA at large intensities is deleterious to nonlinearwaveguide switching.55 Therefore a2(l) is presented inFig. 5(b), which indicates a peak of TPA at l5 800–850 nm. At l . 1000 nm, a2 decreases quickly

Fig. 4. Spectra of (a) the absorption coefficient a and (b) the re-fractive index n of a fresh MEH-PPV film with a thickness of 58nm (solid curve) and after UV irradiation at 365 nm with an in-tensity of 33 mW/cm2 at the sample position for 1 h (dashedcurve) and 2 h (dotted curve).

and does not show any other significant resonance. Toverify these data, we performed two-photon excited fluo-rescence experiments with a solution of MEH-PPV intoluene. As the excitation source, we used the sameNd:YAG laser and OPG as in nonlinear prism-couplingexperiments. At laser wavelengths between 700 and1000 nm the fluorescence intensity scales with the squareof the intensity of the laser pulses, which allows the cal-culation of the imaginary part of the cubic nonlinearityIm x(3) in arbitrary units. The spectral dependences of a2and Im x(3) agree excellently and confirm our new evalua-tion procedure of nonlinear prism coupling.36

The spectrum of n2 is shown in Fig. 5(c). We observe anegative n2 at l , 980 nm. At approximately 980 nm,n2 is zero and becomes positive at longer wavelengths.We observe a maximum of n2 at l 5 1080 nm with n2 aslarge as 2.2 3 10213 cm2/W. From the spectra shown inFig. 5, we conclude that a nonlinear waveguide devicecould be operated favorably in the wavelength range1000–1200 nm. At 1064 nm, we observe agw 5 0.5dB/cm, a2 5 3 3 1029 cm/W, and n2 5 1.8 3 10213

cm2/W. These data are used in the simulations ofswitching effects (see Section 5, below).

Ablation by high-intensity laser pulses is a frequentlyused method for writing desired structures. We studied

Fig. 5. Comparison of the linear and the nonlinear optical spec-tra of MEH-PPV measured at the laser wavelengths l: (a) Lin-ear attenuation loss agw(l) determined from prism coupling atlow intensity (filled circles) and from waveguide-loss experiments(open diamonds). (b) Comparison of dispersions of a2 from non-linear prism coupling (filled circles) and Im x(3) from two-photonexcitation fluorescence spectra (open triangles). The error barsof the Im x(3)-data are similar to the symbol size. (c) Dispersionof n2 from nonlinear prism coupling.


the photoablation of MEH-PPV by using fundamental andharmonic wavelengths of our mode-locked Nd:YAG laserat 1064 nm (pulse duration of tFWHM ' 30 ps), 532 nm(tFWHM ' 21 ps), and 335 nm (tFWHM ' 17 ps). In allcases the laser was focused by a telescope with a focal dis-tance of approximately 2 m on the polymer film. The fo-cused spot size in the Gaussian beam waist was measureddirectly at low intensities with the beam profiler placed atthe sample position. For the specification of the peak in-tensities I0 of the damage thresholds, we use the defini-tion of the Gaussian beam profile

I~x, y, t ! 5 I0 expS 2x2

2sx2 2

y2

2sx2 2

t2

2s t2D , (2)

where

I0 5Ep

~2p!3/2sxsys t(3)

is the peak intensity and Ep is the pulse energy. Thefilms were exposed to series of 100 shots with defined in-tensity. For intensities larger than the damage thresh-old, well-defined holes down to the substrate were ablatedinto the thin films and observed under a microscope. Thediameters of these holes were proportional to the laser in-tensities. Below threshold intensity, however, no damageof the film surface was observed even after exposure tomore than 1000 shots. We obtained the following thresh-old intensities: I0 5 21 GW/cm2 at l 5 1064 nm, I05 2 GW/cm2 at l 5 532 nm, and I0 5 4 GW/cm2 atl 5 355 nm. These data provide limits for ablation pur-poses but show, on the other hand, that MEH-PPV has ex-cellent photostability at the working wavelengths of non-linear switching devices in the NIR.

4. GRATING FABRICATIONBoth setups described in Section 2 were used to writegratings in spin-coated low-loss MEH-PPV films. Twoexcimer lasers, ArF at a wavelength of 193 nm with apulse duration of 20 ns and KrF at a wavelength of 248nm with a pulse duration of 500 fs, were used to fabricateablated gratings by the imaging method that uses aSchwarzschild objective: the third harmonic of aNd:YAG laser at a wavelength of 355 nm with a pulse du-ration of 40 ps was used to fabricate ablated gratings bythe direct-illumination method. Grating periods as smallas 360 nm have been generated with good quality by useof both setups. Time-resolved measurements showedthat the ablation behavior of conjugated polymers is simi-lar to the behavior of metals.56 This similarity could ex-plain why we found that the gratings fabricated with fem-tosecond pulses show the most promising featuresregarding a grating structure and resolution. The grat-ing thickness is controlled by the applied fluence and canbe varied from shallow for fluences just above the ablationthreshold to a thickness that corresponds to the thicknessof the polymer film. The actual thickness can be mea-sured by an atomic-force microscope.

Scanning electron microscope pictures that were takento investigate the structure of the gratings generated inMEH-PPV films of 590-nm thickness when different laser

wavelengths were applied are shown in Fig. 6. Very goodresults of sinusoidal grating structures were achieved byuse of 500-fs pulses at 248 nm [Fig. 6(a)] and 20-ns pulsesat 193 nm [Fig. 6(b)]. Both gratings were fabricated bythe imaging of a chromium grating mask onto the poly-mer surface with a Schwarzschild objective. The gener-ated grating periods are 386 nm [Fig. 6(a)] and 1.0 mm[Fig. 6(b)]. The applied fluences and the numbers ofpulses were 11.2 mJ/cm2 and 50 pulses [Fig. 6(a)] and 350mJ/cm2 and 2 pulses [Fig. 6(b)]. Especially for the fem-

Fig. 6. Grating structures in MEH-PPV waveguides with athickness of 590 nm on quartz substrates and generated by UVphotoablation. The applied wavelength, pulse duration, fluence,number of pulses, and grating period, respectively, were (a) 248nm, 500 fs, 11.2 mJ/cm2, 50, and 386 nm; (b) 193 nm, 20 ns, 350mJ/cm2, 2, and 1.0 mm; (c) 355 nm, 40 ps, 7 mJ/cm2, 100, and 720nm.


tosecond pulses at 248 nm, the intensity window for ob-taining a definite, highly resolved grating structure in thepolymer turned out to be quite narrow. The debris ob-served in Fig. 6(b) could have been caused by the grating-fabrication setup. The film is ablated in a horizontal po-sition instead of a vertical position, like the film shown inFig. 6(a), so some of the ablated particles could havefallen back down onto the sample. Further investigationinto this is under way. It should be noted that all thegratings done with this setup show a highly homogeneousamplitude profile; the grating area is of the order of2.5 3 103 mm2, corresponding to a grating length of ap-proximately 50 mm.

A grating generated by direct illumination with a phasemask is shown in Fig. 6(c). The third harmonic of a 40-psNd:YAG laser at 355 nm was used, and a grating period of720 nm was generated by the application of a fluence of 7mJ/cm2 and 100 pulses. The nearly Gaussian laser beamprofile that was used in this case induced an apodized am-plitude profile in the grating. Thus this approach is mostpromising for applications like gap-soliton switching inwhich the sharp edge of the PBG in the waveguide trans-mission spectrum of an apodized grating favors the obser-vation of nonlinear optical switching effects based on x (3)

nonlinear materials.57 We again observe a good-qualitygrating. Here, the grating reaches areas of severalsquared millimeters and grating lengths of approximately1 cm.

The above results show that the one-dimensional PBGstructures necessary for nonlinear switching applicationsin MEH-PPV films can be fabricated by laser ablation.Bleached gratings have been successfully fabricated insimilar materials,58 so we can assume that this method isalso applicable to MEH-PPV, given its photoreactivityproperties described above. These techniques can easilybe extended to allow patterning of two-dimensional pho-tonic crystals merely by fabrication of an appropriatephase mask. We intend to pursue such a generalizationin the near future.

5. NUMERICAL MODELING OF DEVICEPERFORMANCEIn this section, we numerically simulate gap-solitonswitching effects in the MEH-PPV material. We assumethat a Bragg grating has been grown into a waveguide ofthe polymer material with one of the techniques discussedabove. A Bragg grating is characterized by its period g,in terms of which we can define the Bragg wave number,k0 5 p/g, and the Bragg frequency, v0 5 ck0 /n0 , wherec is the speed of light in vacuum and n0 is the index ofrefraction of the medium in the absence of a grating. Wealso define the vacuum Bragg wavelength, l05 2pc/v0 . We include the effects of linear absorptionby using an intensity-loss parameter, a 5 2pnImv0 /c,where nIm is the imaginary portion of the index of refrac-tion, so the total dielectric constant of the background me-dium is e 5 (n0 1 nIm)2. We also use the familiar TPAcoefficient a2 in units of centimeters per watt.

We now consider the propagation of slowly varying op-tical pulses in a lossy, Kerr nonlinear Bragg grating,where, by slowly varying, we mean that the frequency

width of the pulses is small relative to v0 . We writedown a set of CMEs that describe the dynamics of theslowly varying amplitudes f6(z, t), where the plus-and-minus sign refers to forward and backward propagation.The fields are carried at v0 and are normalized such thatu f6(z, t)u2 gives the intensity in the field. The CMEstake the form59

0 5 6i]f6

]z1 i

n0

c

]f6

]t1 k~z !f7 1 Fs~z ! 1 i

a

2 G f6

1 S n2

v0

c1 i

a2

2 D ~ u f6u2 1 2u f7u2!f6 , (4)

where k(z) is the grating strength, s(z) accounts for achange in the overall background index of refraction, andn2 is the nonlinear index of refraction coefficient. We al-lowed both k and s to vary slowly with z (slowly relativeto the grating period g) so that we can describe apodizedgratings, which are necessary to remove the well-knownFabry–Perot oscillations that occur in the transmissionspectrum of an unapodized grating.60 For a givengrating-fabrication technique the values of k and s can bedetermined by use of a variety of techniques.61

We first discuss the situation in which the CMEs (4)are used to describe an infinite, unapodized medium [k(z)is a constant and s(z) 5 0]. In this situation any fre-quency v for which uv 2 v0u , uck/nu is within the PBGof the system.9 That is, light traveling in the forward di-rection is strongly coupled to light traveling in the back-ward direction, so in the absence of nonlinearity light can-not propagate. The presence of nonlinearity upsets thiscoupling and, if the light is sufficiently intense, can leadto solitary-wave solutions, which propagate withoutchanging their shape.9,11 The intensity required to formthese solitary waves is dependent both on how deepwithin the gap the frequency content of the pulse is andon the frequency width of the pulse: the deeper withinthe gap, the more intensity required; also, the wider thepulse width, the more intensity required. If a pulse of agiven frequency width has more intensity than that re-quired to form a solitary wave it will tend to compress inthe time domain and broaden in the frequency domain.If the pulse has much more intensity than that requiredto form a solitary wave it will tend to break up into mul-tiple pulses. In the finite, apodized experimental struc-tures that we consider it would be difficult to observe asolitary wave conclusively because the structures arequite short. Furthermore, we include the effects of linearabsorption and TPA, so an intense pulse would tend to beattenuated. Nevertheless, we show in the simulationsbelow that the pulse compression and the pulse breakupthat the CMEs predict can likely be observed in gratingsin MEH-PPV materials despite both the TPA and the lin-ear absorption.

In our simulations, we assume that the Bragg wave-length is 1064 nm, which means n0 5 1.6494, n2 5 23 10213 cm2/W, a 5 0.115 cm21, and a2 5 3 3 1029

cm/W. We assume that k is Gaussian apodized

k~z ! 5 k0 expF24 ~ ln 2 !S z 2 L/2

zFWD 2G , (5)


where L is the length of the grating and zFW is the fullwidth at half-maximum value of the apodization profile.We simulate a grating that was grown by use of ablation,which leads to a reduction of the index of refraction of thematerial, so s(z) 5 22k(z). We use L 5 1 cm, zFW5 0.5 cm, and two different values of k0 : kw/2p5 1024 mm21 and ks/2p 5 1023 mm21, where the sub-script w refers to a weak grating, and the subscript s to astrong grating. We plot the cw transmission spectrum atlow intensity for the grating with kw in Fig. 7(a) and forthe grating with ks in Fig. 7(b). We include linear lossbut not TPA or Kerr nonlinearity. The weak grating hasa width of 1.5 3 1024 mm, whereas the strong grating hasa width of 1.9 3 1023 mm. The edges of the strong grat-ing spectrum are much more sharply defined than thoseof the weak grating spectrum because Bragg reflection isa distributed process, so the weak grating would need tobe much longer than 1 cm to have sharply defined edges.Despite the apodization profile, the high-wavelength edgeof both spectra shows some Fabry–Perot oscillations.However, for optical switching with a positive nonlinear-ity, we need to work at the low-wavelength edge of thespectrum, where the group-velocity dispersion is of thecorrect sign to lead to soliton formation. Therefore ourresults are not affected by the oscillations at the high-wavelength edge. Note that, in both Figs. 7(a) and 7(b),the central frequency of the bandgap is not equal to thedesign wavelength, 1.064 mm. This is because the designwavelength was chosen with the assumption that thebackground index of refraction was equal to n0 , but afterUV illumination the background index is slightly modi-fied. The overall change in the background index of re-fraction is described by s(z), and its effect is to slightlyshift the position of the bandgap. Because s(z)5 22k(z), the effect is much larger for the strong grat-ing than for the weak grating. For the weak grating [Fig.7(a)] the gap position was shifted by roughly 0.1 nm fromthe design wavelength of 1.064 mm; for the strong grating[Fig. 7(b)] the gap was shifted by 1 nm.

Fig. 7. Cw spectra for (a) a weak grating and (b) a strong grat-ing. The effects of linear loss a are included, but TPA and Kerrnonlinearity are not. The filled circles indicate the wavelengthsat which the simulations of the pulse transmission as a functionof the incident pulse intensity of Fig. 8 are carried out to demon-strate the switching process.

We now turn to simulations of pulse propagation in thepresence of Kerr nonlinearity and TPA. We numericallyintegrate the CME by using a colocation technique.62

For the strong grating, we use Gaussian pulses whose in-tensity profile is 30 ps (full width at half-maximum).This profile corresponds to a vacuum wavelength spreadof Dls ' 5 3 1025(l0), which is almost 1/20th of the gapwidth. For the weak grating, we use 100-ps Gaussianpulses with a vacuum wavelength spread of Dlw ' 1.53 1025(l0), which is approximately 1/7th of the gapwidth. In Fig. 8, we plot the pulse transmission throughthe weak grating (solid curve) as a function of the peakincident pulse intensity for a carrier wavelength ofl 5 1.06387 mm and through the strong grating for a car-rier wavelength of l 5 1.062145 mm (dashed curve).These wavelengths correspond to the filled circles in Fig.7. In both situations, the maximum transmission peaksat approximately 35%, but the strong grating switch re-quires less energy (250-MW/cm2 peak intensity ratherthan 550 MW/cm2). Furthermore, the strong grating has5% transmission at 100 MW/cm2 and 30% at 200MW/cm2, a difference of 100 MW/cm2; in contrast, theweak grating has 5% transmission at 100 MW/cm2 and30% at 350 MW/cm2, a difference of 250 MW/cm2.

In Fig. 9, we plot the output pulse for the strong grat-ing at input intensities of 200 MW/cm2 (solid curve) and325 MW/cm2 (dashed curve) as functions of time. Thetime value is the delay measured from the point when thepeak of the input pulse enters the grating. We normalizethe output pulse to the input intensity. Both of thesepulses have a transmission of 30%, but the width of thesecond pulse has shrunk from the input value of 30 ps to2 ps (a factor of 1/15), and, correspondingly, its peak in-tensity has gone up by a factor of 3.2. At higher values ofthe input intensity the pulse undergoes even more com-pression and begins to break up into multiple pulses.Similar behavior can be seen for the pulses that propa-gated through the weak grating.

Fig. 8. Transmission plotted as a function of intensity for 100-pspulses in a weak grating (solid curve) and for 30-ps pulses in astrong grating (dashed curve). Switching for the 100-ps pulseoccurs at approximately 300 MW/cm2, whereas for the 30-pspulse it occurs at approximately 200 MW/cm2. These simula-tions include TPA and nonlinearity.


The drop off in transmission at high incident intensi-ties can be attributed to TPA. In the absence of TPA anincrease in intensity would eventually switch the pulse toalmost 100% transmission. However, as noted, at highintensities the pulses tend to compress and then form intomultiple pulses, each of which has a narrow width and ahigh peak intensity. This high intensity leads directly toa large attenuation that is due to TPA.

There are two issues of main importance when consid-ering switching. First, one wants the slope of the switch-ing plot to be as sharp as possible so that a small changein intensity produces a large change in transmission.The slope of the switching plot reflects the sharpness ofthe edge of the cw transmission spectra. Because thestrong grating has a well-defined edge, it switches with amuch smaller intensity differential. The sharpness ofthis edge is affected by two important parameters: grat-ing strength and grating length. A weaker grating mustbe longer to get a sharp edge. Were we to simulate agrating with a gap width of Dl 5 1022l0 but fix thelength at 1 cm, the switching slope would become evenmore well defined. Or, conversely, we could use a weakergrating but make the grating length much longer.

The second issue regarding switching is the size of thepulse that can be made to switch. Ideally, one would liketo switch as short a pulse as possible. Of course, theshorter the pulse, the stronger the required grating.This is because shorter pulses have a larger frequencywidth; hence they require stronger gratings for adequatereflection. Thus, in our simulations, we used 100-pspulses for the weak grating and 30-ps pulses for thestrong grating. If we moved to even stronger gratings,which could be grown by ablation deeper and deeper intothe waveguide, switching could be achieved with muchshorter pulses.

6. CONCLUSIONWe have demonstrated the excellent suitability ofsolution-processable PPV derivatives as novel PBG mate-

Fig. 9. Output pulses for the strong grating for incident inten-sities of 200 MW/cm2 (solid curve) and 325 MW/cm2 (dashedcurve). The intensities are normalized to the incident pulses,and the delay is calculated from the time that the peak of the in-cident pulse entered the system.

rials for nonlinear optics. Our comparison of selectedmaterials clearly indicates that the polycondensation-type MEH-PPV, in particular, is the most promising can-didate for nonlinear all-optical switching applicationsbased on planar PBG structures. Submicrometer Bragggratings in MEH-PPV thin films have been fabricated byapplication of the UV-photoablation method. On the ba-sis of these new material data for MEH-PPV, we have car-ried out numerical simulations to show that we can ex-pect gap-soliton formation and optical pulse compressionat reasonable intensities in such one-dimensional photo-nic crystals—even if linear losses and TPA of the materialare included.

Concluding from the considerable switching contrastobserved in our numerical results, we consider high-speedall-optical switching in planar MEH-PPV PBG structuresclose to its experimental demonstration. PPV deriva-tives turn out to be promising novel PBG materials fornonlinear optics.

ACKNOWLEDGEMENTSFinancial support was given by the Bundesministeriumfur Bildung und Forschung (grants of the Hot Topic pro-gram to M. A. Bader and K. Koynov), by the DeutscherAkademischer Austauschdienst Sciences (Ph.D. stipendto A. Bahtiar), and by the Natural and Engineering Re-search Council of Canada (to S. Pereira). Helpful discus-sions with S. Soria and technical support by G. Herrmannand W. Scholdei for thin-film preparation and opticalspectroscopy are also gratefully acknowledged, as are dis-cussions with J. E. Sipe concerning gap-soliton effects.

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