1080 J. Opt. Soc. Am. B/Vol. 16, No. 7 /July 1999 Wolff et al.

Speed enhancement of photorefractivepolymers by means of

light-induced filling of trapping states

Janpeter Wolff, Stefan Schloter, Uwe Hofmann, Dietrich Haarer, and Stephan J. Zilker

Physikalisches Institut und Bayreuther Institut fur Makromolekulforschung, Universitat Bayreuth,D-95440 Bayreuth, Germany

Received May 27, 1998; revised manuscript received October 12, 1998

The fastest organic photorefractive materials show response times in the millisecond range. The origin of thislimit is not yet fully understood. Charge-carrier generation and transport processes and reorientation of thenonlinear-optical chromophores may play an important role. We characterize a new photorefractive guest–host polymer by cw two-beam coupling, as well as by cw and pulsed four-wave mixing experiments. The latterwere used to determine unambiguously the response time of the material. We show that a decrease of theresponse time by a factor of 1000 can be achieved by illuminating the sample with a strong cw beam duringpulsed exposure. A model of the enhancement process backed by dc photoconductivity measurements is pre-sented. © 1999 Optical Society of America [S0740-3224(99)00507-X]

OCIS codes: 160.5320, 090.2900, 190.5330, 040.5150.

1. INTRODUCTIONPhotorefractive (PR) organic systems have recently at-tracted much attention owing to their potential applica-tions in optical data storage, processing, and imaging (forreviews, see Refs. 1–3). Examples are optical correlationfor verification of security tags on credit cards4 or transil-lumination imaging through scattering media.5,6 How-ever, the response time of organic specimen is so far lim-ited to ;50 ms.7 All-optical correlators are thus not yetable to challenge digital image processing of conventionalvideo camera pictures in many applications.

The current speed limit arises from the basic principleof a photorefractive system. It incorporates three func-tionalities in one material: a sensitizer yieldingelectron–hole pairs under laser illumination, a photocon-ductor for efficient hole transport, and a nonlinear-opticalchromophore that allows for modulation of the refractiveindex owing to the Pockels effect when an electric field isapplied. All three components, however, interact, par-tially blocking one another’s functions. The introductionof chromophore and sensitizer moieties leads to addi-tional traps for the holes,8,9 thus reducing their mobility.Additionally, in many cases the nonlinear-optical chro-mophores align in the space-charge field that is created bythe incident laser beams,10 resulting in an improvementof the dc performance of these systems (orientational en-hancement). Bittner et al.11 recently showed that orien-tational enhancement can limit the speed of PR polymercomposites with glass-transition temperatures aboveroom temperature.

We concentrate our investigation on a photorefractiveguest–host system consisting of a polysiloxane-based pho-toconductor, doped with trinitrofluorenone (TNF) as asensitizer and a nonlinear-optical tolane chromophore.It is shown that one of the above speed limits, the low mo-bility of the holes, can be reduced by illumination of the

0740-3224/99/071080-07$15.00 ©

sample with a strong helium–neon (He–Ne) laser beamduring exposure with two coherent 6-ns pulses from aNd:YAG laser in a so-called holographic time-of-flight(HTOF) experiment.12–14 The cw laser creates a uniformphotocurrent in the sample within 2 ms after exposure.Thus it is not necessary to preirradiate the sample for anextended period. This was confirmed by opening theHe–Ne beam 25 ms before the arrival of the YAG pulses;there was no difference as compared with experimentswith preirradiation times of the order of tens of seconds.The He–Ne laser fills most of the deep traps, leading to alower trap density. Thus the charge carriers produced bythe subsequent Nd:YAG pulses have an effective mobilitythat can be 3 orders of magnitude higher than in the ab-sence of the cw pump beam. A similar effect was also ob-served before in inorganic crystals.15 We present a char-acterization of the material by pulsed and cw four-wavemixing and cw two-beam coupling, as well as by dc pho-toconductivity measurements.

2. MATERIALFigure 1 shows the investigated photorefractive polymercomposite PSX:DMMNT:TNF (composition 68:31:1 %w/w)based on a photoconducting polysiloxane (PSX) with pen-dant carbazole moieties.16,17 The nonlinear-optical chro-mophore 2,5-dimethyl-4-methoxy-48-nitro-tolane (DM-MNT) was doped into the material. Since the tolane doesnot show photoisomerization,8,18 no additional contribu-tions of nonphotorefractive origin to the modulation of therefractive index are observed. A small amount of TNF isadded as a sensitizer for the cw measurements. Thecomposition of the system was selected to yield a lowglass-transition temperature (Tg 5 25 °C) to allow for anin situ poling at room temperature and to maximize ori-

1999 Optical Society of America

Wolff et al. Vol. 16, No. 7 /July 1999 /J. Opt. Soc. Am. B 1081

entational enhancement, which is important for the cwproperties. The details of the sample preparation weredescribed previously.17

3. EXPERIMENTALThe samples were characterized in a HTOF apparatus.In this technique the photorefractive grating is written bytwo time-coincident pulses from a Nd:YAG laser (Spec-tron SL404G with injection seeder) operating at a wave-length of 532 nm (see Fig. 2), at which the samples’ ab-sorption coefficient is a 5 95 cm21. The pulse width ofthe laser is 6 ns, and the energy density of the writingbeams is ;10 mJ/cm2. In the bright areas of the inter-ference grating, electron–hole pairs are generated by thelaser beams. Since PSX is essentially a hole conductor,16

the electrons stay immobile, whereas the holes drift in theexternal electric field E. To yield an electric field compo-nent in the direction of the grating vector, the sample nor-mal is tilted by a 5 60° with respect to the direction ofincidence of the two s-polarized writing beams. Theangle between the latter is 20°, yielding a grating periodof L 5 2.7 mm with an index of refraction of n 5 1.72, asdetermined by spectral ellipsometry. The grating wasprobed by a p-polarized He–Ne laser with variable inten-sity. Its wavelength of 632.8 nm is still well inside thecharge-transfer band (a 5 12 cm21), which extends to aslong as 680 nm. The diffracted signal is passed througha holographic notch filter and monitored with a photomul-tiplier (Hamamatsu R928) that is amplified and fed into adigital storage oscilloscope. The diffraction efficiency hwas typically lower than 1025.

Diffraction appears immediately after the creation offree holes that are drifting in the external electric field.This leads to the buildup of a space-charge field Esc that

Fig. 1. Investigated guest–host polymer consisting of photocon-ducting polysiloxane (PSX) doped with the nonlinear-opticalchromophore DMMNT and the sensitizer TNF.

Fig. 2. Holographic time-of-flight (HTOF) experiment. S,sample; NF, holographic notch filter; PM, photomultiplier tube;BS, 50/50 beam splitter; HV, high-voltage supply; Osc, digitalstorage oscilloscope; P, polarizers; ND, neutral-density filters.

modulates the refractive index by means of the Pockels ef-fect. The diffraction efficiency h(t),

h~t ! ; ~reff Esc!2, (1)

with reff being the effective electro-optic coefficient,reaches its maximum when the holes have drifted to a po-sition of anticoincidence with the immobile electron dis-tribution. In principle, the signal should then oscillate,reaching maxima whenever anticoincidence occurs again.However, the dispersive nature of the charge transportprevents the observation of further maxima.13 From thetemporal position tmax of the diffraction maximum, an es-timate for the hole mobility m can be obtained from therelation

m 5Ldr

E • tmax, (2)

where Ldr is the drift length that is equal to L/(2 cos g),with L being the grating period and g being the angle be-tween the grating wave vector and the external electricfield E. The advantages of using HTOF for the charac-terization of organic photorefractive materials are appar-ent. As opposed to conventional time-of-flight (TOF)experiments,19 HTOF works with rather small driftlengths that may be less than 1 mm, whereas TOF re-quires a larger sample thickness. Thus HTOF is able toinvestigate charge transport on the length scale of thegrating spacing that is the relevant scale for holographicexperiments. TOF is also very sensitive to spacecharges, whereas HTOF will detect only modulated inter-nal electric fields but not uniform charge distributions.

Finally, HTOF is an ideal way to yield the ultimatespeed limitation of an organic PR system, which is thetime tmax elapsing between the light-induced creation ofmobile holes and their drift over half a grating period.This time is unambiguous as compared with the many dif-ferent versions of defining a response time that are usedin cw experiments.

Time-resolved dc photoconductivity measurementswere performed with a He–Ne laser incident upon thesample. The illumination was switched on with a fastshutter, providing an experimental rise time of less than1 ms. The photocurrent across the sample was amplifiedby a SR 570 current amplifier (Stanford Research Sys-tems) and registered with a digital storage oscilloscope.Degenerate four-wave mixing (DFWM) and two-beamcoupling experiments were carried out with a setup de-scribed previously8 with diode lasers at a wavelength of670 nm and a writing intensity of 1.2 W/cm2 per beam.The grating spacing was 3.9 mm.

4. RESULTS AND DISCUSSIONA. Photorefractive cw MeasurementsFigure 3 shows the diffraction efficiencies for s- andp-polarized reading beams versus the external electricfield as obtained from the cw DFWM experiments. For a50-mm-thick sample, external diffraction efficiencies asgreat as 25%, corresponding to a refractive index modula-tion of Dn 5 2.5 3 1023, have been achieved. As a con-sequence of the sample’s low glass-transition tempera-

1082 J. Opt. Soc. Am. B/Vol. 16, No. 7 /July 1999 Wolff et al.

ture, the modulation of the refractive index is dominatedby orientational enhancement.10 The ratio of the bire-fringent to the electro-optic contribution to Dns was de-termined as ABR /AEO 5 26.2 6 1.1 by measurement ofthe diffraction efficiencies of s- and p-polarized readingbeams.

The inset in Fig. 3 depicts the two-beam coupling gaincoefficient G for both polarizations s and p as a function ofthe external electric field. The gain was evaluated ac-cording to the formula20

G 51

d/cos Q@ln~g0b! 2 ln~b 1 1 2 g0!#, (3)

where U is the angle between the writing beams, d is thesample thickness, b is the ratio of the power of the writ-ing beams in front of the sample, and g0 is given by[power of beam 1 (beam 2 on)]/[power of beam 1 (beam 2off )] behind the sample.

The occurrence of a gain proves the photorefractivecharacter of the observed effect. Due to an almost van-ishing absorption of a 5 2 cm21, a net two-beam couplinggain is observed for both polarizations. For Gp a value of60 cm21 was obtained that corresponds to a phase shiftbetween the intensity modulation and the refractive-index grating of ;10°, as determined by the standardgrating translation technique.21,22

We have evaluated the response time of our system byusing single-exponential fits of the form

h 5 hmax expS 22

tt D (4)

to the decay of the diffraction efficiency h after blockingone of the writing beams (see inset in Fig. 4). The timeconstant t corresponds to the decay of the space-chargefield, which is proportional to the square root of the dif-fraction efficiency.1 The factor 2 in the exponent of Eq.(4) takes this into account. A single-exponentialansatz7,23 proved to yield the best fit, as compared withdouble-exponential formulas that are sometimes used inthe literature24 to describe the grating dynamics for casesin which the processes involved—buildup of the space-charge field and orientational enhancement—are wellseparated in time.

The good single-exponential fit indicates that the timeconstants for the buildup of the space-charge field and forthe chromophore reorientation (orientational enhance-ment) are of similar size.

Figure 4 shows the results. The response time exhib-its a strong dependency on the external electric field andextends down to 7 s. In comparison with current photo-refractive state-of-the-art materials, this system showsan average performance. We have also fitted the buildupof the space-charge field, which is approximately a factorof two slower, extending down to 14 s. Below, we com-pare our HTOF results (which are sensitive mainly to thespace-charge field) with the decay dynamics, since inves-tigations of the poling dynamics of nonlinear-optical side-chain polymers in electric fields have shown that thebackrelaxation occurs faster than the initial orientation.25

Thus the decay of the cw diffraction efficiency is less in-fluenced by orientational enhancement than the buildup.

B. Holographic Time-of-FlightFigure 5 shows a HTOF trace obtained at an applied fieldof 72 V/mm with a probe beam having an intensity ofI 5 0.03 W/cm2. The maximum of the curve occurs at atime tmax of ;2 s. This corresponds to a relatively smallmobility, m 5 2 3 10210 cm2/(V s), as calculated from Eq.(2). To test the influence of the probe beam, we repeatedthe experiment with different reading intensities, rangingfrom 1024 to 3 W/cm2. The curve for 3 W/cm2 is shown asan inset in Fig. 5. It exhibits a dramatically different be-havior: The maximum is already reached after ;8 ms,marking an increase of the rise time by nearly 3 orders ofmagnitude [the corresponding value of m is 53 1028 cm2/(V s)]. Figure 6 depicts the value of tmax as afunction of the intensity of the probe beam.

For small reading powers, tmax does not vary with in-tensity (I , 1 mW/cm2). For larger values, however,tmax decreases rapidly from ;22 s to 8 ms. Moreover, adependence of the diffraction efficiency h on the reading

Fig. 3. Diffraction efficiency for s- and p-polarized readingbeams as obtained from the DFWM measurements. The insetshows the two-beam coupling gain coefficient.

Fig. 4. Decay time versus the external electric field, determinedby means of DFWM. The inset shows the temporal evolution ofh at E 5 80 V/mm with the writing beams switched on at t5 0 s (writing) and one beam switched off at t 5 300 s (erasing).

Wolff et al. Vol. 16, No. 7 /July 1999 /J. Opt. Soc. Am. B 1083

intensity I was observed (see Fig. 7). h drops 3 orders ofmagnitude upon our increasing the illumination with theHe–Ne laser. Thus both the temporal evolution and theamplitude of the holographic grating are strongly influ-enced by the reading laser. The wavelength of the latter,632 nm, is well inside the absorption band of the charge-transfer complex between TNF and carbazole and canthus create mobile charges, however, in a spatially un-modulated way.

This is shown in Fig. 8, which depicts the time-resolvedphotocurrent moving through a sample when it isexcited with a He–Ne laser at time t 5 0 (I 5 3 W/cm2,E 5 72 V/mm). The current exhibits a rise time of only 2ms, after which two thirds of its maximum value of 18 nA(occurring at t 5 6 s) is reached. Certainly this photo-current induced by the He–Ne laser will have a direct in-fluence on the space-charge field created by the green la-ser pulses. Three mechanisms can play a role. First,the He–Ne beam may just erase the modulated space-charge field by creating electron–hole pairs throughoutthe whole sample. This is similar to the last part of thecurve in Fig. 4, which depicts the erasure of a cw gratingby blocking one of the writing beams. However, t/25 3.5 s of cw illumination with 1.2 W/cm2 is necessary to

Fig. 5. Diffracted intensity versus time for two different inten-sities of the reading beam, 0.03 W/cm2 and 3 W/cm2 (inset). Theapplied field was 72 V/mm in both cases.

Fig. 6. Temporal position tmax of the maximum of the diffractionefficiency as a function of the intensity of the reading laser (ap-plied field, 72 V/mm).

reduce the diffraction efficiency by a factor of 1/e. Thusthe time scale of light-induced erasure is much longerthan that of the HTOF maxima observed for high readingpower, namely, 8 ms. To substantiate this claim, we per-formed a second type of HTOF experiment: Now theHe–Ne laser was blocked during pulsed recording andwas switched on only after a delay time of 60 s after thegrating recording. Figure 9 shows the result for a read-ing intensity of 3 W/cm2. First, the initial diffraction ef-ficiency is quite high, comparable with the one obtainedat low reading levels for prior He–Ne exposure (see Fig. 7,same units as in Fig. 9). Second, the signal decays veryslowly, reaching 1/e of its initial value after t/2 5 3.7 s.

Consequently, light-induced grating erasure is mostprobably not responsible for the increasing hole mobilitiesobserved in our HTOF experiments under strong cw illu-mination. The second mechanism involves the fact thatpreillumination can increase the number of charge traps,as observed by Silence et al.26 However, preilluminationleads to slower rise times and higher diffraction efficien-cies in cw experiments—exactly the opposite of our obser-vations: cw experiments on 25-mm samples irradiated bycomparable He–Ne powers yielded a decrease of the risetime by 60% as compared with the value of 3.5 s deter-

Fig. 7. Temporal maximum of the diffraction efficiency h(tmax)versus intensity of the reading beam at an applied field of 72V/mm. The fit is explained in the text.

Fig. 8. Time-resolved photocurrent excited by 3 W/cm2 ofHe–Ne radiation at an applied field of 72 V/mm.

1084 J. Opt. Soc. Am. B/Vol. 16, No. 7 /July 1999 Wolff et al.

mined for the nonirradiated case. This is accompaniedby a decrease of the diffraction efficiency by a factor of 5.Therefore trap activation is not taking place; further-more, our HTOF observations cannot be transferred di-rectly to cw DFWM.

C. ModelWe have devised a third mechanism that may be respon-sible for the observed effects: light-induced filling oftraps and excitation from them. As can be seen from Fig.8, 2 ms of irradiation with a He–Ne laser already leads toa photocurrent exhibiting an amplitude of two thirds ofits overall maximum. We believe that the cw laser cre-ates charges uniformly throughout the sample, resultingin a large photocurrent. The substantial amount of drift-ing holes is rapidly filling the deep trapping sites presentin the sample. In this context deep traps are those fromwhich the holes cannot be excited again during the ex-periment, either thermally or by the reading laser. If onetakes the associated average time that a charge is immo-bilized in a trap as a measure for its effective depth, trapsthat are excitable by the reading laser will have a differ-ent effective depth, depending on the presence of irradia-tion by the reading laser. The excitation of charge carri-ers from these traps by the reading laser reduces theaverage time that a carrier is trapped in them and there-fore decreases their effective depth. The filling of thedeep traps effectively leads to a Gaussian transport of thecharge carriers. This can be seen by performing a MonteCarlo simulation of the transport process. Assumingthat the deep traps have been filled, the probability p fora charge to jump from one trap to another is given by thesum of two terms: a constant probability p1 that is asso-ciated with the normal propagation and a probability p2that depends on the intensity of the reading laser I. Thetotal probability p for a jump will then be given by

p 5 p1 1 p2 . (5)

Simulations have been performed with the assumptionthat p2 depends linearly on I,

p2 5 c 3 I, (6)

Fig. 9. Diffracted intensity as a function of time. The He–Nelaser (3 W/cm2) was switched on 60 s after exposure with theNdYAG pulses.

with c being a constant. The result of one particular run( p1 5 7.5 3 1025, 1000 carriers) is shown in Fig. 10,where t is the time required for half of the carriers to haveperformed four jumps. The general features of the ex-perimental data (Fig. 6) agree with the simulated curve.Both show the transition from a more or less constant re-gime to an intensity-dependent regime. The slopes inthe latter are very similar for the experimental and simu-lated data (21.05 and 20.94, respectively). Therefore weare confident that the model—in spite of its simplicity—isa good approximation.

The maximum of h depends on the reading intensity(Fig. 7), since the He–Ne laser is addressing more andmore sensitizer molecules as its intensity increases.They are no longer available when the Nd:YAG pulses ar-rive. Therefore the diffraction efficiency remains highwhen the He–Ne laser is switched on after pulsed expo-sure (Fig. 9).

Already a rather simple model leads to a quantitativeunderstanding of this phenomenon: The amount of holesand electrons that have been created by the reading laseris np and ne , respectively. If one neglects that someholes will be trapped in deep traps (only recombination ofholes and electrons is taken into account), then np 5 ne5 n. The temporal evolution of n is then given by thefollowing equation:

dn~t !

dt5 2kn~t !2 1 P@N0 2 n~t !#, (7)

with k being a constant for the recombination from elec-trons and holes (knpne 5 kn2) and P@N0 2 n(t)# beingthe term that governs the creation of holes through thereading laser. N0 is the number of TNF molecules, and pis proportional to the He–Ne intensity. The fitted valuefor P/N0k for the run depicted in Fig. 10 is 0.03.

In the steady state @dn(t)/dt# 5 0. Thus Eq. (7) leadsto

nss 51

2 H F S P

k D 2

1 4N0S P

k D G1/2

2P

k J , (8)

with nss being the number of created holes in the steadystate. As the diffraction efficiency h is proportional to E2

Fig. 10. Monte Carlo simulation for the temporal position t ofthe maximum of the diffraction efficiency as a function of the in-tensity of the reading laser.

Wolff et al. Vol. 16, No. 7 /July 1999 /J. Opt. Soc. Am. B 1085

and E again is proportional to the created space charge,Eq. (8) leads to the following expression for h:

h~P ! } ~N0 2 nss!2

5 N02S 1 2

1

2 H F S P

N0k D 2

1 4S P

N0k D G1/2

2P

N0kJ D 2

.

(9)

A fit using this equation is shown in Fig. 7. The ex-perimental data agree well with the calculated fit, takinginto account the simplicity of the model used.

How do these observations relate to previously pub-lished investigations? A similar effect has been observedin inorganic crystals of Bi12SiO20 and Bi12GeO20 by Ko-styuk et al.15 Upon filling their trap states with UV ir-radiation, an increase of the mobility by 3–4 orders ofmagnitude was observed in TOF experiments.

Silence et al.26 observed a slowdown of the cw responseof a photorefractive polymer composite in a cw experi-ment that was accompanied by an increase of the diffrac-tion efficiency. This deviates from our pulsed and cw ob-servations. In the pulsed case this deviation can beexplained by the different experimental conditions. Si-lence et al. produce new charge carriers continuously witha cw laser after preirradiation has stopped. We observeour effect under pulsed conditions (charge creation by thewriting laser during only 6 ns) and only in the case whenthe additional irradiation is present, helping to detrapcharge carriers again. The latter condition also appliesto the cw experiments performed by us.

Furthermore, one has to note the high irradiation doseemployed by Silence et al. (4.4 3 104 J/cm2), which leadto irreversibility of the effect. Our samples that wereread out with high laser power exhibit reversibility. Asubsequent experiment with low reading power yields thesame result as a virgin sample. Furthermore, gated ex-periments showed that the full magnitude of the speedenhancement already occurs when the reading laser isswitched on 25 ms before the writing pulses (incident cwdose of 0.075 J/cm2). Here the 25 ms were determined bythe setup. We believe that the required preirradiationtime is supplied in that the rise time of the dc photocur-rent is 2 ms (see Fig. 8). Thus preirradiation is not a nec-essary prerequisite for our effect as in the case of Silenceet al.

5. CONCLUSIONSWe have shown that strong illumination inside thecharge-transfer band of a photorefractive organic compos-ite can lead to faster hole transport in transient holo-graphic experiments. The latter arises from the light-induced filling of deep charge traps and excitation fromless deep traps induced by the cw laser. The illumina-tion, however, saturates the sensitizer entities, leading toa significant reduction of the maximum diffraction effi-ciency h. If the He–Ne laser is switched on after pulsedexposure, then h remains high; furthermore, light-induced erasure of the grating can be observed to occur ontime scales of seconds. This confirms—in combinationwith dc photoconductivity measurements—that light-

induced grating erasure is not responsible for the speedenhancement observed in our experiments.

Unfortunately, one has to emphasize a few limitationsof the speed-up effect. First, it is drastically reduced in acw experiment. This may be due to the greater depen-dence of cw experiments on orientational enhancement orto the fact that in cw experiments a steady state (betweenenhanced conductivity and enhanced grating erasure,both induced by the intensive reading beam) is reached,whereas a steady state is not reached in the transient ex-periments presented in this paper. Thus the speed-upscheme cannot be employed for the improvement of cwphotorefractive devices. Furthermore, the effect seemsto be present only in materials exhibiting a high trap den-sity and subsequently rather poor photoconduction. Ahigh-performance bifunctional model glass based on aphotoconducting starburst molecule, Stilbene3-DCTA,27,28 did not show this effect. This may be relatedto its high hole mobility, which is 2 orders of magnitudehigher than the one of the PSX system presented here.28

Thus speed enhancement is probably an interesting effectfrom the physical point of view, which can give new in-sight into photoconduction in organic PR systems. How-ever, it will not lead to faster PR systems.

ACKNOWLEDGMENTSThe authors thank I. Otto for the preparation of thesamples and the Bayerische Forschungsstiftung(FOROPTO II) and the Bayerisches LangfristprogrammNeue Werkstoffe for financial support.

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