Optical Data Storage in Photorefractive Materials

$Optical Data Storage in Photorefractive Materials$

OPTICAL DATASTORAGEDATA STORAGE IN PHOTOREFRACTIVE MATERIALS

T. A. Rabson, F.F. K.K. TittelTittel and D. M. KimKirn Electrical EngineeringEngineering Department Rice University Houston, TexasTexas 77001

Abstract

The basicbasic physicalphysical theorytheory ofof thethe photorefractivephotorefractive effecteffect willwill bebe presented.presented. Methods will be discussed forfor utilizing the effect for the storage of informationinformation as well as optical displaydisplay ofof information.information. The funda-fundamental limitslimits on writing time,time, accessaccess timetime andand storagestorage densitydensity willwill bebe relatedrelated toto thethe physicalphysical propertiesproperties ofof the materials involved.involved. Particularly promising materials, such as LiNb03LiNbO^ will bebe discusseddiscussed inin detail.detail.

Introduction

The incidenceincidence of lightlight on certain ferroelectricferroelectric crystalscrystals cancan alteralter thethe indexindex ofof refractionrefraction ofof thethe crycry - stalstal(1). ^ ''. This physical phenomenonphenomenon isis knownknown asas thethe photorefractivephotorefractive effect.effect. Although thethe physicalphysical behaviorbehavior of this effect has been well studiedstudied thethe fundamentalfundamental photoexcitationphotoexcitat ion processprocess isis stillstill notnot wellwell understood.understood. This lacklack of understanding doesdoes notnot preventprevent thethe applicationapplication ofof thethe effecteffect toto thethe storagestorage ofof datadata inin crystalscrystals either holographically or by point by point scanning.scanning. The basic characteristicscharacteristics ofof thethe processprocess holdhold outout thethe possibility that memories utilizing the photorefractive effect could bebe builtbuilt with highhigh storagestorage densitiesdensities (> 10810® bitsbits/cm /cm3),3 ), fastfast accessaccess time (< 11 µsec),iisec) , andand highhigh datadata ratesrates (>(> 109 10 9 bitsbits/sec). /sec). The desirable properties forfor aa photorefractivephotorefractive materialmaterial toto bebe usedused inin aa memorymemory systemsystem areare thatthat it have aa high writingwriting sensitivity, aa largelarge decaydecay time,time, aa highhigh persistencepersistence duringduring readread-out, -out, andand highhigh resolution.resolution. Al-Al though no one materialmaterial scores highest for all of these properties,properties, ironiron dopeddoped lithiumlithium niobateniobate lookslooks likelike oneone pf the more promising materials andand forfor thisthis reasonreason itsits propertiesproperties willwill bebe discusseddiscussed inin detail.detail.

Theory of the Photorefractive Effect The photorefractive effect was firstfirst observedobserved asas opticaloptical damagedamage inin crystalscrystals ofof LiNb03LiNbOg apdand LiTa03LiTaOg wherewhere itit produced unwanted scattering and decollimation of lightlight inin nonlinearnonlinear optical experiments(2).experiments' 2 '. The firstfirst stage ofof the photorefractive effect consists of thethe photoexcitationphotoexcitat ion of electronselectrons fromfrom localizedlocalized donordonor sitessites in the crystal. The next stage of thethe processprocess involvesinvolves thethe electronelectron transporttransport inin thethe crystalcrystal andand thethe subse-subse quent retrappingretrapping ofof thethe electrons.electrons. Electric fields are consequently produced because ofof thethe spacespace chargescharges produced and these fields then alter the indexindex of refractionrefraction of thethe crystalcrystal throughthrough thethe electroopticelectrooptic effect.effect.

Figure 11 illustrates these processes. There isis generalgeneral agreement thatthat thethe indexindex changeschanges areare producedproduced asas aa result ofof the space charge fields inin the crystal. There isis stillstill somesome controversycontroversy overover thethe electronelectron photophoto- - excitation and transport processes.processes. In orderorder toto understandunderstand thethe currentcurrent theories of the electron generation and transport, a briefbrief reviewreview of the process isis given below.

XX-Y -Y BEAM DEFLECTOR MODULATOR LASER

PLANE WAVESWAVES

f;\/' V v v INTENSITYINTENSITY INTERFERENCEINTERFERENCE RATTERNWTTERN

UNIFORM DISTRIBUTION PHOTOREFRACTIVE MEDIUM OF TRAPS

r ' PHOTOEXCITATKRTIPHOTOEXCIwN r CHARGECHARGE 1*T——r-fr RETRAPPINGRETRAPpNG - i ——- OF _ OF C^RRjEJRS_j^J^_JRA_NSPO_RT_CARRIERS_JH TRANSPORT J*T_—— J =__OF _ OF CARRERS CARRIERS -I

light modulation pattern apace charge field and A refractive index modulation pattern FLY'S EYEEYE LENSLENS PAGE HOLOGRAPHIC DETECTOR COMPOSER STORAGE ARRAY MEDIUM

Fig. 1. Schematic explanation of the photorefractive Fig. 2.2. Experimental arrangement for a holo-holo effect. graphic readread-write -write memory.

If lightlight ofof sufficientsufficient frequencyfrequency interacts withwith donor sites withinwithin a crystal it willwill have a certain prob-prob ability ofof photoexciting electrons intointo the conduction band. The raterate of excitation of thethe donordonor sitessites isis

SPIE Vol. 128 Effective Utilization ofof Optics Optics in in Radar Radar Systems Systems (1977) / 291291

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as follows:fol lows: , 1a 9=i£'g=hv. (1)

where 11 = IntensityIntensity ofof thethe light,light, ao/ = Attenuation coefficientcoefficient ofof thethe crystal,crystal, h = Planck's constant,constant, v = Frequency of light.light. The continuity equation forfor thethe conductionconduction bandband electronicelectronic concentrationconcentration n is given by £=9(x)-^vJdn dt =g(x) - T + e v j (2) where T = Electron trappingtrapping time,time, j = Current density,density, e = Electronic charge magnitude.magnitude. The current density of the electrons cancan bebe divideddivided intointo driftdrift andand diffusiondiffusion currents.currents.

j = een,n(x) u,n(x) ?E ++eD eD vnvn (3) where p,µ = Electron mobility D = Diffusion constant E = Electric field The firstfirst termterm representsrepresents driftdrift currentcurrent andand thethe secondsecond termterm diffusiondiffusion current.current. Under most circumstancescircumstances diffusion currents inin LiNb03LiNbOg can bebe neglectedneglected asas farfar asas thethe photorefractivephotoref ract ive effecteffect isis concerned,concerned, however,however, there isis general agreement as toto howhow toto calculatecalculate thethe diffusiondiffusion currentcurrent termterm ifif itit isis significant.significant. With regard to thethe driftdrift current term inin order to reconcilereconcile experiment with theorytheory aa constantconstant internalinternal electricelectric field waswas assumed^''.assumd)1). Glass has suggested that thethe photoexcited carriers havehave aa preferredpreferred averageaverage velocityvelocity upon excitationexcitation' '. Under circumstances where thethe trappingtrapping timetime isis shortshort andand thethe darkdark conductivityconductivity small,small, the results of the two theories are equivalentequivalent andarid thethe purposepurpose ofof thisthis paperpaper isis notnot toto choosechoose betweenbetween thethe two theories. If anan equivalent internalinternal field,field, E. ., is included then the drift current becomes

j dr . ft = e^n(x) [E, nt+ e(x)] (4) adrift =eµn(x) [Eint + 8(x)] where E(x)£(x) includesincludes allall realreal electricelectric fielefields. . Another more realistic description of the current is to describe thethe driftdrift currentcurrent asas GlassGlass etet al`` al did as the sum of a bulk photovoltaicphotovoltaic currentcurrent andand aa standardstandard photo current. _ j = H ] ala\ ++ eu,n(x)E(x)eun(x)E(x) (5)

where HIxl is a constant depending on the nature of the absorbing center andand thethe wavelengthwavelength andand independentindependent of the crystal geometry, thethe electrode configuration,configuration, andand thethe impurityimpurity concentration.concentration. One can seesee thatthat

there isis a direct relationrelation betweenbetween thethe equivalentequivalent internalinternal fieldfield EintE| nt andand 741 HJ . Some measurements of the value of EintEj nt for LiNb03LiNb03 will be reportedreported here.here.

Utilization for Data Storage

Figure 2 illustratesillustrates how the photorefractivephotoref ract ive effecteffect cancan bebe utilizedutilized inin aa holographicholographic readread writewrite memory.memory. The system consists of a coherent lightlight source,source, beambeam deflector,deflector, aa datadata inputinput elementelement oror pagepage composer,composer, aa recording medium, and aa detectordetector matrix.matrix. Considerable researchresearch has gonegone intointo eacheach ofof thesethese principalprincipal com-com ponents(4)ponents' ' and considerable progress hashas beenbeen made. This has ledled to thethe development ofof severalseveral memorymemory andand display prototypes but inin order toto developdevelop thethe potentialspotentials ofof holographicholographic opticaloptical storagestorage onon aa commercialcommercial scale, further progress needs toto bebe mademade inin thethe areasareas ofof lowlow-cost, -cost, coherentcoherent lightlight sources,sources, fastfast andand effectiveeffective beam deflectors, highhigh-speed -speed pagepage composers,composers, and optical storage materials. It is obviousobvious that the above described system can also be utilizedutilized asas aa realreal timetime displaydisplay systemsystem byby directlydirectly observingobserving thethe outputoutput wherewhere the photodetector isis located.located. The speed of a memory systemsystem basedbased onon thethe photorefractivephotoref ract ive effecteffect isis notnot limited limited Iiy2 by anyany ofof the time concon- stants involvedinvolved inin thethe effect.effect. The photoexcitationphotoexc i tat ion processprocess occursoccurs inin lessless thanthan 10-10 seconds and thethe mean free time until the first scatteringscattering ofof thethe photoelectronphotoelectron isis alsoalso lessless thanthan this.this. With mode lockedlocked laserslasers enough lightlight intensityintensity cancan bebe generatedgenerated inin lessless thanthan 1010"'' -11 seconds toto write aa discerniblediscernible hologram.hologram. The erasure and readingreading processes are alsoalso possiblepossible with suchsuch pulses.pulses. The timetime limitationlimitation isis currentlycurrently causedcaused by the beam steering and detection processes.processes.

Experimental Studies of Lithium Niobate

Experimental studies have been made of various LiNb03LiNbO^ crystalscrystals usingusing thethe arrangementarrangement shownshown inin FigureFigure 3.3. Although the object beam was planarplanar andand notnot modulated thetne basicbasic principleprinciple isis thethe samesame asas ifif informationinformation werewere contained inin the beam through spatialspatial modulation. The interferenceinterference pattern generatedgenerated byby thesethese twotwo beamsbeams ofof

wavelength 7<\ and intensityintensity IoI Q intersecting atat an angle 2029 is given by l(x)1(x) == I0(1I Q (1 + cos Kx)Kx) (6)

where K == 2rr2-n/A /A is the grating vector, andand A/v = X/2x/2 sinesin0 is the wavelengthwavelength of thethe grating.grating. By observing the diffracteddiffracted beam intensity whenwhen the object beam isis interruptedinterrupted one can obseçobserve the raterate of,of hologramhologram for-for mation inin the crystal.crystal. For a thick sinusoidal grating phase hologram KogelnikKogelnik^ ' has derived the following expression for the diffraction efficiency

292292/SPIE / SP/E Vol. 128128 Effective UtilizationUtilization ofof Optics Optics in in Radar Radar SystemsSystems (1977)

( DiffractedDiffracted . 2 TT d An ==sinsm (X (7) 7 = cose) (7) o where An = Change inin indexindex of refractionrefraction d = Grating thickness Observations of 7]7 as a function of time for various writing intensitiesintensities areare shownshown inin FigureFigure 4.4. These measurements allow one toto inferinfer thethe valuevalue ofof internalinternal equivalentequivalent fieldfield whichwhich mustmust existexist forfor thesethese differentdifferent intensities in orderorder to have the observedobserved chargecharge transport.transport. This fieldfield dependence isis shownshown inin FigureFigure 5.5. This field has a linearlinear dependence onon intensity.intensity.

SPECTRUM MONO- NUMBER OF OSCILLATIONSOSCILLATIONS ININ ,l77 -CURVE-CURVE ANALYZER CHROMATOR 125'25 1.751.75————22

[ ARGON LASER / / ^1 ' ^ /^

UE SHUTTER 15 peom

BEBEAMSPLITTER ANS PL T TER

m SLOPE=l.4lxl0SLOPE = 1.41x1066 V-cm/WV -cm /W SHUTTER INTERCEPT == 8.5 8.5 kV/cmkV /cm Fig. 3.3. Experimental arrangement forfor measuring writing sensitivity.sensitivity. 5 10l0 15 INTENSITYINTENSITY (mW(mW/cm /cm2)2)

Fig. 5. Dependence ofof equivalentequivalent internal field on light1ight v intensintensity. ity.

~^00—————600" 0 200 400 600 TIME (sec)(sec) 80 08

70 06

0.40.9 60 J 0.2 W LA- -I———L——I———I———i———IT2——————24 i C.)40 0 1.2 2.4 3,6 48 60 INTENSITY EXPOSURE (J/cm(J/cm2)2) F- 30 II I` 131 11g 1 1 o 247 mw/cmzmw/cm2

06 J 20 LAI •. 62vt.Gt!9 lew/cm2nut/cm~*, 2 04 1 Io

0.2 7 11111111 Io 20 30 4 200 400 600 800 0300 EXPOSURE (J/cmil(j/cmM TIME (sec)(sec)

Fig. 6.6. 'TheThe electric fieldfield asas aa Fig. 4.4. Diffraction efficiency asas aa functionfunction ofof timetime forfor function of timetime basedbased onon four different writing intensities.intensities. The two upper interferometric measuremeasure- curves are experimental results,results, thethe lowerlower isis basedbased ments. on calculation.

SPIE Vol. 128128 Effective Utilization ofof Optics Optics in in Radar Radar Systems Systems (1977)11977) / / 293

Because the change inin indexindex of refractionrefraction isis aa goodgood measure ofof thethe electricelectric fieldfield producingproducing it,it, wewe carriedcarried out some experiments to monitor the change inin indexindex as aa hologramhologram was written. We usedused aa MachMach-Zehnder -Zehnder inter-inter ferometer inin one arm of which was placed an .05%.0555 ironiron dopeddoped lithiumlithium niobateniobate crystal.crystal. An inputinput beam at 4880 A from an argon ionion laserlaser was splitsplit intointo twotwo beamsbeams andand aa hologramhologram waswas recordedrecorded insideinside thethe crystalcrystal withwith writing angleangle ofof approximatelyapproximately 1212°. °. Simultaneously a weak HeNe laserlaser beam was usedused toto measuremeasure thethe ensuingensuing change in the indexindex of refraction An, of thethe crystal byby measuring thethe fringefringe shiftshift asas aa functionfunction ofof writingwriting time for two different beam intensities.intensities. An can then be attributed to the spacespace chargecharge fieldfield andand thisthis isis equal to the equivalent internalinternal field when steadysteady statestate isis achievedachieved ifif thethe darkdark conductivityconductivity isis sufficient-sufficient ly low. In FigureFigure 66 is presentedpresented the space charge field E thus measured as aa functionfunction ofof writingwriting timetime forfor two different intensities.intensities. Clearly E is enhancedenhanced rather significantly withwith increasingincreasing writing intensity.intensity. Glass et al^al(3)3 ' have reported that the bias external fieldfield necessarynecessary toto reducereduce thethe photocurrentphotocurrent toto zerozero waswas greatly dependent on the inputinput intensity.intensity. For example, inin Fe:LiNb03Fe:LiNb03 withwith oa == 3838 cmcm~', -1, the photocurrent was zero for 9nan externalexternal fieldfield ofof 6060 1sVIsV/cm /cm atat .32w.32w/cm /cm22 input intensity and decreasingdecreasing toto 3838 kVkV/cm /cm forfor 0.08 ww/cm /cm .. We have carried out a similarsimilar experimentexperiment butbut measuredmeasured thethe photocurrentphotocurrent producedproduced throughthrough aa shortshort circuitcircuit when thethe crystalcrystal is illuminated with various lightlight intensities.intensities. The resultsresults areare shownshown inin FigureFigure 7.7. The two straight lines are for a least square fit toto thethe firstfirst fourfour pointspoints andand aa leastleast squaresquare fitfit toto allall points.points. This.indicatesThis,indicates that a significantly higherhigher fieldfield isis producedproduced atat higherhigher intensities.intensities. Figure 8 shows the dependence of the writingwriting sensitivity on wavelength. It cancan bebe concludedconcluded that the sensitivity increasesincreases for shorter wavelengths. In thethe processprocess ofof writingwriting certain beam coupling effects can occur. Those areare illustratedillustrated inin FigureFigure 9.9. This isis manifested by thethe switchingswitching ofof beambeam energyenergy betweenbetween thethe objectobject andand referencereference beamsbeams afterafter theythey havehave ipassedpassed through the crystal.crystal.

1000 1

-- i cw sourcesource - j\ pulsed sourcesource 100

1.0

O.I

30. . 0.01 E-=

0.00I0.001 I 400 500 600GC0 TOO700

o o WAVELENGTH X,X, nm s: 20. . Fig. 8.8. Writing sensitivity asas aa function of wavelength UJ cr. Qi !D O

10-10..

100loo 200 300 INTENSITY lmw,cm2) lmw/cm2 )

Fig. 7.7. Photocurrents as a function of incidentincident lightlight intensity withwith no applied voltage.

294 /SPIE/SP /E Vol. 128128 Effective Utilization ofof Optics Optics in in Radar Radar SystemsSystems (1977)

6.0

BEAM COUPLING ENVELOPE S.O

LO DIFFRACTION EFFICIENCY SAMPLE POINTS

IO IS 2O TIME (MINUTES) (MINUTES)

Fig. 9. Beam coupling effects inin thethe writing process.process.

ConclusionsConclus ions The properties of photorefractive The properties of photorefractive crystalscrystals makemake themthem attractiveattractive candidatescandidates forfor constructingconstructing opticaloptical memory systems with fast memory systems with fast read,read, write andand eraseerase times,times, highhigh storagestorage density,density, andand longlong lifetime.lifetime. Further study is indicated to optimize Further study is indicated to optimize all of these parameters. In addition,addition, fundamental studies of thethe bulk photo- voltaic effect are bulk photo voltaic effect are needed to understand the basic photoexcitationphotoexcitat ion and charge transporttransport processes.processes.

References 1. Chen, F.S., "Optically Induced 1. Chen, F.S., "Optically Induced Change of RefractiveRefractive IndicesIndices inin LiNb03LiNb03 and LiTa03,"LITa0 3> " J.J. Appl. Phys.Phys. Vol.Vol *4040, pppp'3389-3396. 3389 -3396. 1969. 2 Ashkin A , G.D. Boyd, J.M. 2. Ashkin, A., G.D. Boyd, J.M. Dziedzic,Dziedzic, R.G.R.G. Smith,Smith, A.A.A.A. Ballman,Ballman, J.J.J.J. Levinstein,Levinstem, andand K.K. Nassau,Nassau, "Opt'ically Induced Refractive "Optically- Induced Refractive IndexIndex InhomogeneiInhomogeneities t ies in LiNb0LiNb033 and LiTa03,"LiTaQ3 ," Appl. Phys.Phys. Lett.,Lett., Vol.Vol. 9,9, pp 72 -74. 1966. PP 3 " Glass A*M D von der Linde, and 3. Glass, A.M., D. von der Linde, and T.J.T.J. Negran,Negran, "High-"High-Voltage Voltage Bulk PhotovoltaicPhotovoltaic EffectEffect andand thethe Photorefractive'Process Photorefractive Process inin LiNb03."LiNb03 ." Appl.Appl. Phys.Phys. Lett.,Lett., Vol.Vol. 25,25, pppp 233233-235 -235. 19741974. 4 Special 4. Special IssueIssue on Optical Storage of Digital Data, Appl. Opt.,Opt., Vol.13,Vol.13, April 1974.1974. 5. H. Kogelnik, "Coupled Wave 5. H. Kogelnik, "Coupled Wave TheoryTheory forfor ThickThick HologramHologram Gratings,Gratings,"Bel "Bell 1 Syst. TechTech J.,J., Vol.Vol. 48,48, pppp 2909-2909- 2947. 1969.

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