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-funda- mental 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 Downloaded From: http://proceedings.spiedigitallibrary.org/pdfaccess.ashx?url=/data/conferences/spiep/65116/ on 07/19/2017 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx RABSON,TITTEL,RABSON, TITTEL, KIMKIM 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