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Photorefractive Properties of Undoped, Cerium-Doped

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Photorefractive Properties of Undoped, Cerium-Doped Photorefractive properties of undoped,undoped, ceriumcerium-doped,-doped, and iron-dopediron -doped single-crystalsingle-crystal Sr0.6Ba0.4Nb2O6 Sr0 George A. Rakuljic Abstract. We present the results of our theoretical andand experimental studies of Amnon Yariv the photorefractivephotorefractive effect effect in in single single-crystal -crystal SBN:60, SBN:60, SBN:Ce, SBN:Ce, and and SBN:Fe. SBN:Fe. TheThe California Institute ofof TechnologyTechnology twotwo-beam -beam couplingcoupling coefficients,coefficients, response times,times, and absorption coefficients of Department of Applied Physics these materials areare given. Pasadena, California California 9112591125 Subject terms: photorefractive materials;materials; non nonlinear linear optical optical materials; materials; optical optical phase phase con con­ - Ratnakar Neurgaonkar jugation; imageimage processingprocessing; opticaloptical signal signal processing. processing. Rockwell International Corporation Optical Engineering 25(11), 12121212-1216 -1216 (November 1986).1986). Science Center Thousand Oaks,Oaks, CaliforniaCalifornia 9136091360 CONTENTS The point groupgroup symmetrysymmetry of of SBN SBN isis 44 mm,mm, whichwhich impliesimplies 1. IntroductionIntroduction that itsits electroelectro-optic -optic tensortensor is nonzero.nonzero. The dominant electro-electro- 2. MaterialMaterial propertiesproperties optic coefficientcoefficient is r33,r33 , which ranges from 100100 pm/pm/V V inin 3. PhotorefractivePhotorefractive properties SBN:25 to 14001400 pm/ V in SBN:75. In order toto realizerealize thethe largelarge 4. SummarySummary ofof resultsresults values of electro-opticelectro -optic coefficients in SBN crystals, they must, 5. ConclusionConclusion in practice, be poled by first being heated toto above their Curie 6. AcknowledgmentsAcknowledgments points and then being cooled to roomroom temperaturetemperature with an 7. ReferencesReferences applied dc electric field of 5 to 88 kVkV/cm. /cm. 1. INTRODUCTIONINTRODUCTION 3. PHOTOREFRACTIVEPHOTOREFRACTIVE PROPERTIES A givengiven photorefractivephotorefractive material is considered useful for opti­opti- Single crystals ofof SBN:60, SBN:60, SBN:Ce SBN:Ce (Sro (Sr^Bao^NbjCVCe), 6Bao 4Nb206:Ce), and cal processingprocessing applicationsapplications such as phase conjugate optics if itit SBN:Fe (Sr06Ba04Nb2O6:Fe)(Sr^Ba^NbjCVFe) grown grown atat RockwellRockwell Interna-Interna­ possesses threethree importantimportant features: low response time, large tional CorporationCorporation were were studied studied using using the the two two-wave -wave mixingmixing coupling coefficient, andand high optical quality. SpeedSpeed isis neces-neces­ experiment shownshown inin Fig.Fig. I1 toto determine determine their their effectiveness effectiveness asas sary if thethe crystalcrystal is toto be usedused in real-timereal -time applications, applications, and a photorefractive media. In Fig. 11 beamsbeams 11 andand 22 areare planeplane large photorefractive coupling coefficientcoefficient isis required forfor thethe waves thatthat intersect in the crystal and thus form an intensity construction ofof efficient devices. Regardless of itsits speedspeed and interference pattern. ChargeCharge isis excitedexcited byby thisthis periodicperiodic inten-inten­ gain, however, aa crystalcrystal withwith poorpoor optical quality is of little sity distribution into the conduction band,band, wherewhere itit migratesmigrates practical importance. Although a material is yet to be found under the influence of diffusion and drift inin thethe internalinternal elec-elec­ that completely satisfies allall threethree requirements, here we show tric fieldfield and thenthen preferentiallypreferentially recombinesrecombines with traps inin how well SBN:60SBN:60 approximatesapproximates them. regions of low irradiance. AA periodicperiodic spacespace chargecharge isis thusthus created that modulatesmodulates thethe refractiverefractive indexindex byby meansmeans ofof thethe 2. MATERIALMATERIAL PROPERTIES electroelectro-optic -optic effect. effect. This This indexindex grating,grating, being out of phasephase with the intensity distribution, introduces an asymmetry Strontium barium niobate (SBN) belongs to a class of with the intensity distribution, introduces an asymmetry that Strontium barium niobate (SBN) belongs to a class of allows one beam to be amplified by constructive interference tungsten bronze ferroelectrics that are pulled from a solid allows one beam to be amplified by constructive interference tungsten bronze ferroelectrics that are pulled from a solid with light scattered by solution of alkalinealkaline earthearth niobates. The crystal isis transparenttransparent with light scattered by the gratinggrating whilewhile the otherother beambeam isis attenuated by destructive interference and can bebe grown with a varietyvariety ofof ferroelectricferroelectric andand electro-electro- attenuated by destructive interference withwith diffracteddiffracted light.light. This process is shown graphically in optic properties,properties, depending on the specific cationcation ratiosratios intro­intro- This process is shown graphically in Fig.Fig. 2.2. AlthoughAlthough itit isis implicitly assumed here that the only duced into the structure. InIn SBNSBN thethe unitunit cellcell containscontains 1010 implicitly assumed here that the only photocarriers inin SBN:60SBN:60 are electrons, it is acknowledged that holes NbO6 octahedra, with only five alkaline earth cations to fill 10 are electrons, it is acknowledged that holes maymay also partici-partici­ Nb06 octahedra, with only five alkaline earth cations to fill 10 pate in the photorefractive interstitial sites.1sites. 1 " -33 The The structurestructure is thus incompletelyincompletely filled,filled, pate in the photorefractive effect.effect. ExperimentsExperiments are currently under way to resolve this which permitspermits thethe additionaddition of aa widewide rangerange ofof dopants dopants intointo thethe under way to resolve this issue.issue. Mathematically, this two-beam coupling may be host crystal. TheThe general general formula formula for for SBN SBN is SrXBa1_XNb2O6,is SrxBa,_xNb2O6, Mathematically, this two -beam coupling may be described in the so SBN:60SBN:60 representsrepresents Sro Sr 066BaoBa04 4Nb2O6.Nb2O6. in the steadysteady statestate asas follows:follows: dl1 III, Paper 21822182 received Aug. 13,13,1985; 1985; revisedrevised manuscript receivedreceived July 16,16,1986; 1986; = - r- all, (1)(I) accepted for publication JulyJuly 18,18,1986; 1986; received by Managing Editor JulyJuly 29,29, df 1986. ThisThis paperpaper isis aa revision of Paper 567567-04 -04 which was presented at the -I' SPIE conference on Advances in MaterialsMaterials forfor Active Active Optics,Optics, Aug.Aug. 2222-23, -23, 1985, SanSan Diego,Diego, Calif.Calif. TheThe paperpaper presented there appears (unrefereed) in dI2 IlI2 SPIE ProceedingsProceedings Vol.Vol. 567. d= T'I1+12r al,, (2) ©e 19861986 Society of Photo-OpticalPhoto -Optical Instrumentation Engineers.Engineers. 1212 / OPTICALOPTICAL ENGINEERING / / NovemberNovember 19861986 // Vol. 2525 No.No. 1111 Downloaded From: https://www.spiedigitallibrary.org/journals/Optical-Engineering on 7/10/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use PHOTOREFRACTIVE PROPERTIESPROPERTIES OF OF UNDOPED,UNDOPED, CERIUM-DOPED,CERIUM -DOPED, ANDAND IRONIRON-DOPED -DOPED SINGLE-SINGLE -CRYSTAL Sr0.6Ba0.4Nb206 4 - 'e 33 E U tf 2 Z=d 2=0 z = d z 400 500 600 700760 800 Fig. 1.1. Experimental setup for twotwo-beam -beam coupling experiments. X(nm), (nm) Fig. 3.3. Absorption spectrum of SBN:Ce. 12 10io - e8 - E U cos kx) `" 66 C 4 2 500 600 700 800 X(nm) n ~ rEsc Fig. 4.4. AbsorptionAbsorption spectrumspectrum ofof SBN:Fe.SBN:Fe. Fig. 2. The photorefractive mechanism.mechanism. Two laserlaser beamsbeams intersect, forming an interference pattern. Charge is excited wherewhere thethe inten­inten- 0.30 sity is large andand migrates migrates to to regions regions of of low low intensity. intensity. TheThe electricelectric fieldfield associated withwith the resultant spacespace charge operates throughthrough the 0.25 electroelectro-optic -optic coefficientscoefficients to produce a refractive index grating. TE 0.20 o t10.1500.15 where I)Ij and 12I2 are are thethe intensitiesintensities of beams 11 and 2 inside the O.IO0.10 coefficient, a crystal, respectively, Fr isis thethe twotwo-beam -beam coupling coefficient, a 0.05 is the absorption coefficient, and ¿f z/cos0j,z/ cos9i, wherewhere 0 0 < S fe < It - d d/cos0j. /cos9i. TheThe transienttransient behaviorbehavior isis approximated byby 400 500 600 700 800 X(nm) 1,(e;t) = (1 -e `ir)I1(e;t;t oo ) + e- `Irli(e;ttf ;t == 0),0), Fig. 5.5. Absorption spectrum of SBN:60. (3) i =1,2, be obtained from the above equations. It is important toto notenote where r is a characteristic time constant andand that although thethe aboveabove description ofof the transient behavior is not strictlystrictly correct,4correct,4 it does indeed approximate thethe tem-tem­ I ¡(e;t 00)111,(e) (4) poral responseresponse ofof thethe twotwo-wave -wave mixing process in SBN very well since the measuredmeasured waveformswaveforms can bebe accuratelyaccurately de-de­ The solutions ofof thethe aboveabove coupledcoupled-wave -wave equations areare scribed by simple exponentials. Maximum couplingcoupling willwill result inin crystals with large PF but small a. However, a and I'F areare notnot independent.independent. InIn fact, fact, sincesince [I,(0)
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