1997-PRB-Off-resonant spectral hole burning in CaS Eu by tim(3)

shiftsoftheEu2 transitionfre-quenciescanbyobtainedusingpublisheddataoneffectsofanexternalelectric eldEquadraticStark0onthef-dspectrallinesofrareearthions.Theshiftofthef-dZPLofEu2 ions in13cubic 1centrosymmetric8(OForh)sitesinCaF/ V/cm 2Etheperturbednoncen-2is5 trosymmetric10cm02.3 sites,anestimatecanbeobtainedfromresultsonCecentersinCaF2wherethecubicsymmetryisdis-turbedbyaneighboringchargecompensatorwhichdestroystheinversionsymmetry.TheobservedStarkeffectis5 10 5cm 1/ V/cm E0.9TheresultingStarkshiftsinanelectric eldofE 105V/cm,typicalofwhatisexpectedfortheCoulomb eldduetoasingleelectronchargeintheCaSlatticeatmeandistancesbetween1Euionsfor0.1%Eucon-centrations,is 0.1cm forquadratic mainsite and 3GHzlinear and perturbed5cm 1sites 150 GHzStark effects,respectively.Thusthe100-GHzbroadwingsinoursamplescanbeexplainedbythelinearStarkeffect.Thisconclusionissupportedbythefactthatthe2 holewingsaremuchmorepronouncedontheZPLofEuionsperturbedbydefectsandlackinginversionsymmetrythanfortheZPLofthemainsiteswhichhavecubicsymmetry,andthereforeexhibitonlyquadraticStarkeffects.

Tosupportthisassumptionfurther,weappliedelectric eldsofupto22kV/cmtoaholeburnedonthe631.5-nmZPLatzero eld.Weobservedasplittingandbroadeningofthehole,andthemagnitudeoftheobservedStarkeffectisadequatetoexplainthewidthsofthebroadholesontheperturbedsites.Consistentwiththeabsenceofabroadholeonthemainsite,nomeasurableStarkeffectisobservedatthese eldsforholesburnedonthemainsite,asexpectedforacubicsite.WenotethattheseresultsareconsistentwiththepreviouslyreportedresultsofStarkeffectsinCaFandCe3 .8,9

2forEu2 We havedemonstratedthattheselectivephotoionizationofEu2ionsat LcausesadynamicspatialredistributionofelectricchargesinthesystemofEu2 andEu3 ions.ThisredistributionresultsinphotoionizationoftheEu2 ionswhosefrequenciesbeforeirradiationdifferedfrom StarkshiftsresultingLbylargevalues,correspondingtothefromchanginginternalCoulomb elds.Thisistheoriginoftheextendedwings( 100GHz).ThesewingscorrespondtothetransitionsofEu2 ionsexperiencingstrongStarkshiftsduetotheirproximitytoEuionswhosechargestatechanged( e)duringirradiation.Theappearanceofthenarrow 200MHz holeand5-GHzcontouris2 duetothemostrecentselectiveresonantionizationofEuionswhoselocal eldandresultingf-dtransitionfrequencieshavenotyetchangedsinceitsphotoionization.

ThegeneraloriginoftheionizationholeerasureunderopticalexcitationisphotoinducedrechargingofEu2 3 andEu,resultinginspatialredistributionofthe2 and3 chargestatesintheensembleofEuions.Thischargeex-changecanerasetheholesintwoways.Inthe rst,erasureoccursasafrequencyredistribution( 100GHz)oftheholeduetovariationsoftheinternalCoulomb eld,producingStarkshiftsoftheEu2 transitionfrequencieswhichresultsinareductionoftheoriginal‘‘monochromatic’’componentsofthehole.Itisclearthatdissipationofanycomponentoftheinitialholemustoccurfastestforthenarrowestcompo-nents,aswasobserved thenarrow200-MHzfeaturedisap-pears rst,thenthe5-GHzcontour .Duetothelargerangeoffrequencyhops comparablewiththewidthofthe100-GHz-widewings ,theerasureofthesenarrowercomponentsoftheholeoccurswithalmostnobroadeningbutratheraredistributionwithinthe100-GHzfeature.Thusthebroad 100-GHzInthesecondfeaturebutislessnoterased.

likely3 process,someoftheEu2 ionspreviouslyconvertedtoEuandwhoseabsenceisre-sponsibleforthehole,canbeconvertedbacktoEu2 their4f7→4f65dZPLfrequencieswithin 50GHz withofthosebeforeburning resultinginpartialremovalofthebroad( 100GHz)hole.Aholeiscreatedbythoseionsthatwerephotoionizedbythebroadbanderasinglightwhichcre-atedthephotoelectrons,buttheholefrequencycannowbeanywhereintheZPL,sothattheirremovalisnotperceivedasanewhole.ThissecondprocessoccursforonlyasmallfractionofEuions,sinceitisfoundthatthebroadholeisnotef cientlyerased seeFig.5 .

WhatisthemechanismofphotoinducedEu2 Eu3 re-chargingresultingfromone-photonexcitationofEu2?Theerasurespectrum Fig.6 followstheEu2 absorptionspec-trum,andexhibitsalinearpowererasureef ciency.Thelin-earerasureef ciencyindicatesthatthedominantmechanismforerasuremustbequitedifferentthanthatforphotoioniza-tion.Weassociatethiswithtunnelingofanelectronfromtheexcited4f65dEu2 leveltoaneighboringEu3 .Theexis-tenceofsuchtunnelingisindependentlysupportedbytheobservationofaphotocurrentthatislinearinlaserirradiationpoweratT 2K,whoseexcitationspectrumfollowstheEu2 absorptionspectrumintheregionofthemainZPLandshorterwavelengths.Suchprocessesofelectrontunnelingfromimpurityexcitedstateswereobservedalsoinothersys-tems photorecharging1ofCrtances. Even,andionsinruby,10HBoncolorcentersintheyAlef cientlyoccuroverratherlargedis-levelofCr3 islocated2O3:Cr3 very deepruby in—wherethebandthegapexcited2Eandthusitswavefunctionisverycompact—thetunnelingef cientlyoccursoverseveralnanometers.10ForEu2 ionsinCaS,theproximityoftheEu2 4f65dleveltotheCBshouldbetakeninto account,whichstronglyaffectstheextensionoftheEu24f65dlevelwavefunctioninCaS,contrarytothecasesofthesystemssuchasCaF2:Eu2 ,with EorderofseveraleV.

CBontheThetunnelingwhichprovidesthemaincontributiontoholeerasureinCaS:EushouldalsocontributetotheHBwhere,asmentionedabove,two-photonionizationdomi-nates.However,tunnelingisfavoredforholeerasure,sincetheerasinglightisbroadbandandisthereforenonselective,excitingthef-dtransitionforallEu2 ionswithequalprob-ability.ThesystemofEu2 ionscanthenself-selectthoseionswithnearbyEu3 electronacceptorsiteswheretunnel-ingishighlyprobable.Theresultingphotoionizationgener-atestime-varying elds.Incontrast,underselectiveexcita-tionduringtheHB,onlyaverysmallfractionofthoseEu2 ionsresonantwiththelaserwillbeinfavorablelocationsfortunneling,especiallyiftheEuexistpredominantlyintheEu2 stateaswebelieve.However,two-photonionizationto

OFF-RESONANTSPECTRALHOLEBURNINGIN...12997

theconductionbandprovidesmobilitytotheelectron,allow-ingittoseekamoredistantEu3 acceptor,therebymakingphotoionizationavailabletoalargefractionoftheresonantEu2 centers.

V.CONCLUSIONS

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