1997-PRB-Off-resonant spectral hole burning in CaS Eu by time-varying Coulomb fields

PHYSICALREVIEWBVOLUME56,NUMBER2015NOVEMBER1997-II

Off-resonantspectralholeburninginCaS:Eubytime-varyingCoulomb elds

S.A.Basun,*M.Raukas, andU.Happek

DepartmentofPhysicsandAstronomy,TheUniversityofGeorgia,Athens,Georgia30602

A.A.Kaplyanskii

A.F.IoffePhysico-TechnicalInstitute,AcademyofSciencesofRussia,194021,St.Petersburg,Russia

´triePhysique,Universite´JosephFourierandCNRS,Bo tePostale53X,38041GrenobleCedex,FranceLaboratoiredeSpectrome

J.C.VialJ.Rennie

MaterialsandDevicesLaboratories,ToshibaCorporation,1Komukai,Toshiba-cho,Saiwai-ku,Kawasaki210,Japan

W.M.YenandR.S.Meltzer

DepartmentofPhysicsandAstronomy,TheUniversityofGeorgia,Athens,Georgia30602

Received9June1997

Persistentspectralholeburninghasbeenobservedonthezero-phononlinesofthemainsiteandseveralperturbedsitesofEu2 inCaS:Eusinglecrystals.Holeburningoccursbytwo-stepphotoionization,canbestronglygatedwithIRirradiation,andtakesplacebyexcited-stateabsorptionfromthemetastable4f65dexcitedstatetotheconductionbandfollowedbyelectrontransporttoEu3 centerswhicharethedominanttraps.Acomplexholestructureconsistingofanarrowfeature 200MHz ,andbroaderfeatures 5and 100GHz isobserved.Amechanismisdescribedfortheoccurrenceoftheseunusuallybroadfeatures.Time-varyinginternalelectric eldswhichoccurduringtheholeburningduetophotoionizationandtrappingcanleadtoburningofholesatfrequenciesnon-resonantwiththatofthelaser.Inaddition,amechanismforholeerasure,tunnelingbetweenEuions,isdemonstrated.Thismechanismisidenti edfromthefrequencydependenceoftheholeerasurewhichfollowstheEu2 absorption,andthelineardependenceofthephoto-conductivityandholeerasureef ciencyonirradiationpower,bothofwhichindicateerasureinasingle-photonprocess. S0163-1829 97 08544-5

I.INTRODUCTION

Oneoftheimportantmechanismsforpersistentspectralholeburning PSHB inaninhomogeneouslybroadenedzero-phononline ZPL ofimpurityanddefectcentersincrystalsisphotoionizationofthecenters.1Photoionizationcanresultbothfromone-photonandsequentialtwo-stepphotoexcitationofthecenters gatedholeburning .Photo-ionizationholeburning HB isinevitablyaffectedbyspatialseparationoftheelectricchargesinthecrystallatticeoccur-ringinthecourseoftheHB,whichcausestimevaryinginternalelectric eldsmodifyingtheenergypositionsoftheelectroniclevelsandtheopticaltransitionfrequenciesofthecenters.Atthesametime,asnotedinRef.1,wearestillfarfromadetailedunderstandingoftheroleoftheseprocessesintheformationoftheholesandtheirfundamentalproper-ties spectralwidth,thermalandopticalstability,erasure,etc. .Suchanunderstandingisimportantforchoosingsys-temsforphotoionizationPSHBwhichmighthaveapplica-tioninthedomainofopticalstorageofinformation.

ThegoalofthepresentworkistheelucidationoftherolesofelectrontransportprocessesintheformationanderasureofionizationspectralholesintheexampleofCaS:Eu.InSec.II,thespectralandphotoconductivityresultsarede-scribed.InSec.IIItheHBandholeerasurepropertiesare

presented.SectionIVpresentsadiscussionoftheseresultsintermsoftheelectrontransportproperties.

II.CaS:Eu—SPECTRALANDPHOTOELECTRIC

PROPERTIES

Thesinglecrystalsweregrownusinganarcimagingfur-nacewithaxenondischargelampactingastheheatsource.2Theabsorptionspectrum(T 100K)ofCaS:0.1%Eusinglecrystalsshowsanintensebroadbandwithalong-wavelengthZPLat625.5nm 1.99eV associatedwiththetransitionfromthe4f7groundelectronshelltothelowestexcitedlevelofthe4f65dcon gurationofEu2 .Figure1presentsapho-toluminescenceexcitationspectrumofthesampleatT 2Kdetectingemissionfromthe4f65dlevel,togetherwiththephotoluminescencespectrumofthistransitioncon-sistingoftheelectron-phononsidebandandastronglyreab-sorbedZPLat625.5nm.Itcanbeseenfromthephotolumi-nescenceexcitationspectrum Fig.1 thatitalsocontainsaweaklong-wavelengthsatellite-typecontinuumwithseveralclearlyresolvedzero-phononlines e.g.,at631.6and641.3nm correspondingtoEu3 ionsperturbedbydeformationornearbydefects.

AlthoughEuionssubstitutingforCa2 ionsintheCaShostlatticeareexpectedtobedivalent,itisknownthat

0163-1829/97/56 20 /12992 6 /$10.005612992©1997TheAmericanPhysicalSociety

OFF-RESONANTSPECTRALHOLEBURNINGIN...12993

FIG.1.Curvesaandb:photoluminescenceatT 2Kexcitedwiththe514.5-nmAr-ionlaserline.Curvescandd:photolumi-nescenceexcitationspectraexcitedwithtunabledyelaserexcitationforCaS:0.1%Eu.Curvesa,c,andd—singlecrystalsample;curveb—powder,1%Eu.Curvediscurvecexpandedbyfactorof

10.

CaS:Eu3 containsEuionsintwochargestates,Eu2 andEu.3Theinfrared-absorptionspectrumofoursamplestakenwithaBrukerIFS66vinfrared1Fourierspectrometerintheregion1170–1250cm 1showsasinglenarrow( 1cm 1)lineat1204cm tionofoneofthepredictedf-f 8306transitionsnm —closebetweentosublevelstheloca-oftheEu3 ground-statemultiplet,7F0-7F2,whosefre-quency forpowdersamplesatroomtemperature canberoughlyestimatedas1195 6cm 1,basedontheEu3 lu-minescencespectrum.4ThisindicatesthepresenceofbothchargestatesofEuinoursamples.

The4f65demittinglevelofEu2 inCaSislocatedinthebandgapratherclosetotheconduction-band CB bottom.From studiesofthetemperature-dependentquenchingoftheEu2d-fphotoluminescence,whichwereinterpretedontheassumptionthatthequenching6resultsfromthermalioniza-tionfromtheEu2 4f5dexcitedstatetotheconductionband,itwasestimatedthat E lms,dependingontheCBEu 0.039–0.05concentrationeVfor5thinand ECB 0.13eVforbulksamples.6Inthepresentwork,theseparationbetweentheemittingEu2 levelandtheCBwasmeasureddirectlyfromphotoconductivityexcitationspectraonthesingle-crystalCaS:0.1%Eusampleunderopticalirra-diationintotheEu2 f-dabsorptionband.Figure2showsthewavelengthdependenceofthephotocurrents,normalizedtotheincidentphoton ux.Thephotocurrentsarelinearinincidentpower.Atlowtemperatures,whenthermalioniza-tionfromtheexcitedstateissuppressed,thephotocurrentexcitationspectrumrisesabruptly Fig.2,curveb forwave-lengthsshorterthan550nm.WeinterpretthissteepriseasresultingfromdirectexcitationofEu2 totheCB,allowingustoplacelimitson Eand CB,i.e., E 0.3eV,where CB h –h ZPLZPLcorrespondto550and625nm,respectively.

Wecanobtain ECBmoreexactlyfrommeasurementsofthetemperaturedependenceofthephotocurrentexcitedintheregionofthef-dlong-wavelengthabsorptionedge, ture,620basednm.Weonmeasurements ndthatitrisesbetweenexponentially80and300withK,tempera-aresultwhichisconsistentwiththeexpectedtemperaturedepen-dence,j exp( Enon excitationofEuCB2 /kT)forcombined photonpluspho-totheCBviathemetastable4f65d

FIG.2.Stationaryphotocurrentexcitationspectraofa1-mmsinglecrystalofCaS:0.1%Eu.Curvea:T 300Kandcurveb:150K.Thephotocurrentisnormalizedtotheincidentphoton ux.TheexperimentalsetupwasdescribedinRef.11.

level.We nd EfoundfromthethresholdCB 0.3ineV,theinphotocurrentagreementexcitationwiththevaluespec-trum.

III.HB—EXPERIMENT

HBexperimentswerecarriedoutatT 2K,witharingdyelaser CoherentRadiation,model899 resonantwiththeZPL.Gatingorerasurewasaccomplishedwithatunablebroadbanddyelaser CoherentRadiation,model590 orIRlightofa50-Wtungstenlamp lteredbycutoffcolor ltersorbandpassinterference lters.Duringthescans,theEu2 phononsidebandemissionintensity,selectedwithaSchottRG665 lterandnormalizedtothelaserbeamintensity,wasdetectedbyaphotomultipliertube,andrecordedwithadigi-taloscilloscope.Thuswewereabletomeasurequantitativelyanychangesoftheinitialabsorptionpro le,evenwhentheholesweremuchbroaderthanthescanrange 20GHz .PSHBwasfoundtooccuronthemainZPLat625.5nm,aswellasinthewholeregionbetween625.5and655nmcontainingthecontinuous2absorption tailandontheZPL’sbelongingtoperturbedEufeaturesofthePSHBonCaS:Eu seeFig.observed1 .TheoncharacteristictheZPL’sat625.5,character. i PSHB631.6,Thisonandfollowsall641.3thesenmaresummarizedbelow.

fromlinesthehasobservedatwo-photonquadraticstepwisedepen-dence isself-gatingofthemuchsmaller atburningtherateontheringlaserpowerdensitythanbeginningitssaturatedofburningvalue,whenandthealsoholefromdepththeobservationoftwo-colorgatedburningwiththeadditionaluseofIRlight.Figure3,curvecshowsaself-gatedholeat625.5nm,incomparisontothephoton-gatedholeburnedwiththesimultaneousirradiationwitha50-WtungstenlampthroughtheSchottRG715color lter Fig.3,curved dur-ingthesameperiod.Astronggatingeffectisclearlyseen.StronggatingisobservedeveninthepresenceoftungstenlampexcitationthroughaSicutoff lter( 1.2 )implyingthat EobtainedCBfrom 1eV,theconsistentphotoconductivitywiththemeasurementsvalue ECB of0.3Sec.eVII.

differs ii Thebetweenspectraltheshapemainof 625.5theholesnm andisrathersatellitecomplexlines.

andOn

12994S.A.BASUNetal.FIG.3.PersistentspectralholesburnedatT 2Kwithapowerdensityof60mW/cm2incidentonasinglecrystalofCaS:0.1%Eu.Curvesaandb:irradiationat631.5nmfor3min.Curvescandd:irradiationat625.5nmfor0.5min.Curvesaandcareself-gatedholes,curvescanddaregatedwithatungstenlampthroughtheSchottRG715color lter powerdensity 1W/cm2 .Dashedline—spectralholeburnedinCaF2:Eu2 forcomparisonofholeshape Ref.7

.

themainZPL,aswellasonthesatellitelines,acontourwithafullwidthathalfmaximum FWHM of 5GHz seeFigs.3and4 isobserved,whosespectralshapedoesnotchangemarkedlywithholedepth.Onthe631.5-nmZPL,inadditiontothe5-GHzcontour,therealsooccursamuchnarrowerhole 200-MHzFWHM whichdisappearswithincreasedburningandincreasedoverallholedepth.Astheoverall appear5GHzhole ZPL’sremainsdepthofthepracticallyincreases,theperturbedthe‘‘main’’sitessame,at631.6butholeextendedwidthcontourand641.3wingsnm andbecomedeeper seeFig.4 ,extendingoveraratherbroadspectralrangetobothsidesfromtheresonantfre-quency( 50GHz).Theseextendedwingsaswellasthenarrow200-MHzholearemuchlesspronouncedonthemainZPLat625.5nm.

FIG.4.SequentialburningofaholeatT 2Kandat641.3nm incidentlaserpowerdensityof60mW/cm2,gatingwithatungstenlampthroughtheSchottcolor lterRG715 afteratotaltimeof0.5,2.5,5,and10min.Thetoptraceistheluminescenceexcitationspectrabefore

burning.

FIG.5.Sequentialerasing for5mineach oftheholeburnedafter10min bottomtraceofFig.4 withilluminationfromatung-stenlamp lteredthroughdifferentinterferencebandpass lters 10-nmFWHM :656,650,640,630,620,610,600,and590.

sample iii Theisheatedholesarefromthermally2KtostableT 100andK,persistandkeptwhenatthetheelevatedtemperatureforseveraltensofminutes.Aftersub-sequentcoolingdowntoT 2K,boththewidthanddepthofthethe ivsample holeTheremainholesunchanged.

inthecanspectralbeeffectivelyregionoferasedtheEuby2 irradiationabsorption.ofFigure5showstheprocessoftheerasureforthe641.3-nmsiteundersuccessiveirradiation,eachfor5min,withlightofdifferentwavelengths tungstenlampplusinterferenceband lters,10-nmFWHM .Itisseenthateffectiveerasingoc-cursonlyforwavelengthsshorterthantheEu2 ZPLregion.Althoughtheerasureisdonesuccessively,themainpointwhichisillustratedisthewavelengthdependenceoftheera-sureef ciency.Ingeneral,duringerasurethenarrow200-MHzfeaturedisappears rst,thenthe5-GHzholefadesbutalmostnochangeintheextendedwingsisobserved Fig.5 ,i.e.,theluminescenceremainsatalevelwhichislessthan65%ofthatbeforeholeburningoverthewhole20-GHzscanrange. Fig.625.5-nmIn5,ispresentedZPLFig. ,6,calculatedthespectraltogetherfromef ciencyoftheerasingwithdatathephotoluminescencesimilartothatgivenex-incitationspectrum.ThereisaclearcorrelationbetweentheerasurespectrumandtheEu2 absorptionspectrum.ThisfactsuggeststhattheEu3 actastrapswhichbecomecon-vertedtoEu2 bycapturingthephotoionizedelectrons.Itisimportanttonotethattheerasureratewasdirectlypropor-tionaltothepoweroftheerasinglight,i.e.,aone-photonexcitationprocessofEu2 tothe4f65dstateisresponsibleforfrom thev theBurningerasure.

rstoneofresultsadeepinsecondthedisappearanceholeatadifferentofthefrequency rstholeincludingthe 100-GHz-widepedestal.Thissupportstheionizationcharacteroftheholesandtheproposedredistribu-tionofEuionsbetweentheirchargestates,Eu2 andEu3 ,whichoccursduringirradiation seebelow

.

OFF-RESONANTSPECTRALHOLEBURNINGIN...12995

parisonoferasureef ciencyandEu2 excitationspectrum.Squares—relativeerasureef ciencymeasuredonaholeburnedonthemainsiteat625.5nmandatT 2Kbysequentialerasureatdifferentwavelengths 5mineach withabroadbandtunabledyelaser powerdensity10mW/cm2 .Solidline—photoluminescenceexcitationspectrumfromFig.

1.

IV.DISCUSSION

TheseobservationsofPSHBonthef-dEu2 ZPL’spro-videclearevidenceofatwo-stepphotoionizationmechanismofholeburninginCaS:Eucrystals.Thesecondstep,promo-tionofelectronstotheCB,isaccomplishedeitherbythesame monochromaticlight self-gating orbylievegatingare .PhotoelectronspredominantlyareEu3captured ions.Indeed,bytrapsIRthewhichirradiationpresencewebe-ofelectrontrapsshallowerthanEu3 ions—iftheydominated—wouldresultinafasterasureofholesunderirradiationatthewavelengthslongerthanEu2 ZPL’s,con-trarytoobservations.Thus,selectiveexcitationresultspre-dominantlyinaspatialredistributionofEuionsbetweentheirchargestates,Eu2 andEu3 tionofrelativelyshallowelectrontraps althoughshouldsomenotcontribu-beruledout .Thelargebindingenergyofthedominant traps Eu3 ions equaltotheionizationenergyofEu2, 2.3eV(EZPL ECB),explainsthethermalstabilityoftheholes.Thespectralshapeoftheholeshasacomplicatedstruc-turewhichcontainsbothrathernarrowfeatures central200-MHzholeanda5-GHz-wideunderlyingstructure andaverybroad( 100GHz)wings.TheХ5-GHzwidestruc-tureofboththemainholeandholesburnedintheperturbedsites,webelieve,isassociatedwithsideholeswhichresultfromthesmallcrystal- eldsplittinginthegroundstateandhyper neinteractionsoftheEu2 inboththe4f7(8SgroundstateandexcitedstateasobservedinCaF7/2)theholespectrumobservedinCaF2:Eu2 .7Forcomparison,2:Eu2isreproducedasthedashedlineinFig.3,wherethewidthofthesideholestructureisseentobeabout5GHz.TheabsenceofresolvedstructurecomparedtothatofCaF2:Eu2 isas-cribedtotheroleofchanginginternalCoulomb eldswhichalter duetotheStarkeffect theEu2 opticaltransitionen-ergiesduetothephotoionizationandtrappingoftheresult-ingphotoelectronsasdescribedbelow.Zero- eldcrystal- eld2 splittingsof 2–5GHzforthe8Scubicsitesinanumber7/2groundstateofEuarecommoninofhosts.AsinthecaseofCaF2:Eu2 ,7thecontributionofallionstotheholeatthelaserfrequencyresultsintheobservationofthenarrow 200MHz centralfeatureintheholespectrum.Theobservationofthebroad( 100GHz)wingsonthephotoionizationholeisadirectmanifestationofthephoto-

FIG.7.Schematicillustrationoftheroleofchanginginternalelectric eldsintheburningofphotoionizationspectralholes.IonA,whichinitiallycontributestotheholeatfrequency i,hasitsfrequencyshiftedbytime-varyingCoulomb eldsresultingfromtheongoingphotoionizationandtrappingofotherions.Eventually,itsfrequencycomesintoresonancewiththatofthelaser( L),atwhichtimeitisphotoionizedcreatingaholeatitsoriginalfre-quency, i seetextfordetaileddescription .TheionswiththeirlocalCoulomb eldsarerepresentedbythecircleswitharrowsontheleft.

ionizationnatureoftheHBprocess.Weassociatetheap-pearanceofthesewingswiththetime-varyinginternalCou-lomb eldsofEuionswhosechargesarechangedduetothephotoinducedrechargingofionsinthecourseofHB.Toourknowledge,noattentionhasbeenpaidtotheideathatthetimevaryinginternalelectric eldsalsoresultsintheburn-ingofbroadholes,notonlyinthebroadeningoftheinitiallynarrowholes.Suchadynamicredistributionofopticaltran-sitionfrequenciesinvolveslargefrequencyhops,andthusisdistinctfromwhatisusuallymeantbyspectraldiffusion.Theprocessbywhichthechanginginternalelectric eldsresultinthebroadwingsisillustratedinFig.7fortwotypi-calions.Initially Fig.7,curvea ionAisinthe2 chargestatewithitsZPLatfrequency A i,andionBisinthe3 2 chargestate.IonBdoesnotinitiallycontributetotheEuZPL,butwilleventuallytrapthephotoionizedelectronfromionA.Asthelocalelectric eldationAchangesduetootherphotoionizationprocessesonotherionswithassoci-atedmovementofcharge Fig.7,curveb ,aholeiscreatedat i,andabsorption antihole appearsatthecurrentinstan-taneousfrequencyofionAat GHzabout A.Withtime Acanmove 50LwithintheZPLcontourasthelocalelectric eldatAchanges Fig.7 c .Eventuallywhen AL,thephotoionizingtionFig. 7antihole d .The appearsantiholeatislaserthenowfrequency,resonantremovedionizationresultsfrequencyandanewofabsorp-ionBwhichisnowinthe2 chargestateaftertrappingtheelec-tron Fig.7 e .Ingeneral,thision’sresonantfrequencyisoutsidethe100-GHzfrequencyrangeproducedbythetime-varyingCoulomb eldsbecauseofothersourcesofinhomo-geneousbroadening strain,vacancies,dislocations,etc. whichareresponsibleforthe30-cm 1-wideZPL.Thenetresult Fig.7,curvee isthataholeisleftat i,somewherewithin 50GHzof L,andanantiholeappearsin

the

12996S.A.BASUNetal.ZPLfarremovedinfrequencyfrom L.WhenoneconsiderstheensembleofEuionsinitiallyinthe2 chargestate,itisseenthatabroadholeiscreatedwithahalf-widthof 100AnGHzestimatecenteredoftheon StarkL.

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

Infraredphoton-gatedburningofpersistentspectralholesisreportedinCaS:Eu.Theholesexhibitacomplexstructureconsistingofanarrowfeature 200MHz ,andbroaderfea-tures 5and100GHz .Amechanismfortheformationoftheanomalouslybroadspectralholesisdemonstrated;time-varyingCoulomb eldsresultingfromthephotoionizationleadtoholeburningatfrequenciesnonresonantwiththatof

theexcitation.Aholeerasuremechanisminthissystemisunraveled.Holeerasureresultsfromelectrontunnelingbe-tweenEu2 andEu3 ions,consistentwithour ndingsthatholeerasureoccursinaone-photonprocess,andthatthefrequencydependenceoftheerasureratefollowstheEu2 absorptionspectrum.DirectevidenceoftunnelingbetweenEuionsisgivenbyphotoconductivitymeasurements.

ACKNOWLEDGMENT

WeacknowledgethesupportoftheNationalScienceFoundationthroughGrantNos.DMR-9307610,DMR-9321052,andDMR-9424216.

*OnleavefromtheA.F.IoffePhysico-TechnicalInstitute,St.

Petersburg,Russia.

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