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

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 …… 此处隐藏：6179字，全部文档内容请下载后查看。喜欢就下载吧 ……