上转换发光

JOURNALOFAPPLIEDPHYSICS98,113504͑2005͒

EnhancementofblueupconversionmechanisminYLiF4:Yb:Tm:Ndcrystals

LiliaCoronatoCourrol

FaculdadedeTecnologiadeSãoPaulo–LaboratóriodeEspectroscopiaÓpticaCEETEPS,SãoPaulo,Brazil

IzildaMarciaRanieri,LuísVicenteGomesTarelho,SôniaLíciaBaldochi,LaércioGomes,andNilsonDiasVieiraJúnior

InstitutodePesquisasEnergéticaseNucleares–CentrodeLaserseAplicações,CNEN,SãoPaulo,Brazil

͑Received30March2005;accepted20October2005;publishedonline5December2005͒InthispaperwepresentacomparisonbetweenYLiF4:Yb:TmandYLiF4:Yb:Tm:Ndsystemsidentifyingthemostimportantprocessesthatleadtoa20timesenhancementofthuliumblueupconversionemission,underexcitationaround792nm,forthedoublesensitizedsystem.Analysisofthe483nmand1030nmemissionsforthesampleswithdifferentconcentrationsofNd3+ionsshowedthatenergytransferbetweenNd3+andYb3+isthemainmechanismandresponsiblefortheenhancementinupconversion.2005AmericanInstituteofPhysics.͓DOI:10.1063/1.2137462͔

I.INTRODUCTION

Thedevelopmentofcompactbluelasersbasedonthu-lium͑Tm3+͒upconversionprocessesisnowadayswidelystudiedmainlyduetothepotentialofhighperformanceOEMapplications,suchasbiomedicalandanalyticalinstru-mentation,displaysystems,photoprinting.1,2

Inupconversionprocessestheinfraredexcitationiscon-vertedintoshorterwavelengthintotheultraviolet͑UV͒andvisibleranges.3Inthuliumdopedmedia,theefficiencyofthisprocessisverylow,butitwasnoticedthatthemecha-nismcouldbemadeoneortwoordersofmagnitudemoreefficientbyusingytterbium͑Yb3+͒4orneodymium͑Nd3+͒5assensitizerions.

Inthispaper,wecomparethethuliumsensitizationbyYbandYb/NdintheYLiF4͑YLF͒host.

InYLF,theneodymiumupperlaserlevelpresentsaverylongstoragetime,anaturalbirefringence,andarelativelyweakthermallensing.ThesecharacteristicsmaketheYLF:Ndcrystalaveryimportantlasermedium.6Recently,singledopedYLFfiberswereproducedshowingaveryin-terestingnewwaytothedevelopcompactlow-lossandlow-costlasersystems.7

WepresentherethespectroscopiccharacterizationofthenewYLF:Yb:Tm:Ndcrystal,underpumpingat792nm.Thispump,intheNdabsorptionband,resultsinablueup-conversionemissionthatcanbeusedtomakeaYLFfiberlaser.

FortheabsorptionmeasurementsaspectrometerCary17D-Oliswasused.Foremissionmeasurements,thesampleswereexcitedbyaSDLdiodelaserat792nm,andobservedbya0.5mSpexmonochromator,Stanfordchopper,PAR-EG&Glockin,Hammamatsus-20PMTandGermaniumde-tector.III.RESULTS

Figure1showstheabsorptionspectraofthesetwosamples.Figure2showsthepolarizedspectraofthesample.WhenYLFsamplescontainingTm3+co-dopedwithYb3+orYb3+andNd3+,areexcitedat792nm,astrongblueemissionisobserved.Itisimportanttomentionthatthesameexcita-tionofYLF:1%Tmdonotresultinanydetectableblueemissions.

Theblueemissionobservedinthesespectracanbeun-derstoodreferringtotheenergylevelsdiagramshowninFig.3.Whentheco-dopedYb/Tmsampleisexcitedat792

nm

II.MATERIALSANDMETHODS

InthisworkweusedYLFcrystalsgrownbytheCzho-chraskimethodindifferentcompositions:YLF:1%Tm;10%Yb;1%Tm;and20%Yb3+,0.5%Tm3+,x%Nd3+.Threesampleswerecutfromthislastcrystalfromthebeginning͑#2͒,half͑#5͒,andend͑#8͒thathavealittleNdconcentra-tionchangeduetosegregationcoefficient.Thesampleswerecutandpolishedwith2mmthickness.

0021-8979/2005/98͑11͒/113504/3/$22.50

FIG.1.PolarizedabsorptionspectraofYLF:Yb:Tm:Nd͑a͒andYLF:Yb:Tm͑b͒samples.

2005AmericanInstituteofPhysics

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TABLEI.EnergytransferparametersfortheYLF:Yb:Tm:Ndcrystal.Theuncertaintyinthecrosssectionandlifetimevaluesare5%and10%inenergytransfermicroparametervalues.Parameters

Values

5.7ϫ10−20cm26.0ϫ10−19cm22.1ϫ10−21cm22.1ϫ10−21cm27.3ϫ10−21cm23.5ϫ10−21cm22ms15ms1ms2.1ms750␮s570␮s

͑CYbTm=13.7ϫ10−40cm6/s;Rc=12.0Å͒͑CTmYb=9.1ϫ10−40cm6/s;Rc=11.2Å͒͑CNdYb=5.5ϫ10−40cm6/s;Rc=9.8Å͒

FIG.2.PolarizedemissionspectraofYLF:Yb:Tm:Nd͑a͒andYLF:Yb:Tm͑b͒samplesexcitedat792

nm.

␴abs͑␲͒͑Nd͒͑792nm͒␴em͑␲͒͑Nd͒͑1047nm͒␴em͑Nd͒͑960nm͒␴abs͑Yb͒͑960nm͒␴absb͑␲͒͑Tm͒͑792nm͒␴em͑Tm͒͑475nm͒␶͑2F7/2͒␶͑3F4͒␶͑3H5͒␶͑3H4͒␶͑1G4͒␶͑4F3/2͒fcb

thefollowingprocessesaЈ,,andfoccurandpopulatetheTm3+1G4levelthatprocessesare:͑a’͒GroundstateabsorptionofTm͑␴aTm͒;͑c͒EnergytransferTm-Yb:

Tm͑F4͒+Yb͑F7/2͒→Tm͑H6͒+Yb͑F5/2͒;͑d͒

Cross-relaxationYbϫTm:

Yb͑2F5/2͒+Tm͑3F4͒→Yb͑2F7/2͒+Tm͑1G4͒;͑f͒

BacktransferYb-Tm:

Yb͑2F5/2͒+Tm͑3H6͒→Tm͑3H5͒+Yb͑2F7/2͒.

Thesameprocessesarecomplementedbytheprocesses,g,,andwiththeadditionofNdasco-dopantin͑a͒͑b͒

GroundstateabsorptionofNd͑␴aNd͒;CrossrelaxationNdϫYb,

Nd͑4F3/2͒+Yb͑2F7/2͒→Nd͑4I11/2͒+Yb͑2F5/2͒;͑e͒

CrossrelaxationNdϫTm:

Nd͑4F3/2͒+Tm͑3F4͒→Nd͑4I11/2͒+Tm͑1G4͒;͑g͒

BacktransferYb-Nd

Yb͑2F5/2͒+Nd͑4I9/2͒→Yb͑2F7/2͒+Nd͑4I15/2͒;͑h͒

EnergytransferNd-Tm:

3

2

3

2

Nd͑4F5/2͒+Tm͑3H6͒→Nd͑4I9/2͒+Tm͑3F4͒;͑i͒

EnergytransferTm-Nd:

Tm͑3F4͒+Nd͑4I9/2͒→Tm͑3H6͒+Nd͑4F5/2͒.

TableIshowstheenergytransfersparametersobtainedbyoverlap-integralbetweensensitizeremissioncrosssec-tionsandreceptorabsorptioncrosssectionbands.8Theen-ergytransferprocessesf,c,andgaremoreimportantthatthebacktransferethislastoneinvolvesabsorptionofphononsleastprobablethatcreationofphonons.

TheenergytransferratebetweenTm-Ybinthesamplecontaining10%Yb:1%Tmis660s−1andinthesamplecontaining20%Yb:1%Tmis1020s−1.4Theenergytransferefficienciesare47%and57%inthesamplescontaining10%Yband20%Yb,respectively.ItwasobservedthatbacktransferTm-Ybisalsoanimportantmechanism,andthatTm-YbenergytransfergrowswithYbconcentration.

WeobservethatasmallNdconcentrationvariationintheYLF:Yb:Tm:NdsamplesresultsinanenhancementoftheTmblueemissionasitcanbeseeninFig.4.TheNdconcentrationvariationreflectstooinanenhancementintheYbemissionbandintheinfraredascanbeseeninFig.5.Thisindicatesthattheprocesshashighefficiency.

TheblueemissioncomesfromtheTm3+1G4levelsandincreaseswiththepumpingintensitywithaslopeof1.6

in

FIG.3.EnergylevelsschemeandtheenergytransfermechanismoftheYb/Tm/Ndsystem.

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FIG.4.VariationofTmblueemissionintensityinthethreesamplesYLF:Yb:Tm:NdcontainingdifferentNdconcentrations.TheinsidefigureshowstheabsorptioncoefficientofthethreedifferentsamplesoftheYLF:Yb:Tm:Nd

crystal.

FIG.6.Dependenceoftheblueemissionsignalwiththepumpingintensity.

boththeYb:TmandYb:Tm:Ndsamples͑Fig.6andRef.4͒,confirmingthattheupconversionprocessfromthe3F4statetothe1G4stateisatwophotonprocess.IV.CONCLUSIONS

ComparingthetwodifferentYLFcompositionsYb/TmandYb/Tm/Ndwecanconcludethat:

͑1͒Bothcompositionareefficientandgenerateblueemis-sionbyatwophotonprocessmechanismarisingfromthe792nmexcitation;

͑2͒TheYLF:Yb:Tm:Ndsamplehasahigherabsorption

coefficientat792nmthantheYLF:Yb:Tmsample,andconsequentlyahigherabsorptioncrosssection;

͑3͒SinceTmionsaredirectlypumpedby792nmwave-length,thegroundstatecanbedepleted,andtherefore,

thetransition1G4→3H6,atϳ483nmcanbeusedforgeneratingstimulatedemissionusinggroundstatedeple-tionorpump-resonantexcitationmethods;

͑4͒Thecross-relaxationNdϫTmleadstoagrowthinthe

1

G4population;

͑5͒Averyimportantenergytransfermechanismbetween

NdandYbionswasnoticeconsideringthatnoNdemis-sionisobservedininfraredregion,thisfactalsoim-peachestheTm3F4populationdecreasesduetoTm-Ndinteraction.TheenhancementinblueemissionisproportionaltotheenhancementinNdabsorption.

Inconclusion,anenhancementinϳ483nmTm3+emis-sionofalmost20timeswasobservedinthesamplesYLF:Yb:Tm:NdcomparedtotheYLF:Yb:Tmsample.ThissystemcouldbeinterestingfordevelopmentofcompactlasersystemsusingdopedYLFfiberswithabout5mminlengthand300␮mindiameter,underadiodelaserpump.

1

FIG.5.͑a͒VariationoftheinfraredYbemissionbandoftheYLF:Yb:TmandYLF:Yb:Tm:Nd,͑b͒inthethreedifferentYLF:Yb:Tm:Ndsamples,and͑c͒variationwith

polarization.

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