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
98,113504-1
113504-2Courroletal.J.Appl.Phys.98,113504͑2005͒
TABLEI.EnergytransferparametersfortheYLF:Yb:Tm:Ndcrystal.Theuncertaintyinthecrosssectionandlifetimevaluesare5%and10%inenergytransfermicroparametervalues.Parameters
Values
5.7ϫ10−20cm26.0ϫ10−19cm22.1ϫ10−21cm22.1ϫ10−21cm27.3ϫ10−21cm23.5ϫ10−21cm22ms15ms1ms2.1ms750s570s
͑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.
abs͑͒͑Nd͒͑792nm͒em͑͒͑Nd͒͑1047nm͒em͑Nd͒͑960nm͒abs͑Yb͒͑960nm͒absb͑͒͑Tm͒͑792nm͒em͑Tm͒͑475nm͒͑2F7/2͒͑3F4͒͑3H5͒͑3H4͒͑1G4͒͑4F3/2͒fcb
thefollowingprocessesaЈ,,andfoccurandpopulatetheTm3+1G4levelthatprocessesare:͑a’͒GroundstateabsorptionofTm͑aTm͒;͑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
GroundstateabsorptionofNd͑aNd͒;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.
113504-3Courroletal.J.Appl.Phys.98,113504͑2005͒
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.ThissystemcouldbeinterestingfordevelopmentofcompactlasersystemsusingdopedYLFfiberswithabout5mminlengthand300mindiameter,underadiodelaserpump.
1
FIG.5.͑a͒VariationoftheinfraredYbemissionbandoftheYLF:Yb:TmandYLF:Yb:Tm:Nd,͑b͒inthethreedifferentYLF:Yb:Tm:Ndsamples,and͑c͒variationwith
polarization.
W.P.Risk,T.R.Gosnell,andA.V.Nurmikko,CompactBlue-GreenLasers,CambridgeStudiesinModernOptics͑CambridgeUniversityPress,Cambridge,2003͒.2
LeeLaser,DPSS473-nmMicrochipBlueLaser,PhotonicsMiniMaga-zine͑2003͒.3
A.Wnuk,M.Kaczkan,Z.Frukacz,I.Pracka,G.Chadeyron,M.-F.Jou-bert,andM.Malinowski,J.AlloysCompd.341,353͑2002͒.4
X.X.Zhang,P.Hong,M.Bass,andB.H.T.Chai,Phys.Rev.B51,9298,͑1995͒.5
N.Rakov,G.S.Maciel,M.L.Sundheimer,L.deS.Menezes,A.S.L.Gomes,Y.Messaddeq,F.C.Cassanjes,G.Poirier,andS.J.L.Ribeiro,J.Appl.Phys.92,6337͑2002͒.6
E.P.Maldonado,I.M.Ranieri,S.P.Morato,andN.D.Vieira,Jr..inTrends,OpticsandPhotonicsSeries,TOPS,AdvancedSolidStateLaser,editedbyC.R.PollockandW.R.Bosenberg͑OpticalSocietyofAmerica,Washington,D.C.,1997͒,Vol.10,pp.444–447.7
A.M.E.Santo,I.M.Ranieri,G.E.S.Brito,B.M.Epelbaum,S.P.Morato,N.D.Vieira,Jr.,andS.L.Baldochi,J.Cryst.Growth275,528͑2005͒.8
T.Förster,Ann.Phys.2,55͑1948͒;D.L.Dexter,J.Chem.Phys.21,836͑1953͒
.