Table Of ContentTIME-RESOLVEDINFRAREDSPECTROSCOPYATTHENSLSU12IR
BEAMLINE
BY
JOSEPHDONALDLAVEIGNE
ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOL
OFTHEUNIVERSITYOFFLORIDAINPARTIALFULFILLMENT
OFTHEREQUIREMENTSFORTHEDEGREEOF
DOCTOROFPHILOSOPHY
UNIVERSITYOFFLORIDA
1999
ACKNOWLEDGMENTS
I would liketooffersincere thanks my adviser, Professor David B. Tanner,
forhisencouragement and adviceduringmygraduatecareer. Hissupport and
guidance have been invaluableto me and I willcontinue to look to him as an
exampleofanidealmentorandcolleague.
IwouldalsoliketothankProfessorDavidReitze,amemberofmycommittee,
forhisadviceandassistancewiththetime-resolvedproject. IalsothankProfessors
S.Hershfield,J.GraybealandR.Singhforservingonmycommitteeandreading
thisdissertation.
IwouldliketothankmycollaboratorsatBrookhavenNationalLab, L. Garr
andR.Lobo,fortheirassistanceandconversations. Iwouldliketothankallmy
pastandpresentcolleaguesinTanner’sgroupandthosewhoworkedinhislabfor
theirconversations, friendshipandcooperation: H.L. Liu,A. Zibold,A. Ugawa,
L.Gasparov,G.Porter,D.John,N.L.Tache,V.Boychev,S.Hong,J.Hwang,S.
Moore,andR.Kumar.
I would like to express my gratitude for technical support to the members
ofthe machineshop, electronicsshop and cryogenicsteamofthe Universityof
Floridaphysicsdepartment,aswellasthemembersoftheexperimentalshopat
theNSLS.Iwouldalsoliketoacknowledgetheinvaluableassistanceofthephysics
departmentstaff,especiallyC.Bright,inhandlingthebureaucraticdetailsatUF
whileIwasinBrookhaven.
Iwouldliketothankmyparents,DonandMidgeLaVeigne,fortheirsupport,
bothemotionalandfinancialduringmyentireeducationalcareer. Finally,Igive
11
special thanks to my wife, Yao-yi Tseng. For her constant love, support and
patience,Iamverygrateful.
TABLEOFCONTENTS
ACKNOWLEDGMENTS
ii
ABSTRACT vii
CHAPTERS
1 INTRODUCTION 1
1.1 History 1
1.2 Motivation 1
1.3 Organization 2
2 EXPERIMENTALMETHODS 5
2.1 Introduction 5
2.2 Spectroscopy 5
2.2.1 Monochromators 6
2.2.2 Interferometers 8
2.3 TheoryofFTS 9
2.3.1 DisadvantagesofFilmBeamSplitters 15
2.3.2 WavefrontSeparation 17
2.3.3 Aliasing 18
2.3.4 PhaseCorrectionandApodization 20
2.3.5 FastScanInterferometry 22
2.3.6 Time-ResolvedMeasurementsUsingInterferometers 23
2.4 SynchrotronRadiation 25
2.4.1 Introduction 25
2.4.2 SynchrotronRadiationTheory 25
2.4.3 InfraredSynchrotronRadiation 30
2.4.4 ComparisonofSynchrotronandThermalSources 32
2.4.5 PulsedNatureoftheSource 37
2.4.6 CoherentEmission 38
2.5 OpticalPropertiesofMaterials 39
2.5.1 DielectricResponseFunctione(uj) 39
2.5.2 DeterminationofFilmThicknessFromInterferenceFringes. 42
2.5.3 ModelingAbsorption 43
2.5.4 SumRule 44
IV
3 EXPERIMENT 45
3.1 Introduction 45
3.2 NSLSVUVRing 46
3.3 GeneralBeamlineDescription 49
3.4 PumpLaserSystem 52
3.4.1 Mode-lockedTi:sapphireLaser 57
3.4.2 PhotoindiicedMeasurements 60
3.5 LamellarGratingInterferometer 62
3.5.1 Description 62
5 3.5.2 LaserInsertion 67
3.5.3 MeasurementMethods 68
3.5.4 SourceGomparison 70
3.6 Bruker113VFastScanInterferometer 72
3.6.1 Description 72
3.6.2 LaserInsertion 74
3.6.3 MeasurementMethods 75
3.6.4 SourceGomparison 78
4 TIME-RESOLVEDIRSTUDIESOFHg^Gdi_^Te 81
4.1 Introduction 81
4.2 OpticalPropertiesofMGT 82
4.2.1 Transmission 82
4.2.2 Time-resolvedMeasurements 83
4.2.3 EstimatedGarrierDensity 85
4.2.4 Time-resolvedSpectroscopy 89
4.2.5 SumRule 95
4.3 Summary 97
TIME-RESOLVEDIRSTUDIESOFPbFILMS 98
5.1 Introduction 98
5.2 Superconductivity 99
5.2.1 FIRTransmissionofThinSuperconductingFilms 100
5.2.2 Non-equilibriumSuperconductors 100
5.3 Experiment 104
5.3.1 FilmGrowthandGharacterization 104
5.3.2 ZPDDecay 106
5.3.3 TemperatureDependenceoftheQuasiparticleLifetime ... 112
5.3.4 Time-resolvedSpectroscopy 119
5.3.5 Pbo.75Bio.25 122
5.4 Summary 124
V
6 CONCLUSION 126
APPENDICES
A COHERENTEMISSION 130
A.l Introduction 130
A.2 Observations 131
A.3 Summary 135
B USINGABOLOMETERASAMIDIRDETECTOR 137
B.l Introduction 137
B.2 Experiment 138
B.3 Summary 144
C LASEROPERATION 146
C.l Description 146
C.2 Manual 148
C.3 OptimizationoftheDownstreamOptics 150
REFERENCES 152
BIOGRAPHICALSKETCH 157
VI
AbstractofDissertationPresentedtotheGraduateSchool
oftheUniversityofFloridainPartialFulfillmentofthe
RequirementsfortheDegreeofDoctorofPhilosophy
TIME-RESOLVEDINFRAREDSPECTROSCOPYATTHENSLSU12IR
BEAMLINE
By
JosephDonaldLaVeigne
August1999
Chairman: DavidB.Tanner
MajorDepartment: Physics
Synchrotronradiationhasbeenusedasalightsourceformanydecades. First
usedasasourceforultravioletandxrays,inthe1970sand1980spioneeringwork
wasdoneintheuseofsynchrotronradiationasaninfraredsource. Asynchrotron
sourcehasseveraladvantagesoverathermalsource. Althoughithaslowertotal
powerthanathermalsonceovermostoftheIRspectrum, thesynchrotronhas
higherbrightness. Atverylowfrequencies,thesynchrotronproducesmorepower
than a standard black body source. The least used advantage ofsynchrotron
radiationisthepulsednatureofthesource. Theelectronsintheringarecontained
inbunchescausingthesynchrotronradiationtobeemittedinpulseswhichhave
ahighrepetitionrate. Thisadvantageisputtousetoperformtime-resolvedor
pump-probeinfraredspectroscopy.
The U12IR beamline ofthe National Synchrotron Light Source (NSLS) at
Brookhaven National Lab (BNL) is anewlyconstructed beamlinededicated to
time-resolvedandfarinfraredspecroscopy. Itisoperatedbyacollaborationbe-
tweentheNSLS,thephysicsdepartmentofBNLandtheDepartmentofPhysics
attheUniversityofFlorida.
vii
Thisdissertationpresentsadetaileddescriptionofthebeamlineincludingtwo
endstations(spectrometers) andthepumplasersystem. Thesynchrotronpulses
haveadurationof0.3-1.5ns(FWHM),withamaximumtimebetweenpulsesof
~180ns,allowingphenomenawithtimescalesfromhundredsofpstohundredsof
nstobeinvestigated. Datafromtwoseriesexperimentsarepresented,includinga
studyofelectron-holepairrelaxationinHgiCdi_xTehlmsandpairrecombination
insuperconductingleadandleadbismuthhlms.
TheMCThlmsshowahighrelaxationrate,consistentwithalargenumberof
defectsatthesurfaceofthehim. Farinfraredstudiesshowfreecarrierabsorption
atlowfrequencies. MidIRstudiesshow'adecreaseinabsorptionneartheabsorp-
tionedgeofthebandgap,consistentwithstatesbeingremovedbythepromotion
ofcarriersfromthevalencebandtotheconductionband. Adeviationfromthe
sumruleforoscillatorstrengthsisfoundandexplainedbyinsufficientdatainthe
midIRregion.
ThesuperconductingPbandPbBihlmsshowanincreaseinabsorptionintheir
FIRspectraduetopairbreakingbytheincidentpumppulse. Therecombination
timeofthequasiparticlesisresolvedatthelowesttemperatures. Thetemperature
dependenceofthelifetimeisinferredfromsignalchangeswiththepumpandprobe
pulsescoincident.
Thedissertationconcludesw'ithasummaryoftheperformanceofthebeamline
andadiscussionofsomeproposedenhancementstothecurrentexperiment.
viii
CHAPTER
1
INTRODUCTION
1.1 History
The equations describingsynchrotron radiationwere first published in 1949
by Schwinger [1], Since that time, synchrotrons have been used assources for
anumberofopticalexperiments. The first applicationsmade use ofthex-ray
or ultraviolet (UV) portionsofthe spectrum for which other sources were not
available. In the 1970s and early 1980s, pioneeringwork was done in the use
ofsynchrotron radiationasasourceofinfrared (IR) light [2]. Sincethat time,
severalbeamlinesdedicatedtoIRradiationhavebeenconstructedatsitesaround
theworld [3,4, 5,6]. AlthoughthroughoutmostoftheIRspectrumthepower
emittedbymostringsislessthanatypicalglowbarsource, it’shighbrightness
makesitveryusefulforsurfacescienceandmicroscopy[7,8].
1.2 Motivation
The high brightness ofa synchrotron source is not its only benefit. They
canproducemorepowerthanatypicalthermalsourceatverylongwavelengths.
Synchrotronsalsoproducelightinaseriesofpulsesatahighrepetitionrate. It
isthepulsednatureofsynchrotronradiationthatwillbeputtouseinthework
ofthisdissertationto performtime-resolvedspectroscopy. Synchrotron sources
1
2
have been used to perforin time-resolved experiments in the past [9, 10]. The
U12IRbeamlineat the NationalSynchrotron Light Source (NSLS) is afacility
currently undergoing commissioningand is dedicated to far infrared and time-
resolvedinfraredspectroscopy. Thebeamlineprovidesauniqueopportunityfor
measuringtransientphenomenaontimescalesinthe100sofpsto100sofnsrange
overabroadregionoftheelectromagneticspectrum. Thebroadbandnatureofthe
sourceallowstheuseofwelldevelopedinterferometrytechniquesandisthemajor
advantageofusingsynchrotronsoverterahertzspectroscopy[11].
Thefacilityprovidesatoolforstudyingmanynon-equilibriumsystemsinsolid
statephysics. Systemsstudied inthisdissertation includeelectron-holeplasma
relaxationinsemiconductorsandquasiparticlerecombinationinnormalsupercon-
ductors. Manyothersystemsarealsoofinterestsuchasphotogeneratedcarriers
inconductingpolymersaswellasquasiparticlerecombinationandphotoinduced
dopinginhigh-Tpsuperconductors.
1.3 Organization
ThisdissertationdescribesexperimentsusingtwoendstationsontheU121R
beamlineoftheNSLSVacuumLdtraviolet(VUV)ring. Theseexperimentsinvolve
tirne-resolvedspectroscopyandcoverthefrequencyrangeof2-10,000cm“L Much
oftheworkcenteredaroundtheconstructionofvariousdevicesforcouplingthe
synchrotronradiationintothesample,orbringingthelaserpumppulsestothe
samplewithadequatepowderandatthecorrecttimerelativetothesynchrotron
probepulses.
