超快光学 超快光谱

UltrafastLaserSpectroscopyHowandwhyultrafastlaserspectroscopy?GenericultrafastspectroscopyexperimentTheexcite-probeexperimentLock-indetectionTransient-gratingspectroscopyUltrafastpolarizationspectroscopyOpticalheterodynedetection(OHD)Spectrallyresolvedexcite-probespectroscopyUltrafastlaserspectroscopy:Why?Mosteventsthatoccurinatomsandmoleculesoccuronfsandpstimescalesbecausethelengthscalesareverysmall.Fluorescenceoccursonanstimescale,butcompetingnon-radiativeprocessesonlyspeedthingsupbecauserelaxationratesadd:Biologicallyimportantprocessesutilizeexcitationenergyforpurposesotherthanfluorescenceandhencemustbeveryfast.Collisionsinroom-temperatureliquidsoccuronafew-fstimescale,sonearlyallprocessesinliquidsareultrafast.Semiconductorprocessesoftechnologicalinterestarenecessarilyultrafastorwewouldn’tbeinterested.1/tex=1/tfl+1/tnrUltrafastlaserspectroscopy:How?Ultrafastlaserspectroscopyinvolvesstudyingultrafasteventsthattakeplaceinamediumusingultrashortpulsesanddelaysfortimeresolution.Itusuallyinvolvesexcitingthemediumwithone(ormore)ultrashortlaserpulse(s)andprobingitavariabledelaylaterwithanother.Thesignalpulseenergy(orchangeinenergy)isplottedvs.delay.Theexperimentaltemporalresolutionisthepulselength.SignalpulseenergyDelayExcitationpulsesVariablydelayedProbepulseSignalpulseMediumunderstudyWhat’sgoingoninspectroscopymeasurements?Theexcitepulse(s)excite(s)moleculesintoexcitedstates,whichchangesthemedium’sabsorptioncoefficientandrefractiveindex.Theexcitedstatesonlyliveforafinitetime(thisisthequantitywe’dliketofind!),sotheabsorptionandrefractiveindexrecover.UnexcitedmediumExcitedmediumUnexcitedmediumabsorbsheavilyatwavelengthscorrespondingtotransitionsfromgroundstate.Excitedmediumabsorbsweaklyatwavelengthscorrespondingtotransitionsfromgroundstate.ThesimplestultrafastspectroscopymethodistheExcite-ProbeTechnique.Excitethesamplewithonepulse;probeitwithanotheravariabledelaylater;andmeasure

thechangeinthetransmittedprobe

pulseenergyoraverage

powervs.delay.Theexciteandprobepulsescanbedifferentcolors.ThistechniqueisalsocalledthePump-Probe

Technique.Theexcitepulsechangesthesampleabsorptionseenbytheprobepulse.ChangeinprobepulseenergyDelay,t0Epr(t–t)ExcitepulseVariabledelay,tEex(t)SamplemediumDetectorEsig(t,t)ProbepulseModelingexcite-probemeasurementsLettheunexcitedmediumhaveanabsorptioncoefficient,a0.Immediatelyafterexcitation,theabsorptiondecreasesbyDa0.Excitedstatesusuallydecayexponentially:

Da(t)=Da0

exp(–t

/tex)

fort>0wheretisthedelayafterexcitation,andtexistheexcited-statelifetime.Sothetransmittedprobe-beamintensity—andhencepulseenergyandaveragepower—willdependonthedelay,t,andthelifetime,tex:Itransmitted(t)=Iincident

exp{–[a0–

Da0exp(–t

/tex)]L}where

L

=samplelength

=Iincident

exp{–a0L}exp{Da0exp(–t/tex)L}

[Iincident

exp{–a0L}]{1+Da0exp(–t

/tex)L}assumingDa0L<<1

Itransmitted(-){1+Da0exp(–t

/tex)L}DT(t)

/T0

=[Itransmitted(t)

-

Itransmitted()]/Itransmitted()Modelingexcite-probemeasurements(cont’d)Therelativechangeintransmittedintensityvs.delay,t,is:Changeinprobe-beamintensityDelay,

t0Itransmitted(t)

Itransmitted(-){1+Da0exp(–t

/tex)L}DT(t)

/T0

Da0exp(–t

/tex)LModelingexcite-probemeasurements(cont’d)Morecomplexdecaysoccurifintermediatestatesarepopulatedorifthemotioniscomplex.Imagineprobinganintermediatetransition,whosestatestemporarilyfillwithmoleculesontheirwaybackdowntothegroundstate:ExcitetransitionProbetransition0123Excitedmoleculesinstate1:absorptionofprobeChangeinprobe-beamtransmittedintensityorpowerDelay,t00Excitedmoleculesinstate2:stimulatedemissionofprobeLock-inDetectiongreatlyincreasesthesensitivityinexcite-probeexperiments.Thisinvolveschoppingtheexcitepulseatagivenfrequencyanddetectingatthatfrequencywithalock-indetector:ChoppedexcitepulsetrainProbepulsetrainLock-indetectionautomaticallysubtractsoffthetransmittedpowerintheabsenceoftheexcitepulse.Withhigh-rep-ratelasers,itincreasessensitivitybyseveralordersofmagnitude!Theexcitepulseperiodicallychangesthesampleabsorptionseenbytheprobepulse.ChopperLock-indetectorThelock-indetectsonlyonefrequencycomponentofthedetectorvoltage—chosentobethatofthechopper.Excite-probestudiesofbacterio-rhodopsinRhodopsinisthemainmoleculeinvolvedinvision.Afterabsorbingaphoton,rhodopsinundergoesamany-stepprocess,whosefirstthreestepsoccuronfsorpstimescalesandarepoorlyunderstood.Excitationpopulatesanewstate,whichabsorbsat460nmandemitsat860nm.Itisthoughtthatthisstateinvolvesmotionofthecarbonatoms(12,13,14).Anartificialversionofrhodopsin,withthoseatomsheldinplace,revealsthischangeonamuchslowertimescale,confirmingthistheory!NativeArtificialProbeat460nm(increasedabsorption):Probeat860nm(stimulatedemission):Zhong,etal.,UltrafastPhenomenaX,p.355(1996).Excite-probemeasurementsinDNAPecourt,etal.,UltrafastPhenomenaXII,p.566(2023).DNAbasesundergophoto-oxidativedamage,whichcanyieldmutations.Understandingthephoto-physicsoftheseimportantmoleculesmayhelptounderstandthisprocess.Excite-probemeasurementsofHypericin,ananti-viralsubstanceWhenexcitedbylight,HypericindeactivatesHIV.Soitwouldbenicetounderstandhowitworks.RelativechangeinabsorbanceThesecurves(fortwodifferentsolvents)showtherisetimeforaproton-transferprocessimportantinitsbiologicalactivity.M.J.Fehr,etal.,UltrafastPhenomenaIX,pg.462(1994).Excite-probemeasurementsofTerawattfemtosecondUVpulsesinwaterPommeret,etal.,UltrafastPhenomenaXII,p.536(2023).High-intensityUVultrashortpulsesmaysomedaybeusedinsurgery.Sounderstandingwhatthesepulsesdotowaterisimportant.

Hydratedelectronsareformedinveryhighconcentrations(0.01molar).Theinducedabsorptionseenhereisveryhigh.lex=266nmlpr=800nm(Perchloricacid)Excite-probereflectionspectroscopyExcitingasurfaceandprobingitsreflectivitylaterrevealssurfacephysics.Here,aquantumwireisstudiedusingultrashortpulsesina

near-fieldscanningopticalmicroscopetoyield200nmspatialresolution,too!Emiliani,etal.,UltrafastPhenomenaXII,p.256(2023).DR/Rvs.xandyforadelayof10psExcite-probemeasurementscanrevealquantumbeats:TheoryExcitation-pulsespectrum012ExcitepulseProbepulseSinceultrashortpulseshavebroadbandwidths,theycanexcitetwoormorenearbystatessimultaneously.Probingthe1-2superpositionofstatescanyieldquantumbeatsintheexcite-probedata.Excite-probemeasurementscanrevealquantumbeats:ExperimentHere,twonearbyvibrationalstatesinmoleculariodineinterfere.Thesebeatsalsoindicatethemotionofthemolecularwavepacketonitspotentialsurface.AsmallfractionoftheI2moleculesdissociateeveryperiod.Zadoyan,etal.,UltrafastPhenomenaX,p.194(1996).Quantumbeatsinpolymersusing5-fspulsesExcite-probemeasurementsinpolydiacetyleneshowseveraldifferentfrequencies,implyingseveral(vibrational)stateswereexcited.Kobayashi,UltrafastPhenomenaXII,p.575(2023).lprThecoherencespikeinultrafastspectroscopyWhenthedelayiszero,

othernonlinear-opticalprocessesoccur,ainvolvingcoherent4WMbetweenthebeams

andgenerating

additionalsignalnotdescribedbythesimpleDamodel.Asinautocorrelation,it’scalledthecoherencespikeorcoherentartifact.Sometimesyouseeit;

sometimesyoudon’t.Alternatepicture:thepulsesinducea

gratingintheabsorptionand/orrefractiveindex,

whichdiffractslightfromeachbeamintotheother.IntensityfringesinsamplewhenpulsesarrivesimultaneouslyProbepulseSampleExcitepulseThisspikecouldbeaveryveryfasteventthatcouldn’tberesolved.Oritcouldbeacoherencespike.Takingadvantageoftheinducedgrating:theTransient-GratingTechnique.Twosimultaneousexcitationpulsesinduceaweakdiffractiongrating,followed,avariabledelaylater,byaprobepulse.Measurethediffractedpulseenergyvs.delay:Thismethodis

background-free,butthediffractedpulseenergygoesasthesquareofthediffractedfieldandhenceisweakerthanthatinexcite-probemeasurements.Diffracted

pulseenergyDelay,t0DelayExcitepulse#1SampleExcitepulse#2ProbepulseDiffractedpulseIntensityfringesinsampleduetoexcitationpulsesAtransient-gratingmeasurementmaystillhaveacoherencespike!Whenallthepulsesoverlapintime,who’stosaywhicharetheexcitationpulsesandwhichistheprobepulse?Atransient-gratingexperimentwithacoherencespike:Diffracted

beamenergyDelay,t0DelayExcitepulse#1(actingastheprobe)Excitepulse#2Probepulse(actingasanexcitepulse)IntensityfringesinsampleduetoanexcitationpulseandtheprobeactingasanexcitationpulseWhatthetransient-gratingtechniquemeasuresItmeasuresthePythagoreansumofthechangesintheabsorptionandrefractiveindex.Thediffractionefficiency,,isgivenby:Thisisincontrasttotheexcite-probetechnique,whichisonlysensitivetothechangeinabsorptionanddependsonitlinearly.DiffractedbeamintensityDelay,t0TransmittedintensityAbsorption(amplitude)gratingRefractiveindex(phase)gratingH.Eichler,Laser-InducedDynamicGratings,Springer-Verlag,1986.Iftheabsorptiongratingdominatesandtheexcite-probedecayisexp(-/ex),thentheTGdecaywillbeexp(-2/ex):Youmightthinkthatagratingcanbeinducedonlybyasinusoidalintensitypattern(causedbytheinterferenceoftwoparallel-polarizedbeams).Butorthogonallypolarizedbeams,whichhaveaconstantintensityvs.position,alsoinduceagrating!Anorientationgrating.Variationoftheelectricfieldvs.position:TransientorientationgratingsOrientationgratingscanalsodecayduetoorientationalrelaxation.Inducedgratingscanalsodecaybydiffusion.Diffusioncanwashoutaninducedgrating.Sometimesdiffusionisfasterthanexcited-statedecay.Diffusionoccursonatimescalethatdependsonthegratingfringespacing.Ifthefringesarecloselyspaced,diffusionisveryfast;ifthefringesarefarapart,thenit’smuchslower.Varyingthegratingfringespacingcandeterminethetimescalesforbothdecaymechanisms.Position(x)Excited-statedensityt=0t>>0LwhereD=diffusioncoeffTransient-gratingmeasurementsinmultiplequantumwellsBothconcentration(amplitude)andorientation(spin)gratingsinducedbyexcitebeamswithparallelandperpendicularpolarizations.Theorientationgratingdecaysmuchfaster.AmplitudegratingOrientationgratingOrientationgratingTime-resolvedfluorescenceisalsouseful.FluorescentbeampowerDelayExcitingasamplewithanultrashortpulseandthenobservingthefluorescencevs.timealsoyieldssampledynamics.Thiscan

bedonebydirectlyobservingthefluorescenceor,

ifit’stoofast,bytime-gatingit

withaprobepulsein

aSFGcrystal.DelaySlowdetectorExcitepulseSampleLensProbepulseSFGcrystalFluorescenceTime-resolvedfluorescencedecayWhendifferenttissueslookalike(i.e.,havesimilarabsorptionspectra),lookingatthetime-resolvedfluorescencecanhelpdistinguishthem.Here,amalignanttumorcanbedistinguishedfromnormaltissueduetoitslongerdecaytime.NormaltissueMalignanttumorSvanberg,UltrafastPhenomenaIX,p.34(1994).UltrafastPolarizationSpectroscopyIt’salsopossibletochangethe

absorptioncoefficientdifferentlyforthetwo

polarizations.Thisiscalledinduceddichroism.Italsorotatesthe

probepolarizationandcanalsobeusedtostudyorientationalrelaxation.Delay45°polarizedexcitepulseSampleProbepulseHWP0°polarizer90°polarizerA45º-polarizedexcitepulseinducesbirefringenceinanordinarilyisotropicsampleviatheKerreffect.Avariablydelayedprobepulsebetweencrossedpolarizerswatchesthebirefringencedecay,revealingthesampleorientational

relaxation.NicefeaturesofultrafastpolarizationspectroscopyIt’seveneasiertosetupthanexcite-probe(justcrosstwobeamsinspaceandtime,andcrossthepolarizerstomoreeasilyseethesignalbeam).It’salmostbackground-free(crossedpolarizerstransmitaslittleas10-6oftheincidentlight).Unlikeexcite-probe,itmeasuresbothabsorptionandphaseeffects.Itcanuselock-indetection.Andsimultaneously,itcanuseopticalheterodynedetection,whichoptimizesthesignal-to-noiseratio.HeterodynedUltrafastPolarizationSpectroscopyThistrivial(seeminglyinappropriate!)changecanactuallyimprovethesensitivitybymanyordersofmagnitude!OpticalHeterodyneDetection(OHD)polarizationspectroscopyinvolvesslightlyuncrossingthepolarizers.Thisallows

someoftheprobepulsetoleakintothedetector

andcombinecoherentlywith

thesignalpulse.1°polarizerSetupisexactlythesame,exceptforthispolarizer!Delay45°polarizedexcitepulseSampleProbepulseHWP90°polarizerHeterodynedultrafastpolarizationspectroscopyAslongastheleakedprobeintensity>>thesignalintensity,wecanneglectthelatter:Heterodynedpolarizationspectroscopyaddsasmallamountoftheprobepulse,dEprobe(t),tothe(evensmaller)signalpulse.Asaresult,wenowdetectthesquaredmagnitudeofthesumofthesetwofields:Aslongastheprobeintensityisstable,thisyieldsahugeimprovementinsensitivity—especiallywhencombinedwithlock-indetection.ThisyieldsasignaltermproportionaltoEsig(t),whichismuchlargerthanitssquaredmagnitude.Anditalsoyieldsitsphase.usualPSsignalHeterodynedultrafastpolarizationspectroscopyofliquidsCastnerandChang,UltrafastPhenomenaX,p.296(1996).Usingdifferentcolorsfortheexciteandprobepulses,thistechniqueiscalledtheOpticalHeterodyneDetection-Raman-InducedKerrEffectSpectroscopy(OHD-RIKES).Samplemediaarevariousamides.Noticehowverycleanthedataare.Theexcitepulsecaninduceachangeintherefractiveindexseenbytheprobepulse,whichisenhancedwhenwex–wpr

=w10ExcitepulsewexProbepulsewpr012w10HeterodynedultrafastpolarizationspectroscopyofCS2Loughnane,etal.,UltrafastPhenomenaX,p.304(1996).OHD-RIKESstudyofCS2atdifferenttemperatures.TemporallyandspectrallyresolvingthefluorescenceofanexcitedmoleculeExcitingamoleculeandwatchingitsfluorescencerevealsmuchaboutitspotentialsurfaces.Ideally,onewouldmeasurethetime-resolvedspectrum,eq