Quantum Electronics 43(1) 113 (2013) 2013 KvantovayaElektronikaandTurpionLtd

Abstract. Basic methods of laser photothermal spectroscopy, which are used to study photoinduced absorption in various media, are briefly considered. Comparative analysis of these methods is per-formed and the latest results obtained in this field are discussed. Different schemes and examples of their practical implementation are considered.

Keywords:photothermal spectroscopy, photothermal methods (tech-niques), photoinduced absorption, laser-induced absorption, review.

1. Introduction

Photoinduced(induced)absorptionincreaseswithanincreaseinthelaserintensity.Thisabsorptioncanbecausedbyirradi-atingamaterialwithanintenselightsource(pumpradiation)andbeobservedbysensingthesamplewithproberadiationatanotherwavelength.Thespectraldependenceofphotoin-ducedabsorptionyieldsalargeamountofdataonthestruc-

tural defects of thematerial studied, which determine to agreatextentitscharacteristics[1].

The investigation of photoinduced absorption spectraappearstobeveryimportanttogaininformationaboutdefectcentresinsemiconductormaterials,forexample,inthinfilmsofamorphousandpolycrystallinesilicon,siliconcarbide,etc.[24]. In recentyears,muchattentionhasbeenpaid to thelight-inducedabsorptionanditsrelationshipwiththephoto-electric properties of films ofwide-gapmetal oxides (TiO2,ZnO,etc.),whichareusedinsolarbatteriesandinphotoca-talysis [59]. Numerous studies of induced absorption inphotorefractive materials have been performed; they werestimulated by wide application of these materials in holo-graphicdatastoragedevices,nonlinearfilters,phase-match-ing devices, and optical switches [1014]. Photoinducedabsorptionincrystalsusedinnonlinearopticsandsomeothereffectsareactivelystudied[1517].

Thestudyofphotoinducedabsorptionisbasedonmod-erntechniquesformeasuringlowopticalabsorptioninopti-cally transparentmaterials and coatings. The conventionalmethodsformeasuringabsorptioninbothbulkmaterialsandthin filmsare spectrophotometric techniques (whichhavearatherlargeerror)[18]andmoreexactcalorimetricmethods[19].Multipasscellswithmultiplereflection[20,21]andintra-cavitytechniques,bothactiveandpassive[21],withsensitiv-ityontheorderof103104,havebeendevelopedtomeasure

Laser photothermal spectroscopy of light-induced absorption

L.A.Skvortsov

L.A. Skvortsov InstituteofCryptography,Communications,andInformatics,Michurinskiiprosp.70,117602Moscow,Russia;e-mail:lskvortsov@gmail.comReceivedReceived22June2012Kvantovaya Elektronika 43(1)113(2013)TranslatedbyYu.P.Sinkov

REVIEW PACSnumbers:79.10.Ca;78.20.nb;42.62.FiDOI:10.1070/QE2013v043n01ABEH014912

Contents1.Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.Briefreviewoftechniquesbasedonthephotothermaleffect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2.1.Photoacoustictechnique2.2.Photothermaldefelction(mirage-effect)spectroscopy2.3.Thermallenstechnique2.4.Surfacethermallensing2.5.Photothermalradiometry2.6.Photothermalreflectancetechnique2.7.Photothermalinterferometry

3.Measurementoflight-inducedabsorptionbyphotothermaltechniques(caseoflowintensities). . . . . . . . . . . . . . . . . . . . . . . . . . 63.1.Necessityofmodifiedphotothermaltechniquestomeasurelight-inducedabsorption3.2.Modifiedphotoacoustictechnique3.3.Modifiedphotothermalradiometry3.4.Modifieddeflectiontechnique3.5.Modifiedthermallenstechnique3.6.Measurementoflight-inducedabsorptionbythephotothermalreflectancetechnique

4.Measurementoflight-inducedabsorptionbyphotothermaltechniques(caseofhighintensities) . . . . . . . . . . . . . . . . . . . . . . . . . 94.1.Inducedabsorptionundermultiphotoneffect4.2.Measurementofinducedabsorptionundertwo-photonexcitationusingthethermallenstechnique4.3.Measurementofinducedabsorptionundertwo-photonexcitationusingthedeflectiontechnique

5.Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106.References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

L.A.Skvortsov2

low absorption. However, this sensitivity is insufficient todetectphotoinducedabsorptioninthinfilmsor(inthecaseoflowexposures)bulksamples.

Laserphotothermalspectroscopyisanalternativetocon-ventionalabsorptionspectroscopytechniques.Photothermalmethodshavetheactivecharacter;theyarenondestructiveand, inmost cases, contactless. A characteristic sign of allphotothermaltechniquesisthattheyyieldinformationaboutthe properties and composition of a medium under studybasedondirectdetectionofthepowerabsorbedinitfromthecorresponding changes in the physical and thermodynamicparametersofthematerial[2226].Adirectconsequenceofthisisthefollowingsetoffeatures:(1)zero-referencedmea-surements(theoutputsignaliszerointheabsenceofabsorp-tion), (2) an increase in sensitivity with an increase in theradiationpower(uptotheabsorptionsaturationmode),and(3)fundamentallimitationofthelimitingsensitivitybyther-malfluctuationsinthemediumunderstudy.

Photothermal techniques can be implemented in twoways.Inthefirstversion,asampleisexposedtocwlaserradi-ationperiodicallymodulatedinamplitude(theso-calledpho-tothermalmodulation spectroscopy).The second version isthe pulsed photothermal spectroscopy, where periodicallyrepeated laserpulsesareused. Inbothcases,absorptionoflaserradiationbytheobjectunderstudyleadstoexcitationofthermalwavesinit,asaresultofwhichthetemperatureandrecordedsignalbecomemodulated.Havinganalysedthetimedependenceoftherecordedsignalanditsphase,onecangaininformationaboutvariousphysicalpropertiesoftheobject,including the magnitude and dynamics of photoinducedabsorption.

Aswasmentionedabove,photothermalspectroscopyisamodulationmeasurement technique,which impliesperiodicperturbationofsomeparameterofthematerialunderstudybymodulated(pulsed)laserradiation.Thebasicadvantagesof photothermal spectroscopy, as compared with conven-tionalspectroscopy,areduetothepossibilityofeliminatingthebackground(structureless)componentofsignalandmea-suring differential spectra. The reason is that the signalrecordedisdirectlyrelatednottotheparametertobemea-suredbuttoitschange,i.e.,tothederivativeofthisparameterwithrespecttotemperature.Therefore,photothermalspectrahavefinefeatures,analysisofwhichyieldsdataontheprop-ertiesofmaterialthatcannotalwaysberevealedbyconven-tionalspectroscopy.Inaddition,lock-indetectionofasignalatamodulationfrequencyimprovessignificantlythesignal-to-noiseratio,thusreducingthemeasurementerror.

Dependingonthewayofdetectingmodulatedsignal,themost widespread methods are photoacoustic spectroscopytechniques, photothermal deflection spectroscopy (mirage-effect), thermal lens technique (thermal lens spectroscopy),photothermal radiometryandphotothermal interferometry,and thermoreflectance technique. All these methods weredescribedinanumberofreviewsandmonographs[2232].

Inthispaperwewillnotgointodetailsofnumerousorig-inalstudies,as,forexample, in [2426],whereanextensivebibliography on different photothermal spectroscopy tech-niquescanbefound.Wewillrestrictourselvestobriefconsid-erationofthebasic(inouropinion)laserphotothermaltech-niques,whicharemostwidespreadandpromisingforpracti-calanalysisofphotoinducedabsorptioninvariousmedia.Wewillrefertotheoriginalstudiesthatwebelievetobeoffunda-mentalimportancefortheproblemunderconsiderationandtoanumberofnewstudiesthathavenotbeenconsideredin

theaforementionedreviews.Emphasiswillbeonthespecificfeaturesofthetechniquespresentedandonthediscussionofthe latest results obtained generally with the use of newdesigns.

Becauseofthelimitedvolumeofthepaper,theresultsaredescribed only briefly.Details can be found in the originalstudies, reviews, andmonographswe refer to; they containextensivedataonseparateaspectsoftheproblemundercon-sideration.

2. Brief review of techniques based on the photothermal effect

2.1. Photoacoustic technique

The essence of the photoacoustic technique is as follows[33,34].Asampleunderstudyisplacedinaspecialcellandexposedtomodulatedlaserradiation,whichiscompletelyorpartiallyabsorbedinthesampletoheatit.Theheatreleasedistransferredtothegasmediumlocatedintheacousticcell,asaresultofwhichthepressureinthecellperiodicallychanges.Thethusexcitedacousticwavesarerecordedbyahighlysen-sitivemicrophone,built-inintothecell(Fig.1).Onecanalsoperform contact measurement of the photoacoustic signalwhenstudying,forexample,solidswiththeaidofapiezoelec-tric sensor, positioned directly on the sample surface.Currently open photoacoustic systems with a sensitive ele-mentintheformofaquartztuningforkhavebeendeveloped;theyareconsideredtobepromisingdevicesforremotedetec-tionofchemicalcompoundsonsurfacesofsolids.

High sensitivityof photoacoustic spectroscopymakes itpossible to advance significantly in the study of weaklyabsorbingmedia[35,36].Highlysensitivephotoacousticmul-ticomponentgasanalysershavebeendevelopedtosolveenvi-ronmental protectionproblems [3740].Thephotoacousticeffect was used as a basis for measuring thermodynamicparametersofgaseous,liquid,andcondensedmedia[4144].Photoacousticmicroscopyandflawdetectionarewidelyusedinmodernmaterialsscience,biology,andmedicine[4548].Recentlymuchattentionhasbeenpaidtothephotoacousticspectroscopyasatoolofremotedetectionoftracesofchemi-cal compound [4951] etc.Many reviews andmonographsweredevoted to theapplicationofphotoacoustic techniqueforstudyingmatterindifferentaggregatestates[22,25,30].

2.2. Photothermal defelction (mirage-effect) spectroscopy

In this technique modulated excitation radiation heats thesurface of a sample studied. Heat transfer causes periodic

Laser

Microphone

Gas

DP

Modulator

Photoacoustic cell

Figure 1. Schematicofthephotoacoustictechnique[34].

3Laserphotothermalspectroscopyoflight-inducedabsorption

heating of the surface layer of the medium contacting thesample. The formation of a temperature gradient in themediumnearthesurfaceandtherelatedchangeinitsrefractiveindexarerevealedbythedeviationoftheprobebeamfromtheplane parallel to the sample surface (the mirage effect) andrecordedbyaposition-sensitivephotodetector(Fig.2).

Toincreasethesensitivityofthistechnique,thesampleisgenerallyplacedinacellcontainingatransparentliquid,therefractive indexofwhichdepends stronglyon temperature.Usingthisscheme,onecanmeasuretemperatureincrementsonthesamplesurfaceatalevelof104K,whichcorrespondtoaprobebeamdeviationangleof~109rad[52].Thissensi-tivitymakesitpossibletodetectabsorptionlossesinsolidsatalevelof~107cm1[53].Thistechniqueiswidelyappliedtomeasure optical (absorption coefficient [54, 55]), thermal(thermalconductivity[5658],temperature[59]),andelectri-cal [60]propertiesofmaterials.Extensivedataonthismea-surementtechniquecanbefoundinanumberofreviewsandmonographs[25,26].

2.3. Thermal lens technique

Ina simplified form, theessenceof this technique isas fol-lows. Irradiation of an absorbingmediumby a laser beamwithaGaussianintensitydistributioninthetransversecrosssection inducesa temperaturegradient in itdue tononuni-formheating.Inturn,achange intemperaturecauses localchangesintherefractiveindexofthemedium,incorrespon-dencewiththelaserbeamintensitydistribution.Theoccur-renceoftherefractiveindexgradientinthemediumleadstotheformationofanopticalelement,whichactsasascatter-ing/collectinglens;thiselementwasreferredtoasathermallens.Sincetherefractiveindexofmostmaterialsinthetrans-parencyrangedecreaseswithanincreaseinthetemperature,the thermal lens is generally scattering; i.e., the transverselaser-beamsizeincreaseswhenthemediumisheated.

Inthemoregeneralcasevariationsintherefractiveindexofthemediumcanbecausedbychangesinboththetempera-tureTanddensityrofthemedium:

n

Tn T n

P TrrD D D= +c cm m .

The thermal lens technique can be implemented usingbothsingle-anddouble-beammeasurementschemes.Inthesingle-beamschemeradiationofonelaser issimultaneouslyusedforexcitation(generationofathermallens)andprob-ing.Beingabsorbed,laserradiationheatsthesample,andthe

changeinitsintensityyieldsinformationaboutabsorptioninthemedium. Themeasurement technique generally impliesrecordingthetemporalshapeofphotothermalsignalfromaphotodetectorwithadiaphragm,locatedinthefar-fieldzone.

Thedouble-beamschemeismoreuniversal.Here,ather-mallensinducedbyexcitation(pump)radiationisrecordedbymeasuring defocusing of an additional probe beam. Itsmain advantage is that it allows one to study the spectraldependence of the absorption of materials; this cannot bedonewithinthesingle-beamscheme.Oneofconfigurationsofthe classicaldouble-beammeasurement scheme is shown inFig.3.

Thedouble-beamschemeisimplementedineitherlongi-tudinal (Fig. 3)or transverse [22, 61] configurations; in thelattercase,theexcitationbeamisfocusedintothesampleper-pendicularlytotheprobebeam.Inmostcasesthelongitudi-nal (collinear) version is used. In this case, applying, forexample,adichroicmirror,onemakesbothbeamspropagatecoaxiallyinthesample,duetowhichthemaximuminterac-tion length is provided. Here, the so-called mode-matchedand mode-mismatched configurations are distinguished. Intheformercasethewaistsofbothbeamscoincideinthemea-surementcell(sample),whereasinthelatterversiontheyarespatiallyseparated(Fig.4).

Themeasurementschemebasedonthemode-mismatchedconfiguration provides higher sensitivity. In this case, theabsorptioncoefficientofthesampleatthepumpwavelength,whichisrelatedtothethermallyinducedphaseshift,isdeter-minedfromtheexperimentallyobserved(recordedbyapho-todetector) time dependence of the intensity of the centralpart of the probe beam transmitted through a round dia-phragmbyfittingittothetheoreticaldependenceorbycali-

Laser

Modulator

Detector

Detector

Sample

He Ne laser

Position-sensitive detector

Figure 2. Schematicofthedeflectiontechnique[52].

Pump laser

Probelaser

Filter Photodetector

Sample DiaphragmDichroic mirror

Mechanicalchopper

Figure 3. Thermal lenstechnique(longitudinalversion):double-beammeasurementscheme;thepumpandprobebeamspropagatecoaxially[22].

Pump radiation

Probebeam

Sample

l/2 l/2

L

z2v0

r

0

2r0

2d

Figure 4. Mode-mismatchedconfiguration(measurementschemewithshiftedwaists)[61].

L.A.Skvortsov4

brating with the aid of a standard (reference) sample withknownabsorption.

Thepumplasercanoperateinbothpulsedandcwmodes,whiletheproberadiationintensityisgenerallychosentobeconstant; thus, its change on the diaphragm can only becausedbyinducedmodulationfromthepumplaser.Inprac-tice,thethresholdsensitivityofthistechniqueismainlylim-ited by laser power fluctuations.At a fluctuation level notmorethan0.5%1%,themeasuredabsorptionthresholdis106107cm1.Thiscorrespondstothedetectionthresholdof~108 for refractive index variations and a temperaturevariationthresholdintherangeof106107K[22].Lateritwasshownthatonecanmeasuretheabsorptioncoefficientatalevelof~108cm1usingZscanning(scanninginthelongi-tudinaldirection),whichisakindofthermallenstechnique[62].

Recently photothermal common-path interferometry(PCI)hasbecomeverypopular inmeasuringweakabsorp-tion in transparent media; this technique is based on thethermo-opticaleffectandisinessenceamodifiedthermallenstechnique[16,6367].ToprovidehighspatialresolutionofthePCItechnique,theprobeandpumpbeamsareintersectedinthesampleatasmallangle.Thisconfigurationalsomakesitpossibletoeliminatetheeffectofspurioussignalsfromthesurfacesofopticalelementsandthesample.Theradicaldif-ferenceofthismethodfromtheclassicalfar-fieldthermallenstechniqueisintheuseofnear-fieldsignaldetectionscheme.Inthiscase,perturbedandunperturbedpartsofthesamecol-limatedprobebeam interfere.Specifically thiscircumstancemakesthePCIsensitivitybeclosetothatofinterferometrictechniquesformeasuringabsorption.Inotherwords,PCIisathermallenstechniqueinwhichtheprobesignalisdetectedinthe near-field zone. A schematic of PCI measurements isshowninFig.5.

Todate,thethermallenstechniqueanditsmodifications(mode-mismatched configuration, PCI, Z scanning) arewidelyusedtomeasureweakabsorptioninsolidsandliquids[6874],toanalysetraceamountsofmaterials[75]andkinet-icsofchemicalprocesses[7678],innonlinearspectroscopy[7981], etc. Details of the thermal lens technique and itsmodificationscanbefoundin[8285].

2.4. Surface thermal lensing

Thistechniqueisanalternativetothedeflectionphotother-malspectroscopy[86,87].Beinghighlysensitive,ithasaddi-

tionallyatleasttwoimportantadvantagesovertheconven-tional deflection technique; they are related to the ratio ofprobe-beamandpump-beamdiameters(Fig.6).Ifthesizeoftheregionirradiatedwithaprobelaserbeamexceedsthesizeoftheregionheatedbypumpradiation,therearenostringentrequirementsontuning(aligning)themeasurementscheme.Moreover,whendetectingadiffractionpatternwithaCCDcamera, one can obtain much more complete informationaboutthecharacterofsurfacedeformation.Thisisanimpor-tant advantage over the conventional deflection technique,wheretheprobebeamisfocusedintoasmallsurfaceregionsubjected todeformation.Aconsequence isamuchshortertimenecessaryformappingasurfaceunderstudy,forexam-ple,whenmeasuringabsorptioninthin-filmcoatings,wherethesurfacethermallensingiswidelyused[87,88].

2.5. Photothermal radiometry

Foralongtimethetemperatureofaheatedbodywasdeter-minedbymeasuringtheradiationfluxfromthisbody.Thistechnique has certain advantages over others (for example,thosebasedontheuseofathermocouple),becauseitdoesnotrequire direct physical contact with the object studied.However,ithassomedrawbacks;themainonesaretheinflu-enceofreflectedradiationfluxesfromotherbodiesandthatonemusthavedataontheemissivepowerofthebodyanal-ysed.

Photothermal radiometry (PTR) is an active method,whicheliminatestheinfluenceofbackgroundradiationfromforeignobjectssurroundingthebodystudied.Theessenceofthistechniqueisrathersimple(Fig.7).Duringmeasurementsa sample is exposed to cw laser radiationwith an intensitymodulatedaccording toaharmonic lawor is irradiatedbyperiodiclaserpulses.Partialabsorptionoflaserradiationandthecorrespondingheatreleaseleadtomodulationofthesam-ple surface temperature and the recorded heat flux at thepulse repetition (modulation) frequency. Having measuredthetimedependenceofthedynamiccomponentofrecordedthermalsignalanditsphase,onecanobtaindataondifferentphysicalpropertiesoftheobjectunderstudy.

A modification of photothermal radiometry is the so-calledphoto-carrierradiometry(PCR),whichisusedtomea-

Pumplaser

Probelaser

Chopper Sample

Absorber

Diaphragm

Photodetector

Figure 5. SchematicformeasuringabsorptionlossinopticalmaterialsbythePCItechnique[16].

Ar laser

HeNe laser Detector

Mirror Mirror

Power meter

Power meter

Modulator

Sample

Beamsplitter

Diaphragm

Sample

Probe beam

Pumpbeam

a

b

Figure 6. (a)Schematicdiagramofmeasurementsbythesurfacether-mallensingand(b)aschematicoftheexperimentalsetup[87].

5Laserphotothermalspectroscopyoflight-inducedabsorption

sureelectricalcharacteristicsofsemiconductors,forexample,carrierlifetime,concentration,andmobility[89,90].Thereisnoestablishedtermthatwouldadequatelyrevealtheessenceof thisphenomenon in theRussian scientific literature.WewillrefertoitastheradiometryofnonequilibriumIRradia-tion fromphotoexcited carriers; inouropinion, thisdefini-tion reflects most exactly the essence of this technique, inwhichasampleisexposedtoradiationwithaphotonenergyexceedingthebandgapofthesamplematerial.Here,asignalrecordedcontainsbothathermalcomponentandanonequi-libriumplasmacomponent,whichisduetotherelaxationofphotoexcitedchargesintheconductionband.Obviously,tomeasurephotoinducedabsorption,itisnecessarytoconsideronlythethermalcomponent.Atlowfrequenciestheplasmacomponentissmall,andthesignalrecordedisproportionaltotheheatingtemperatureand,therefore,totheabsorptioncoefficient. Itwas reported in [91] thatmodification of thePCRtechniqueduetotheuseofthesecondlaserwithphotonenergylowerthanthebandgapimprovesthedetectingabilityforsubsurfacedefects.

ConcerningPTR,itisusedtomeasurelowabsorptionnotonlyinbulkmaterialsbutalsointhinfilmsandsurfacelayers[33,9296].Thismethodisusedforremotemeasurementofthetemperatureofbodiesandtheirthermophysicalparame-ters(thermalconductivityandthermaldiffusivity)[97105],toinvestigateelectronicpropertiesofsemiconductormateri-alsandmonitortheirdefectstructure[103105],forremotespectral analysis in different technological problems whenstudyingsurfacesofmaterialsandcoatings[79,106],andinthermal-wavemicroscopyandthermography[107].

2.6. Photothermal reflectance technique

Photothermalreflectancetechnique(photomodulationreflec-tancespectroscopy),beingoneofthemostwidespreadmeth-odsofmodulationspectroscopy [108], is intensivelyusedtostudytheenergystructureofsemiconductormaterials,includ-ingquantum-size structures [109119].Thismethod isverysensitive to theenergy structure; itallowsone todeterminethelevelenergieswithanerroroffewmillielectronvoltsandrecord changes in reflection [DR(l)/R(l)] or transmission

[DT(l)/T(l)] spectra of this probe radiation that are due tophotomodulation:changeintheopticalstructuralparametersunderadditionalirradiationbylightwithawavelengthlyingintherangeofintrinsicabsorptionofsemiconductor(pumpradiation).Therelativechangeinthereflectioncoefficientisdefinedas

RR

RR R

off

off onD = - ,

whereRoff andRon are the reflection coefficients of proberadiationintheabsenceandinthepresenceofpumpradia-tion, respectively. Application of phase-sensitive detectionmakesitpossibletorecordrelativechangesinthereflectioncoefficientatalevelof106107[25].OneofpossibleschemesofthephotothermalreflectancetechniqueisshowninFig.8.

Along with the aforementioned original papers, moredetailsonthistechniqueandfieldsofitsapplicationcanbefoundin[25,26,118,120].

2.7. Photothermal interferometry

Aswasnotedabove,variationsintherefractiveindexofirra-diatedmediumchangethephaseofaradiationwavepassingthroughit.Themostsensitivemethodofphasechangedetec-tionistheinterferometric(phase)technique,whereamediumunderstudyisplacedinoneofinterferometerarms(Fig.9).AvariationintherefractiveindexbyDnleadstoachangeinthewavephaseinthisarmby

Laser

Sample IR detector

Filter

Thermal radiation

Parabolic mirrors

Chopper

Power meter

Splitter

Figure 7. Schematicoftheexperimentalsetupformeasurementsbyla-serPTR[103].

Halogen lamp

Monochromator Pumplaser

Modulator

Filter

Detector

Sample

DR, R

VAC

VDC

Lock-in amplifier

Figure 8. Oneofpossible schemesofmeasurementsbyphotothermalreflectancetechnique[118].

Mirror

Photodetector

Probe laser

Cell with a gas

Pumpradiation

Figure 9. Michelsoninterferometerwithamediumstudiedinacell(therearcellwallistransparentforthepumpradiationandmirror-reflectivefortheprobelaserradiation)[22].

L.A.Skvortsov6

Dj =(2pL/lp)Dn,

whereL is the lengthof the spatialalignment region in thesample for theexcitation (pump)radiationandprobewaveandlpistheprobewavelength.

Achangeintheprobewavephaseshiftstheinterferencepatterninthedetectionplane;thisshiftisdetectedbymeasur-ingthechangeintheradiationpowerwiththeaidofaphoto-detectorwithadiaphragm.Dependingonthemeasurementconditions, one can determine variations Dn using manyknown interferometric schemeswith insignificantmodifica-tion(forexample,MachZehnder,FabryPerot,Michelson,andJamininterferometers)[121,122].Incomparisonwiththedeflection and thermal lens techniques, the phase methodprovideshighersensitivity[22,61].Forexample,thesensitiv-ity of the interferencemeasurements in gaseswas~10 ppt[123].Nevertheless,thephasemethodisrelativelycomplexinboth design and operation; in addition, it requires reliablevibroacoustic isolation.Apparently, ithasnotbecomeverypopularforthesereasons.

3. Measurement of light-induced absorption by photothermal techniques (case of low intensities)

3.1. Necessity of modified photothermal techniques to measure light-induced absorption

Themechanismofinducedabsorptionisgenerallyrelatedtotheoccupationof trapsby electrons excitedby light to theconductionbandfromdeepdonorlevelsorfromthevalenceband.Withallowanceforthisphenomenon,measurementofinduced absorption calls formodifying photothermal tech-niquesinthefollowingway.

Whenelectronsareexcitedfromthevalencebandtotheconductionband, the sample under studymust be exposedsimultaneouslytoatleasttwolightbeams.Oneofthem,withawavelengthfallingintherangeofintrinsicabsorptionofthematerial(pumpwave),generateselectronholepairsbothinthebulkofthesampleandonitssurface.Acwlaserisgener-allyusedasasourceofthisshort-wavelengthradiation.Thefreeelectronsformedintheconductionbandcanbecapturedby traps before recombination. The second radiation flux,withphotonenergybelowthebandgap(probewave),leadstotrapdepletion,heatreleasethroughrelaxationelectronsintheconductionband,andtheirfurtherrecombination.Thus,havingmeasured(atthelong-wavelengthpulserepetitionfre-quency) the variable (dynamic) component of the signalinducedbyperiodicvariationintemperature(orsomerelatedparameter,forexample,therefractiveindexofmaterial),onecanmeasuretheinducedabsorptionatthiswavelength.Thespectral dependence of photoinduced absorption is studiedusing such radiation sources as, for example, xenon lamps(whose light is transmitted through a monochromator) orwavelength-tunable lasers.The latterhaveahigher spectralbrightnessandthusprovidehighermeasurementsensitivity.Notethatinthecaseunderconsiderationthemodifiedphoto-thermaltechniquesaredesignedforstudyingthinfilmsandsurfacelayersofmaterials.Thisisduetothefactthatpumpradiation is absorbed at a depth la af1, where af 105106cm1 is theabsorptioncoefficientrelatedtofunda-mentalabsorption.

Atthesametime,depletionofdeepdonorlevelsbypumpradiationalsoleadstooccupationofelectronictraps.Inthiscase,thepumpphotonenergymaybemuchlowerthanthebandgap;thissituationoccurs,forexample,whenstudyingthephotoinducedabsorption in thebulkofphotorefractivecrystalsandotheropticallytransparentmaterials.

Thus, in both cases the above-considered conventionalschemes used to measure photoinduced absorption shouldcontainanadditionalpumpchannelinordertoformexcitedcentres,whoseabsorption is tobemeasured.Thechoiceofthepumpwavelengthisdeterminedtoagreatextentbynotonlythenatureofmaterialsbutalsobythespecificconditionsoftheirapplication.

3.2. Modified photoacoustic technique

Asfarasweknow,themodifiedphotoacoustictechniquewasappliedforthefirsttimein[5],wherethespectraldependenceofphotoinducedabsorption inpowder samplesof titaniumdioxide(amaterialpromisingforphotocatalysis)wasinvesti-gated.Thisstudyrevealedastrongphotochromiceffectinthesamples:reversiblechangeinabsorptionunderirradiationbyUVlight.Afterswitchingoffpumping,thephotoacousticsig-nalrelaxedtoits darkvalueforseveralminutes.Basedontheanalysisofthemeasuredspectraldependencesofphotoin-duced absorption in the range 3501000 nm (Fig. 10), thenatureofthedefectsresponsibleforthiseffect(Ti3+ions)wasrevealedandtheirconcentrationwasestimatedin[5].

These investigationsweredeveloped in [6],whereanewmethod fordeterminingquantitatively the concentrationofdefects(Ti3+ions)inpowdertitaniumdioxidewaselaboratedbasedonmodifiedphotoacousticspectroscopy.Itwasstatedin[6]thatthismethodisarealalternativetothephotochemi-cal and electron spin resonance (ESR)methods, which aretraditionallyusedforthesepurposes.

3.3. Modified photothermal radiometry

The fruitfulnessofphotothermal techniques in thestudyofthe photochromic effect in thin films and surface layers ofvariousmaterialswasalsodemonstratedby theexampleofmodifiedphotothermalradiometry[79].Aschematicoftheexperimental setup formeasuring photoinduced absorptionbymodified photothermal radiometry is shown in Fig. 11.Theobjectsofstudyintheaforementionedworksweresingle-

0

0.2

0.4

0.6

0.8

1.0

1

2

300 500 700 900 l/nm

1 only probe beam (UV filter, 530 nm)2 probe beam + UV excitation

Nor

mal

ised

pho

toab

sorp

tion

sig

nal

Figure 10. Spectral dependence of photoinduced absorption in TiO2films[5].

7Laserphotothermalspectroscopyoflight-inducedabsorption

layertitaniumdioxidecoatings~100nmthick,depositedonfused silica substrates by electron-beam evaporation. Thesource of modulated probe 1064-nm radiation was a cwYAG:Nd3+ laser with an average power of 20 W.Simultaneouslythesamplewasexposedtoshort-wavelengthnitrogenlaserradiationwithawavelengthof337nm(pumpwave). The maximum average pump power was 3 mW.Measurementswereperformedbothinairandinvacuum.Aheaterwasusedtovarythesubstratetemperatureintherangefrom290to500K.

Notethefollowingmainfeaturesofphotoinducedabsorp-tion in the near-IR spectral range for the objects studied.First,theoccurrenceofphotoinducedabsorptionisarevers-ibleprocess.AfterswitchingoffUVlight,theabsorptioninthenear-IRrangerelaxestoitsinitialdarkvalue(Fig.12).Notethatthecharacteristicrelaxationtimeinvacuumexceedsthatinairbyafactorof5.

Second,photoinducedabsorptiondependsontheshort-wavelengthradiationintensityandtheenvironmentpressure,reachingsaturationathighpumppowers(Fig.13a).Finally,thetemperaturedependenceofinducedabsorptionisofspe-cialinterest(Fig.13b).Uponsampleheatingto~150Cthe

absorptiondecreasestothedarkvalue,whichisconstantintheentiretemperaturerange.

Apossiblemechanismoftheoccurrenceofphotoinducedabsorption in polycrystalline titanium dioxide films in thenear-IRspectralrange(l=1064nm)wasproposedin[9].ThedecisiveroleoftheenergylevelsassociatedwithTi3+ionswasindicated.Theprocesswasconsideredwithin themonomo-lecularrecombinationapproximation;thismodelwasfoundtobe ingoodagreementwiththeexperimentaldata.Itwasshownin[8]thatmodifiedphotothermalradiometrymakesitpossibletorevealnonstoichiometricdefectswithaconcentra-tionNmin1015cm3inthinlayers.

3.4. Modified deflection technique

Deflectionspectroscopywasalsousedtostudyphotoinducedabsorption [2]. It was modified in the following way.Frequency-tunable radiation was obtained using a 1000-Wxenonlamp,whoselightwasfocusedontheinputslitofthemonochromator.Thewidthofthespectrallineattheoutputwas20nm.Theradiationwasmodulatedinamplitudeusingamechanicalchopperandfocusedonthesurfaceofasampleplaced in a cell filled with liquid carbon tetrachloride. Torecord photoinduced absorption, the measurement scheme[124] was supplementedwith a pump radiation source; thepumpphotonenergyexceededthebandgapofthematerialunderstudy(Fig.14).

Theobjectsstudiedin[2]weresamplesofthinhydroge-nated amorphous silicon (a-Si:H) films deposited on glasssubstrates. Films were obtained by decomposition of puresilaneinahigh-frequencyglowdischarge.Thefilmthicknesswas10mm,thebandgapofthefilmmaterialwas1.75eV,andthevolumehydrogencontentwas510at%.Figure15showsaroom-temperaturedependenceoftherelativevalueofpho-toinduced absorption in undoped a-Si:H samples on theprobe photon energy. Analysis of this dependencemade it

Modulator

Lock-inamplifier

IR la

ser

UV laser

Computer

Cell withan evacuationsystem and heater

SampleIR objective

IR detector

Preamplifier

Ge windows

Figure 11. Schematicof theexperimental setup formeasuringphoto-induced absorption in thin films and surface layers of materials bymodifiedphotothermalradiometry[9].

0 50 100 150 200 250 t/s

A(%)

0.05

0.10

0.15

0.20

0.30

0.25

Figure 12. Time dependence of photoinduced absorption in titaniumdioxide filmsafter switchingoff thepumpradiation (airatmosphere,roomtemperature)[9].

0

0

200 400 600 800

0.2

0.2

0.1

0.3

0.4

0.4

0.6

0.8

1.0

A (%)

A (%)

P/mW cm2

275 325 425375 T/K

a

b

1

2

Figure 13. (a)DependencesoftheinducedabsorptionontheUVradia-tionintensityforasubstrateplacedin(1)airand(2)vacuumand(b)thetemperaturedependenceofphotoinducedabsorption[9].

L.A.Skvortsov8

possible todetermine theenergy levelsof thedefects corre-spondingtotheneutralandnegativelychargedstates.Itwasnoted in [2] that the sensitivity of the proposed techniqueexceedsthatofconventionalabsorptionspectroscopymeth-odsbyatleastfourordersofmagnitude.

3.5. Modified thermal lens technique

Thermallenstechniqueinamodifiedform(specifically,mod-ifiedPCI)ismostwidelyusedtostudythemechanismsandnature of photoinduced absorption in transparent opticalmaterialsandcoatings.Wewillrestrictourselvestothecon-sideration of only few (most characteristic in our opinion)examplesofapplicationof this techniquefor theaforemen-tionedpurposes.

Inparticular,thismethodwasusedtostudythephotoin-duced absorption in magnesium-doped lithium niobate(LiNbO3) crystals, as well as in lithium tantalate (LiTaO3)andpotassiumtitanylphosphate(KTiOPO4,TP)crystals,whicharewidelyusedinlasertechnique[1517].Green-light-inducedinfraredabsorption(GLIIRA)inthesematerialswasinvestigated.Later, the effectofblue-light-induced infraredabsorption (BLIIRA)was revealed inmostof ferroelectrics[14]. A scheme for measuring photoinduced absorption bymodifiedPCIispresentedinFig.16.

Itcanbeseenthatthisversionisbasedontheuseoftwopump beams. In particular, in the case of lithium niobatecrystal, the dynamics of absorption of IR pump radiation(1064nm)wasstudiedinthepresenceofaweakerpumpbeamatthesecond-harmonicwavelength(532nm)ofaNd:YAGlaser.Periodicinterruptionofthegreenlightbeamwasper-

formedmanually.AHeNelaserservedasaprobeone.TheGLIIRAdynamics for lithiumniobate crystals is shown inFig.17.Thesignalrise(falloff)timewaslessthan30ms.

WeshouldnotesuccessfulapplicationofthethermallenstechniqueinthestudyofGLIIRinKTPcrystals[17].Inthiscase,measurementswere also performed using a common-pathinterferometer.However,theconfigurationofthemea-surement schemewas such that all threebeams (twopumpbeams and one probe beam) propagated collinearly in thecrystals.Aswasstatedin[17],thistechniquecanbeusedtomeasureinducedabsorptionatalevelof~105cm1.

AlongwithPCI,thereareothertechniquesbasedonthethermal lenseffect.Forexample, thephotoinducedabsorp-tioninphotorefractiveBaTiO3crystalsexposedtolaserradi-ation at differentwavelengthswas investigated in [12] by atechnique inwhich twowavespropagating inacrystalatasmallanglewithrespect toeachotheraremixed.Themea-surementswereperformedaccordingtoasomewhatmodifiedscheme,similartothatdescribedin[71].Itwasstatedin[12]that photoinduced absorption may significantly affect therecordingofholographicgratingsinphotorefractivecrystals.

3.6. Measurement of light-induced absorption by the photothermal reflectance technique

Therelativechangeinthereflectioncoefficientofproberadi-ation,measuredbythistechnique,DR(l)/R,isdirectlyrelatedtothepermittivityperturbation:

ComputerLock-inamplifier

Xe lamp

Position-sensitive detector

Modulator

Monochromator

Pum

pla

ser

Pro

bela

ser

Sample holder

CCl4

Sample

Figure 14. Schematicformeasuringthespectraldependenceofphoto-inducedabsorptionbydeflectionspectroscopy[2,124].

0.5 1.0 1.5 2.0Photon energy/eV

DA/A

Figure 15. Dependenceof the relativeabsorption induced in thinhy-drogenatedamorphoussiliconfilmsontheprobephotonenergy[2].

Pumplaser 2

Pumplaser 1

Probe laser

Absorber

Modulator

Sample Diaphragm

Photodetector

Figure 16. Schematic for measuring photoinduced absorption usingmodifiedPCItechnique[16].

8

6

4

2

2

0

0

0 10 20 30 40 t/s

532 nm on off offon

IR a

bsor

ptio

n co

effi

cien

t /10

3 cm

1

1

3

Dnn /108

Figure 17. GLIIRinaLiNbO3crystalwithaclose-to-stoichiometriccomposition[16]forpumpintensitiesof3.6kWcm2(l=532nm)and21.0kWcm2(l=1064nm).

9Laserphotothermalspectroscopyoflight-inducedabsorption

R/R=a(e1,e2)e1+b(e1,e2)e2,

where a and b are the Seraphin coefficients [125], whichdependonthepermittivitye(l)=e1(l)+ie2(l);e1ande2are,respectively,thechangesintherealandimaginarypartsofthepermittivity,whicharelinkedbytheKramersKronigrelation.

Figure 18 shows as an example the photomodulationreflectionspectraR/R; thetransmissionspectraT/T;andthe spectral dependence of the variation in absorption,(aL) (calculated in correspondence with the results of[111]) [112].These spectra are typical of the InGaAs/GaAsquantumwell.Analysisof the reflection spectrummakes itpossibletoobtainthephotomodulationabsorptionspectrumofthestructureunderstudy:a=a(l)/a;itisnothingbutthe relative change in the pump-induced absorption.Therefore,itisnotnecessarytomodifythistechniquetomea-surephotoinducedabsorption.Moreover,accordingto[112],the intensity of the photoreflectance peak and its spectralpositioncanbeusedtodeterminetheinhomogeneityofquan-tumwellwidthandcomposition.

4. Measurement of light-induced absorption by photothermal techniques (case of high intensities)

4.1. Induced absorption under multiphoton effect

Insomematerialsphotoinducedabsorptionarisesasaresultof multiphoton (in particular, two-photon) absorption oflight [126128]. Thesematerials arewidely used as opticallimiters [129], in laser microlithography at two-photonrecording[130],intwo-photonfluorescentmicroscopy[131],

intwo-photonphotodynamictherapy[132],etc.Two-photonabsorptioncanbemeasuredusingseveralconventionalmeth-ods,forexample,two-photonexcitationfluorescenceornon-lineartransmissiontechnique.

Inducedmultiphoton absorption spectroscopy,which isbasedonapplicationofphotothermaltechniques,isanalter-native to theconventional techniques.The theoryofmulti-photonthermallensspectroscopywasdevelopedin[133]andin the later study [134]. In turn, the theory of two-photondeflection spectroscopywas considered for the first time in[135].Thehighsensitivityofthethermallensanddeflectiontechniquesmakesthempromisingformeasuringweaktwo-photon absorption in optically transparent materials[136144]. The measurement of two-photon absorption inmaterialsisgenerallyhinderedduetotheabsenceofnonlin-earreferencesandcomplexityindeterminingthelightinten-sityinthesampleregionstudied.Anumberofmeasurementschemesbasedondifferentversionsofthethermallenstech-niqueweredevelopedtoovercomethesedifficulties[145148].

4.2. Measurement of induced absorption under two-photon excitation using the thermal lens technique

Toincreasethesensitivityofmeasuringnonlinearabsorption,anewflexibleschemebasedonthedouble-beamthermallenstechniquewasproposedin[148].Incomparisonwiththepre-vious Z-scanning schemes [145], the probe beam in it isstrongly focused into the sample, whereas the pump beamremainscollimated(Fig.19).

Nonlinearabsorptionplaysanimportantroleintheopti-cal breakdown of transparent materials. The thermal lenstechniquewasusedin[126]tomeasurethecoefficientsoftwo-photonabsorptionforanumberofborosilicateglasses,whicharegenerallyappliedasopticalcomponentsoflasersoperat-inginthevisiblespectralrange.

The measurements in [126] were performed using thesametechniqueasin[136],wheretwo-photonabsorptioninbenzenewasstudied.Thecoefficientsoftwo-photonabsorp-tionweredeterminedfromtheslopeofthedependencesofphotothermal signal on the squared light intensity. Theobservedlineardependenceindicatestwo-photoncharacterofabsorption.The two-photonabsorptioncoefficientbofBK-7 glass, measured at a wavelength of 532 nm, wasfoundtobe71011cmW1.

AsanexampleFig.20showstheabsorptionspectrumofBK-7glass,inducedbytwo-photonirradiation.Itcanbeseen

5

10

5

0

0

0

5

5

5

10

10

0.98 1.00 1.02 1.04 1.06 l/mm

D(aL)/104

DTT /10

4

DRR /10

4

Figure 18. Typical photomodulation reflection (DR/R) and transmis-sion(DT/T )spectraandthecalculated(accordingto[111])spectralde-pendenceof the change inabsorption [D(aL)] foran InGaAs/GaAsquantumwell[112].

Pump laserProbe laser

Detector

Filter

MirrorMirror

Mirror

Diaphragm

Sample

AmplifierOscilloscope

zDichroic mirror

AO

mod

ulat

or

Figure 19. Schematicoftheexperimentalsetupformeasuringnonlin-earabsorptionbythethermallenstechnique[148].

L.A.Skvortsov10

that induced absorption manifests itself in a wide spectralrangeandhasalongtail,extendingtothenear-IRrange.Itwassuggestedin[126]thattwo-photonabsorptionatawave-length of 532 nm induces some types of optical-absorption(colour)centresinthesamplesstudied.

4.3. Measurement of induced absorption under two-photon excitation using the deflection technique

Insomeioniccrystalsnonlinearabsorptionwasmeasuredbypulseddeflectionspectroscopy[127].ThemeasurementswereperformedusingexcimerXeCl laser radiationwithawave-lengthof308nmandpowerdensityclosetothelaserdamagethresholdofthecrystalinthetransversemeasurementscheme:theprobeHeNelaserbeampropagatedparalleltothesur-faceofthesampleunderstudy(perpendicularlytothepumpbeam) at a distance of several micrometers from it. Thedependencesofthephotodeflectionsignalonthelaserinten-sityforanumberofmaterialsareshowninFig.21.

It can be seen that the experimental data obtained areapproximated well by a straight line. The correspondingdependence for CaF2 crystals (is omitted) exhibits a kink,whichwas explained in [127] by the dominant influence ofthree-photon absorption at radiation intensities above50MWcm2.Inaddition,colourcentreswereinducedinthecrystals under study, which were assigned to defects andimpurities.Itwasnotedin[127]thatphotothermaltechniquesare a unique tool, allowing one to measure multiphoton

absorptioncoefficientsforwide-gapcrystalswithahighaccu-racy.

5. Conclusions

Theaboveexamplesoftheuseofphotothermaltechniquestoanalysethespectraldependenceofphotoinducedabsorptionindicate their efficiency for studying the defect structure ofthin films, surface layers, and bulk materials, which isextremelyimportantforimprovingtechnologiesofmanufac-turingmaterialsandmonitoring theirqualityduringmanu-facture.

However,thephotothermaltechniquesconsideredabovehaveanumberofdrawbacks.Inouropinion,themainonesare as follows. In photoacoustic spectroscopy, a hermeticacousticcellmustbeusedandcontactmeasurementsmustbeperformed toobtainhigh sensitivity. Inphotothermal radi-ometry,onemustusephotodetectorscooledtoliquidnitro-gentemperatureandhigherpowerradiationsources.Ascom-pared with the other photothermal techniques consideredabove, photothermal radiometry has the least sensitivity atroomtemperature.Inthedeflectiontechnique,samplesmustbeplacedinacellwithaliquidhavingextremelylowabsorp-tion and high thermo-optical coefficient in order to reachhighsensitivity.Thephotothermalinterferometryisnotonlydifficultinoperationbutalsocallsforcarefulvibroacousticprotection.Concerningthethermallenstechnique,whenitisappliedtostudyphotoinducedabsorption,onehastouse(asinthecaseofGLIIRA)threeradiationsources,whichleadstoobviouscomplicationofthemeasurementscheme.

Nevertheless,asfollowsfromthisreview,muchattentionis currentlypaid to thedevelopmentofphotothermal tech-niques of nondestructive diagnostics of materials and theequipment necessary for their realisation. Advances in thisfieldarerelatedtoacertainextenttothesuccessfulapplica-tion of photothermal technologies in the semiconductorindustry[anexampleistheTherma-ProbeTMmetrologytools(KLA-TencovCorporation)]andtotheproductionofPCI-based photothermal measurement systems by the SPTS(StanfordPhoto-ThermalSolutions)corporation.

Table1containsalistofthemostwidespreadphotother-maltechniques,themain(inouropinion)fieldsoftheirappli-cation,andmanufacturersofequipmentforthetechnologiesbasedonthesetechniquesanddesignedtomeasurephotoin-duced absorption. Unfortunately, we have to admit that,despitea largenumberof laboratory studies,photothermaltechnologies and the corresponding commercial equipmentare not so widespread in the market of control-measuringequipmentasexpectedtobe.Alikelyreasonisthecomplexityoftheseprecisetechniques,despitetheseemingsimplicityofdesignsproposed.

6. References 1. EmelineA.,SakinaroA.,RyabchukV.,SerponeN.Int

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l = 532 nm

200 400 600 800 1000 l/nm0

0.05

0.15

0.10

0.20P

hoto

indu

ced

abso

rpti

onco

effi

cien

t (r

el. u

nits

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Figure 20. PhotoinducedabsorptionspectrumofBK-7glass[126].

KBrNaClKCl

10

1000

2030

50

100

200300

500

Intensity/GW cm2

Pho

tode

flec

tion

sig

nal (

rel.

unit

s)

0.01 0.10.03 0.3 1

Figure 21. Dependencesofthesignalsrecordedbydeflectionspectros-copyontheradiationintensityforanumberofioniccrystals[127].

11Laserphotothermalspectroscopyoflight-inducedabsorption

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Table 1. Photothermaltechniques,mainfieldsoftheirapplication,andtheexistenceofcommercialequipment.

ExistenceExistence

ExistenceofBasic

Mainfields ofpublications

ofcommercial commercialequipment

photothermalofapplication

onmeasuringequipmentfor

formeasuring Manufacturertechniques photoinduced

basicmethods photoinduced

absorption absorption

Photoacoustic AnalysisofweaklyYes

Yes

Nodata

technique absorbingmedia

Deflection AnalysisofweaklyYes

No

Nodata

technique absorbingmedia

Thermallens AnalysisofweaklyYes(PCI)

Yes(PCI)

Yes(PCI)

StatfordPhoto-technique absorbingmedia ThermalSolution

Surface Surface thermallensing absorption Nodata No Nodata mapping

Remoteanalysis ofabsorbingmedia, Photothermal measurementand radiometry monitoringof Yes Yes Nodata semiconductor parameters

Photothermal Studyof reflectance semiconductor

Yes

Yes

Nodata

technique materials

Visualisation Interferometric ofthermaland

Nodata

No

Nodata

technique acousticfields

L.A.Skvortsov12

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