regardedtonsion

1CoherentGenerationofPhoto-Thermo-AcousticWavefromGrapheneSheetsYichaoTian1,*,HeTian2,*,Y.
L.
Wu1,L.
L.
Zhu1,L.
Q.
Tao2,W.
Zhang1,Y.
Shu2,D.
Xie2,Y.
Yang2,Z.
Y.
Wei1,X.
H.
Lu1,Tian-LingRen2,Chih-KangShih3&JiminZhao1ManyremarkablepropertiesofgraphenearederivedfromitslargeenergywindowforDirac-likeelectronicstatesandhavebeenexploredforapplicationsinelectronicsandphotonics.
Inaddition,strongelectron-phononinteractioningraphenehasledtoefficientphoto-thermoenergyconversions,whichhasbeenharnessedforenergyapplications.
Bycombiningthewavelengthindependentabsorptionpropertyandtheefficientphoto-thermoenergyconversion,herewereportanewtypeofapplicationsinsoundwavegenerationunderlinedbyaphoto-thermo-acousticenergyconversionmechanism.
Mostsignificantly,byutilizingultrafastopticalpulses,wedemonstratetheabilitytocontrolthephaseofsoundwavesgeneratedbythephoto-thermal-acousticprocess.
Ourfindingpavesthewayfornewtypesofapplicationsforgraphene,suchasremotenon-contactspeakers,optical-switchingacousticdevices,etc.
Sincegraphenewasdiscoveredadecadeago1,itsremarkablepropertieshavebeenutilizedfornoveldevicesandtechnologicalapplications2–10.
TheoutstandingpropertiesofgrapheneprimarilyoriginatefromitsDirac-particle-likeelectronicstates11–15.
Earlyworkfocusedonitsexceptionaltransportprop-ertiesemployingstatesneartheDiracpoint16–21.
TheexistenceofDirac-likeelectronicstatesoveralargeenergywindowalsoresultedinmanynovelopticalpropertiesspanningacrossalargefrequencyrange4,10,22.
Theinterplayofitsuniqueelectronicstructuresandlatticevibrationsunderphoto-excitationcanalsoleadtointerestingproperties,whichcanbeharnessedforenergyapplications.
Inthepresentarticle,weintroduceanotherinnovativeapplication:coherentgenerationofacousticwavesinambientenvironments.
Specifically,byusingultrafastlaserpulses,wedemonstratethegenerationofacousticpulsesfromamulti-layergraphene(MLG)sheetthroughaphoto-thermo-acoustic(PTA)process.
Mostintriguingly,weshowthephasecoherencebetweentheacousticpulsesthroughthephaseinterferencesbetweensequentiallygeneratedacousticpulses.
WenotethatgrapheneandMLGhavealreadybeenusedforthegenerationofthermo-acousticwaves(i.
e.
soundwaves)inambientair23–25.
Nevertheless,thesestudiesusedgraphenesheetsormonolayergraphenesolelyassheetresistors,similartothinmetalsheets,forthethermo-acousticgenerationofsoundwaves,ratherthanemployinggraphene'scharacteristicproperties.
InthePTAprocessutilizedhere,theuniqueelectronicpropertiesofgrapheneplayakeyrole.
ResultsPhoto-thermo-acousticwavegenerationinMLG.
TheexperimentalsetupisschematicallyshowninFig.
1a,whichdisplaystheultrafastlaserpulsesusedtogenerateandphase-controlacousticsound1BeijingNationalLaboratoryforCondensedMatterPhysicsandInstituteofPhysics,ChineseAcademyofSciences,Beijing100190,China.
2InstituteofMicroelectronics&TsinghuaNationalLaboratoryforInformationScienceandTechnology(TNList),TsinghuaUniversity,Beijing100084,China.
3DepartmentofPhysics,TexasUniversityatAustin,Texas78712,USA.
*Theseauthorscontributedequallytothiswork.
CorrespondenceandrequestsformaterialsshouldbeaddressedtoT.
R.
(email:RenTL@tsinghua.
edu.
cn)orC.
K.
S.
(email:shih@physics.
utexas.
edu)orJ.
Z.
(email:jmzhao@iphy.
ac.
cn)received:27June2014accepted:17April2015Published:08June2015OPEN2wavesinaMLGsheetsample(seeMethods).
WenotethattheelectricalcontactsseenonthesampleinFig.
1aareusedforotherexperimentsandtheyarenotincludedinaclosedcircuit.
Thesoundgen-erationisperformedthroughaPTAmechanism,illustratedinFig.
1b.
Itcomprisesatwo-stageprocessintemporalsequence:aphoto-thermal(PT)process,followedbyathermo-acoustic(TA)processasdescribedbelow.
WhenlaserpulsesilluminatetheMLGmembrane,theabsorbedphotonsexcitethevalencebandelectrons,generatingfreecarriersintheconductionband(electrons)andthevalanceband(holes).
Suchexcited-statefreechargecarriersrelaxtothegroundstatebyemittingopticalandacousticphonons26,27(Fig.
1c).
Extensiveinteractionbetweenelectronsandlatticevibrationsleadstoahotlat-ticetemperature(witharadialthermalgradientasshowninFig.
1b).
Ithasbeenwell-establishedthattheenergyrelaxationofhotcarrierstothehotlatticetemperatureismediatedbytheelectron-phononinteraction28–31,leadingtoanefficientPTprocessoccurringatarelativelyfasttimescaleoftheorderofpicoseconds(seeSI1)11,32–34.
Thesamplethenheatstheambientairatomsthroughvibrationsandcollisions,whichmodifiesairpressureaccordingly35,leadingtothegenerationoflongitudinalsoundwavesintheair(Fig.
1b).
Inessence,thissecondstageisaTAprocess36.
Thelongitudinalsoundwaves,whichareplainwavesinthenearfield,becomesphericalwavesinthefarfield(Fig.
1b;forquantitativecharacterization,seeSI2).
Inthismanner,thetrainofinputopticalpulsesproducesatrainofacousticpulsesdetectedinthefarfield.
Mostinterestingly,eachopticalpulsegeneratesanacousticpulsewithhigh(anharmonic)acousticfrequencywithawell-definedphase(Fig.
2–4),whichenablesustoachievephasecontrolofthesoundgeneration.
WefirstinvestigatedthePTAprocessasafunctionoftheexcitationwavelength.
Weusedboth800nmand400nmlaserpulsestoexaminethesoundgenerationefficiency.
TheexperimentalresultsareshowninFig.
2,whichdisplaysthetemporalsignaltraceofthegeneratedacousticsound.
ThemajorpeaksFigure1.
Schematicdiagramoftheexperimentalsetupandsoundgenerationmechanism.
(a)Ultrafastlaserpulsesofdifferentwavelengths,timedurations,andrepetitionratesareirradiatedontothegraphenesheetsample.
(b)Ultrafastlaserpulsesgenerateathermalgradientwhichleadstoacousticsoundwavegeneration.
Thetimeintervalbetweenpulsesphase-controlsthesoundamplitude.
(c)MLGunderultrafastlaserpulseexcitation.
Theelectron-phononinteractiongeneratesthermalheatduringtheultrafast(ps)relaxationprocess,whichfurtherproducesacousticsoundatamuchlonger(μs)timescale.
TheconesareusedtomimicthebandstructureofMLG.
3areseparatedby1ms,correspondingtothelaserpulserepetitionrateof1kHz,wherethelaserpulsewidthis130fs.
Furthermore,higherfrequencyoscillationsareobservedbetweenthemajorpeaks.
Thesefasteroscillationscorrespondtothecharacteristicfrequencyoftheacousticsoundwave,aswewilldiscussinthefollowing.
InFig.
2weshowthefrequencydomainanalysisofthecorrespondingtimedomaindatawithinoneperiod.
Itisevidentthatthetimeandfrequencydomainanalysesshowindis-tinguishableresultsforthe800nmand400nmopticalexcitations.
Moreover,theefficiencyforsoundgenerationisalsoindependentofthelaserwavelength,sincethegeneratedsoundpressureshavethesamepowerdependence(Fig.
2).
Onecanalsoquantitativelydetermineasoundgenerationefficiencyof0.
012%(seeSI3).
Thisphoton-energy-independentfeaturecanbeattributedtotwofactors—thephoton-energy-independentabsorptioncoefficientinthevisibletothenearIRrange37,38,duetoalargeenergywindowoftheDirac-likeelectronicstates;andaveryefficientenergyrelaxationchannelforthehotelectrons(holes)toreachequilibriumwiththelatticetemperature—bothofwhichhavebeenregardedashallmarksoftheremarkablepropertiesofgraphene.
ThePTAconversionefficiencyof0.
012%isnearlyidenticaltotheefficiencyoftheTAprocessinvestigatedearlierusingpureJouleheating(SI3),implyinganalmostidealenergyconversionefficiencyofthephoto-thermalprocessintheMLGsheet.
Subsequently,weinvestigatedtheeffectofthelaserpulsedurationonthegenerationefficiency.
Threelaserbeamswithdurationsof130fs,190ps,and230ns(seeMethods)wereemployed,allat1kHzrepe-titionrate.
Themicrophonedetectiondistancewas25mm.
TheexperimentalresultsareshowninFig.
3,whichdemonstratesthatwithintwoordersofmagnitudedynamicrangeofthelaserpower,theslope,andthusthesoundgenerationefficiency,isnearlyidenticalforthethreepulsedurations.
Moreover,thelineshapeoftheacousticwavesisindependentoftheexcitationpulseduration.
InFig.
3bweshowtheFouriertransformofthetimedomaindata(Fig.
3binset)thatistakenforexactlyoneperiod.
Apeakisclearlyobservedaround6kHz.
Forallthreepulsedurations,thefrequencycomponentsandtheiramplitudesareidentical.
Unlikethe1kHzrepetitionrateobservedinFig.
2lowerrightpanel,this6kHzanharmonicsignalismoreinteresting,whichhasneverbeenreportedbefore.
Oneneedsapulsedexcita-tionsourcetoobservethisanharmonicsignal.
Weshowthatthis6kHzcharacteristicfrequencyorigi-natesfromtheinteractionbetweenthesampleandtheambientgasmolecules.
Bychangingtheambientcondition(e.
g.
usingHeliumgasinanenclosure)weobservedthatthis6kHzfrequencychangedto~2kHz(FigureS2inSI4).
ConsideringtheresultsshowninFig.
3,wewereabletoascribethesoundgenerationtoaPTAmechanism,atwo-stepprocesscomprisinganultrafastPTprocessfollowedbyaslowerTAprocess.
Firstweeliminatedthepossibilityofadirectphoto-acoustic(PA)mechanism.
InthePAmechanism,thephoto-excitedelectronsinteractdirectlywiththeambientairmolecules.
Theultrafastdynamicsofthefreecarriers,thephonons,andtheirinteractionsallhavetheircharacteristictimescales,rangingFigure2.
Effectofthephotonenergy(laserwavelength)ontheefficiencyofsoundgeneration.
Thelaserpulseswith400nmand800nmcentralwavelengthshavesimilareffectsonthesoundgenerationefficiencyinthetimedomain(withoffset),inthefrequencydomain,andalsoinintensity(withoffset).
4fromtensoffemtosecondstopicosecondstosub-nanoseconds(seeSI1).
IfadirectPAmechanismwasinvolved:(1)thegenerationefficiencywillbehigherforthe130fspulses,becausefor190psand230nspulsesaprominentportionoftheabsorbedphotonenergiesareinevitablydissipatedthroughelectron-phononscattering(asthermalenergy,insteadofacousticenergy);(2)thepeakwidthoftheacousticwaveshouldbesmallerforthe130fsand190pscases,sinceitisonlylimitedbytheultrafastelectron-airmoleculescatteringrate.
Thisiscontrarytoourexperimentalresults.
Theabovetworeasonsaresummarizedinatableinthesupplementaryinformation(seeSI5).
OurcarefulexperimentinbothFigure3.
Effectofthelaserpulsedurationonthesoundgenerationefficiency.
(a)Theblue,black,andreddotsindicatethe130fs,190ps,and230nslaserpulses,respectively.
Foralargedynamicalrangetheslopeofthethreearethesame.
(b)Thefrequencydomainamplitudesandthetimedomainsignals(inset,withoffset)ofthesoundwaves,producedwithdifferentpulses.
5thetemporalandthefrequencydomainwithdifferentpulsewidths(Fig.
3)isadirectexperimentalproofofthePTAmechanism.
Ourmethodalsoappliestoothersystemsofsimilarmaterials.
ThePTAmecha-nismthatwefoundisinconsonantwiththephoto-thermal-electric(PTE)ratherthanthephoto-voltaic(PV)mechanismintheelectronictransportpropertiesofgraphene39–41.
TheultrafasttimescaleofthePTprocesseffectivelycreatesadelta-functionliketemperaturepulseonthesample.
Thissharp(intime)temperaturepulsegeneratessoundwavesattheair/grapheneinterface,whichthenpropagatethroughtheairandaredetectedinthefarfield.
Coherentphase-controlofthePTAsoundwaves.
AninterestingaspectofthesePTAgeneratedacousticwavesisthewell-definedfrequency(~6kHz,differentthanthelaserrepetitionrate)andthewell-definedphaseinthetimedomain.
Thisintroducestheinterestingprospectofcoherentlycontrollingtherelativephasebetweenacousticpulses,leadingtoconstructiveordestructiveinterferences.
Inordertoinvestigatethisthoroughly,weusedlaserpulsesof532nmwavelength,400nsduration,andafixedenergy,thustheaveragelaserpowerincreasedlinearlywiththerepetitionrate.
InFig.
4aweshowthetime-resolvedacousticwaves,whichexhibitconstructiveanddestructiveinterferenceeffects,asafunctionofthelaserrepetitionrate(fortuningtherepetitionrate,seeMethods).
Therelativephasebetweentwoconsecutiveacousticwavepacketsinthetimedomainisdirectlyrelatedtotherepetitionrate.
InFig.
4bweshowanumericalsimulationofsuchaninterferenceeffect,bytakingtheacousticresponseofasinglepulseandapplyingstrictlythewavesuperpositionaccordingtothelaserrepetitionrate.
Itisevidentthatthenumericalsimulationsusingwavesuperpositionaccuratelyreproducetheexperimentalresults.
InFig.
4cweshowthefalsecolormappingoftheresultinFig.
4atoclearlyillustratethephase-controleffect.
Owingtothefinitenumberofdiscretevaluesofrepetitionrates,theinterpo-lationisimplementedbetweenthemeasureddata.
Thephasetuningismarkedbywhitedashedcurvesandtheinterferenceeffectismanifestedbythehorizontalredandbluecolorstripes.
Atlowrepetitionratestheinterferenceeffectissmall,andatrelativelyhighrepetitionratestheinterferencebecomesmorepronounced.
ThequantitativeanalysisofsuchaninterferenceeffectisfurtherdescribedinthediscussionFigure4.
Interferenceeffectandphasecontrol.
(a)TimedomainsignalofMLGsoundatdifferentlaserrepetitionrates.
Additionaloffsethasbeenappliedtothecurvesforclarity.
(b)Numericalsimulationofthephasecontrol.
Thesuperpositionoftwoconsecutiveacousticwavesgivesthewell-simulatedsignaldisplayedin(a).
(c)Falsecolormappingofthetime-domainsignalatthedifferentlaserrepetitionratesshownin(a).
(d)Analyticalresultforthephasecontrol.
Constructiveanddestructiveeffectsarecontrolledbytuningtherepetitionrate.
Thedotsaretheexperimentalresultsshownin(a),andthesolidcurveisaplotofourtheoreticalequation.
Theinsetshowstheresultwithafixedaveragelaserpower.
6section.
Asverifiedinadditionalexperiment(resultsnotshownhere),tuningtherepetitionrateatmuchlowerthan1000Hz(forexample,from1Hzto1000Hz)hasverylittleeffectonthesoundamplitude.
However,asthelaserrepetitionrateincreases,thesoundamplitudedisplaysapronouncedincreaseanddecreasealternately(Fig.
4c,d).
Thismodificationcanbeconstructiveordestructive,dependingontherelativephasebetweentheconsecutiveacousticwavepackets.
InFig.
4ctheredstripecorrespondstoconstructiveinterferenceandthebluestripetodestructiveinterference.
DiscussionWefurthermoreperformedanalyticalanalysisofthesoundamplitudeasafunctionofvaryinglaserrep-etitionrate.
AssumingasinusoidalfunctionsuperimposedonasingleexponentialdecayforindividualacousticpulsesasA(P)sin(ωt)exp(βt),thesuperpositionoftwoconsecutivepulsescanbeexpressedasΘωβωβtAPttAPsintTtTsinexp[]exp[]10whereTistheperiodofthelaserpulserepetition(i.
e.
thetimeintervalbetweentwopulses),whichissimplycontrolledbytuningtherepetitionrate,j0isafittingparameterthataccountsfortheinitialadditionalphasebetweenthetwooscillations,A(P)istheamplitudeasafunctionofthelaserpowerforeachacousticwavepacket,andωandβarethefrequencyanddecayconstants,respectively.
Consideringproportionalitybetweenthelaserpower,rate,andamplitude,afterasimplecalculationwederivedtheinterferenceasΘκβπβπβ)(/)+(/+)(/)tAfsinfttexp2cos2exp12exp2G02G0whereisthelaserrepetitionrate,G=ω/2πisthegraphene'sanharmonicoscillationfrequency,arctanffsin2expcos2exp1GG00=πβπβ(/)(/)+,andAPAPAA000ζζξκ=+,withζ,ξ,andκbeingconstantcoefficients.
ThetotalamplitudeoftheoscillationΘ(t)canthusbecontrolledbythelaserrepetitionrateasfollows:κβπβ()=(+)(/)+(/+)(/)AAfexp2cos2exp13G020Inordertocomparewiththeexperimentalresult,weplottedboththeexperimentaldataandthefittedtheoreticalcurveinFig.
4d.
Toobtaintheexperimentalamplitudevaluewehavesubtractedtheminimumamplitude(wavevalley)fromthemaximumamplitude(wavepeak)foreachcurve.
ThebestfittingparametersforthecalculationwereA0=0.
075V,κ=0.
0163VkHz1,G=6.
5kHz,j0=1.
92,andβ=3.
78ms1.
Itisobviousthatthetheoreticalcalculationcurvecompareswellwiththeexperimentaldata(Fig.
4d);thereforethisanalyticalcalculation,albeitusingharmonicwavesofasinglefrequency,isprovedtoadequatelydescribetheamplitudeasafunctionoftherepetitionrate.
Insummary,wehavedemonstratedtheprecisephasecontrolofacousticsoundwavegenerationingraphenesheetsusingultrafastopticalpulses.
Inthephasecontrol,theconstructiveanddestructivegen-erationefficiencywaspreciselyandeasilycontrolledbytuningthelaserrepetitionrate.
Ourinvestigationpavesthewaytothedevelopmentofenergyapplicationsusinggraphenematerials.
Bothvisibleandultravioletopticalpulsescanbeusedtogeneratesoundwavesingraphenesheets,showingthepotentialforenergyharvestingfarfromtheFermisurface.
Anharmonicsoundwavegenerationhasbeenclearlyobservedandforthefirsttimethoroughlyinvestigated,revealingaPTAphysicsmechanism.
Ourworkdemonstratesanopticalsoundgenerationdevicebasedongraphenesheets,whichhasnon-contactandremotecontrolcapability.
Ourinvestigationcanbeeasilyextendedtoelectricalinterferencecontrolandothersoundgenerationapplications,suchasopticalswitchingofacousticsoundgeneration.
MethodsSoundgenerationusingultrafastlaserpulses.
Weusedmultipleultrafastlasersystemsastheexcitationsource.
Lightpulseswithtunabletemporalpulsewidth(70fs,130fs,190ps,230ns,and400ns),repetitionrate(0–8kHz),andphotonenergy(with800nmand400nmwavelength)wereusedasexcitationsourceswithanormalincidentgeometry.
Theopticalbeamwasexpandedtoadiameterof10mmonthesamplesurfaceusingalenssystem.
Thesoundsignalwasdetectedwithamicrophoneandamplifiersystemandquantitativelyrecordedwithanoscilloscope.
Whenthelaserpowerwasincreasedto50mW,acousticsoundcouldbeheardbytheearsat10cmawayfromthesample.
Whenthelaserbeamwasblocked,thesounddisappeared;asthelaserpowerwasincreased,thesoundvolumeincreasedaccordingly.
Toensurethatthesoundwasproducedbythegraphenesheetsinsteadofthepapersub-strate,acontrolexperimentwasperformedonthebarepapersubstrate.
Underthesameconditionsanduptothemaximumlaserpower(595mWforthe130fslaserbeam,2Wfor190pslaserbeam,and1.
59Wforthe230nslaserbeam),nosoundsignalwasdetected.
OurMLGsheetonthepapersubstratehada1*1cm2areaandanaveragethicknessof60nm.
Thesample(Fig.
1)wasfabricatedbyCVDonNi,7withdetailsdescribedinRef.
25.
TheX-rayDiffractiondataofoursampleisshowninSI6.
Theelectri-calcontactsseeninFig.
1areusedforotherexperimentsandtheyarenotincludedinaclosedcircuit.
Acousticsoundwavedetection.
Thesoundintensitywasdetectedusingamicrophone(TM-12,TongShengInc.
http://www.
tonsion.
com.
cn/productInfo.
aspxtypeid=23&id=58),whoseoutputwassentintoapreamplifier(JX-01B,JuLongInc.
)beforeitwasinputintotheoscilloscope(DPO4000,Tektronix,Inc.
),whichhadasamplingrateof5GS/sandbandwidthof1GHz.
ToobtainthedatashowninFig.
4a,thesamemicrophonewasplacedatadistanceof2.
5cmandanangleof45°fromthesample,inordertocollectthesoundsignalandtoconvertitintoelectricalsignal;thelatterwasthenamplifiedbyadifferentpreamplifier(KX-2A,KesuosiInc.
)beforebeingrecordedbyanoscilloscope(DSO7104B,AgilentTechnologiesInc.
).
UltrafastLaserSystem.
Ourlasersystemwasanin-housebuiltchirped-pulseamplifier(CPA),whichconsistedofafemtosecondTi:sapphireoscillator,apumplaser,astretcher,aringregenerativeamplifier,andacompressor.
Initially,stablefemtosecondlaserpulsesasshortas40fsweregeneratedfromtheTi:sapphireoscillatoratarepetitionrateof80MHz.
Subsequently,agratingstretcherwasusedtostretchthepulsedurationto190ps.
Followingthestretcher,thelaserpulsewasinjectedintotheregen-erativeamplifier,whichwaspumpedbyacommercial527nmpumplaserwithapulsedurationof230nsatarepetitionrateof1kHz.
Byoptimizingthetimedelaybetweentheseedingandpumpingpulses,thechirpedlaserbeamwasamplifiedprogressivelyandcontinuallyuntilsaturatingatthemaximumgainbeforebeingextractedfromthecavity.
Finally,thefullyamplifiedchirpedlaserpulsewascompressedto70fsbyusingasinglegratingcompressorafter4-passdiffraction.
Thetypicalenergywasapproxi-mately3mJandthebandwidthwas18.
4nm(FHWM).
Weutilizedthelaserfromthe527nmpumplaser(230ns),theamplifiedpulsebeforecompression(190ps),andthefinalcompressedpulse(70fs),respectively,forourexperiment.
Forthepulsedurationinvestigation,thelaserpulsesweresuppliedbythechirped-pulseamplifierlasersystemandthe130fscommerciallasersystem.
Tuningoftherepetitionrate.
BesidestheultrafastlasersystemsdescribedaboveandillustratedinFig.
1,twoadditionalultrafastsystemswereusedinourexperiment.
Theyhadasinglewavelength,butwereabletoprovideatunablelaserpulserepetitionrate.
Oneofthemcouldbetunedfrom1Hzto1000Hzandtheotherfrom1000Hzto10kHz.
Thetuningofthelatteronewaschallenging,becauseeachtuningrequiredopeningthelasercavityandre-calibratingthesystem.
References1.
Novoselov,K.
S.
etal.
Electricfieldeffectinatomicallythincarbonfilms.
Science306,666–669(2004).
2.
Bae,S.
etal.
Roll-to-rollproductionof30-inchgraphenefilmsfortransparentelectrodes.
Nat.
Nanotechnol.
5,574–578(2010).
3.
Bunch,J.
S.
etal.
Electromechanicalresonatorsfromgraphenesheets.
Science315,490–493(2007).
4.
Liu,M.
etal.
Agraphene-basedbroadbandopticalmodulator.
Nature474,64–67(2011).
5.
Xia,F.
N.
,Mueller,T.
,Lin,Y.
M.
,Valdes-Garcia,A.
&Avouris,P.
Ultrafastgraphenephotodetector.
Nat.
Nanotechnol.
4,839–843(2009).
6.
Lee,E.
J.
H.
,Balasubramanian,K.
,Weitz,R.
T.
,Burghard,M.
&Kern,K.
Contactandedgeeffectsingraphenedevices.
Nat.
Nanotechnol.
3,486–490(2008).
7.
Mueller,T.
,Xia,F.
N.
A.
&Avouris,P.
Graphenephotodetectorsforhigh-speedopticalcommunications.
Nat.
Photonics4,297–301(2010).
8.
Shi,S.
F.
,Xu,X.
D.
,Ralph,D.
C.
&McEuen,P.
L.
Plasmonresonanceinindividualnanogapelectrodesstudiedusinggraphenenanoconstrictionsasphotodetectors.
NanoLett.
11,1814–1818(2011).
9.
Yan,J.
etal.
Dual-gatedbilayergraphenehot-electronbolometer.
Nat.
Nano.
7,472–278(2012).
10.
Wu,R.
etal.
Purelycoherentnonlinearopticalresponseinsolutiondispersionsofgraphenesheets.
NanoLett.
11,5159–5164(2011).
11.
Sun,D.
etal.
UltrafastrelaxationofexcitedDiracfermionsinepitaxialgrapheneusingopticaldifferentialtransmissionspectroscopy.
Phys.
Rev.
Lett.
101,157402(2008).
12.
CastroNeto,A.
H.
,Guinea,F.
,Peres,N.
M.
R.
,Novoselov,K.
S.
&Geim,A.
K.
Theelectronicpropertiesofgraphene.
Rev.
Mod.
Phys.
81,109–162(2009).
13.
Zhang,Y.
B.
etal.
Directobservationofawidelytunablebandgapinbilayergraphene.
Nature459,820–823(2009).
14.
Li,Z.
Q.
etal.
Diracchargedynamicsingraphenebyinfraredspectroscopy.
Nat.
Phys.
4,532–535(2008).
15.
Novoselov,K.
S.
etal.
.
Two-dimensionalgasofmasslessDiracfermionsingraphene.
Nature438,197–200(2005).
16.
Han,M.
Y.
,Zyilmaz,B.
O.
,Zhang,Y.
B.
&Kim,P.
Energyband-gapengineeringofgraphenenanoribbons.
Phys.
Rev.
Lett.
98,206805(2007).
17.
Bolotin,K.
I.
etal.
Ultrahighelectronmobilityinsuspendedgraphene.
SolidStateCommun.
146,351–355(2008).
18.
Winnerl,S.
etal.
CarrierrelaxationinepitaxialgraphenephotoexcitedneartheDiracpoint.
Phys.
Rev.
Lett.
107,237401(2011).
19.
Zhang,L.
M.
etal.
Determinationoftheelectronicstructureofbilayergraphenefrominfraredspectroscopy.
Phys.
Rev.
B78,235408(2008).
20.
Han,M.
Y.
,Brant,J.
C.
&Kim,P.
Electrontransportindisorderedgraphenenanoribbons.
Phys.
Rev.
Lett.
104,056801(2010).
21.
Yao,W.
,Yang,S.
Y.
A.
,&Niu,Q.
Edgestatesingraphene:fromgappedflat-bandtogaplesschiralModes.
Phys.
Rev.
Lett.
102,096801(2009).
22.
Sensale-Rodriguez,B.
etal.
Broadbandgrapheneterahertzmodulatorsenabledbyintrabandtransitions.
Nat.
Commun.
3,780(2012).
23.
Tian,H.
etal.
Graphene-on-papersoundsourcedevices.
AcsNano5,4878–4885(2011).
24.
Suk,J.
W.
,Kirk,K.
,Hao,Y.
F.
,Hall,N.
A.
&Ruoff,R.
S.
Thermoacousticsoundgenerationfrommonolayergraphenefortransparentandflexiblesoundsources.
Adv.
Mater.
24,6342–6347(2012).
25.
Tian,H.
etal.
Single-layergraphenesound-emittingdevices:experimentsandmodeling.
Nanoscale4,2272–2277(2012).
26.
Bistritzer,R.
&MacDonald,A.
H.
Electroniccoolingingraphene.
Phys.
Rev.
Lett.
102,206410(2009).
827.
Strait,J.
H.
etal.
Veryslowcoolingdynamicsofphotoexcitedcarriersingrapheneobservedbyoptical-pumpterahertz-probespectroscopy.
NanoLett.
11,4902–4906(2011).
28.
Yan,J.
,Zhang,Y.
B.
,Kim,P.
&Pinczuk,A.
Electricfieldeffecttuningofelectron-phononcouplingingraphene.
Phys.
Rev.
Lett.
98,166802(2007).
29.
Yan,J.
,Erik,A.
,H.
,Kim,P.
&Pinczuk,A.
Observationofanomalousphononsofteninginbilayergraphene.
Phys.
Rev.
Lett.
101,136804(2008).
30.
Efetov,D.
K.
&Kim,P.
Controllingelectron-phononinteractionsingrapheneatultrahighcarrierdensities.
Phys.
Rev.
Lett.
105,256805(2010).
31.
CastroNeto,A.
H.
&Guinea,F.
Electron-phononcouplingandRamanspectroscopyingraphene.
Phys.
Rev.
B75,045404(2007).
32.
Dawlaty,J.
M.
,Shivaraman,S.
,Chandrashekhar,M.
,Rana,F.
&Spencer,M.
G.
Measurementofultrafastcarrierdynamicsinepitaxialgraphene.
Appl.
Phys.
Lett.
92,042116(2008).
33.
Kumar,S.
etal.
Femtosecondcarrierdynamicsandsaturableabsorptioningraphenesuspensions.
Appl.
Phys.
Lett.
95,191911(2009).
34.
George,P.
A.
etal.
Ultrafastoptical-pumptetrahertz-probespectroscopyofthecarrierrelaxationandrecombinationdynamicsinepitaxialgraphene.
NanoLett.
8,4248–4251(2008).
35.
Balandin,A.
A.
etal.
Superiorthermalconductivityofsingle-layergraphene.
NanoLett8,902–907(2008).
36.
Xiao,L.
etal.
Flexible,Stretchable,Transparentcarbonnanotubethinfilmloudspeakers.
NanoLett.
8,4539–4545(2008).
37.
Nair,R.
R.
etal.
Finestructureconstantdefinesvisualtransparencyofgraphene.
Science320,1308(2008).
38.
Mak,K.
F.
etal.
Measurementoftheopticalconductivityofgraphene.
Phys.
Rev.
Lett.
101,196405(2008).
39.
Gabor,N.
M.
etal.
Hotcarrier-assistedintrinsicphotoresponseingraphene.
Science334,648–652(2011).
40.
Xu,X.
D.
,Gabor,N.
M.
,Alden,J.
S.
,vanderZande,A.
M.
&McEuen,P.
L.
Photo-thermoelectriceffectatagrapheneinterfacejunction.
NanoLett.
10,562–566(2009).
41.
Song,J.
C.
W.
,Rudner,M.
S.
,Marcus,C.
M.
&Levitov,L.
S.
Hotcarriertransportandphotocurrentresponseingraphene.
NanoLett.
11,4688–4692(2011).
AcknowledgementsThisworkwassupportedbytheNationalBasicResearchProgramofChinaMOST(2012CB821402,2015CB352100),theExternalCooperationProgramofChineseAcademyofSciences(GJHZ1403),theNationalNaturalScienceFoundationofChina(11274372,60936002,61025021,61434001),theNationalKeyProjectofScienceandTechnology(2011ZX02403-002),theNSFDMR-1306878andWelch-1672.
AuthorContributionsJ.
Z.
,T.
L.
R.
andC.
K.
S.
conceivedandsupervisedtheproject.
Y.
T.
performedexperiments.
H.
T.
,Y.
Shu,D.
XieandY.
Yangmadethesample.
Y.
W.
,L.
Z.
andL.
T.
assistedonexperiments.
W.
Z.
andZ.
Y.
W.
preparedpartialofthelasersystems.
H.
T.
preparedonesectionofS.
I.
X.
Ludouble-checkeddataanalysis.
J.
Z.
andC.
K.
S.
analyzedthedataandwrotethepaper.
AdditionalInformationSupplementaryinformationaccompaniesthispaperathttp://www.
nature.
com/srepCompetingfinancialinterests:Theauthorsdeclarenocompetingfinancialinterests.
Howtocitethisarticle:Tian,Y.
etal.
CoherentGenerationofPhoto-Thermo-AcousticWavefromGrapheneSheets.
Sci.
Rep.
5,10582;doi:10.
1038/srep10582(2015).
ThisworkislicensedunderaCreativeCommonsAttribution4.
0InternationalLicense.
Theimagesorotherthirdpartymaterialinthisarticleareincludedinthearticle'sCreativeCom-monslicense,unlessindicatedotherwiseinthecreditline;ifthematerialisnotincludedundertheCreativeCommonslicense,userswillneedtoobtainpermissionfromthelicenseholdertoreproducethematerial.
Toviewacopyofthislicense,visithttp://creativecommons.
org/licenses/by/4.
0/