We report the ultrafast photocarrier dynamics and coherent phonon excitation in type-Ⅱ Dirac semimetal plati-num ditelluride(PtTe2)thin films via femtosecond(fs)pump-probe spectroscopy at room temperature.Quantitative analysis revealed that the incoherent electronic relaxation consists of two components:a subpico-second fast relaxation process and a slow component with a time constant of hundreds of picoseconds(ps).Furthermore,the launch of a coherent acoustic phonon(CAP)in the 20 nm film but absence in the 6.8 nm film uncovers the dominant role of temperature gradient in producing a strain wave.The sound velocity and Young's modulus in the thick PtTe2 are determined to be 1.736 km/s and 29.5 GPa,respectively.In addition,the coherent optical phonon(COP)with a frequency of 4.7 THz corresponding to Te atoms out-of-plane A1g vibration has been well resolved in all films,which is ascribed to displacive excitation of coherent phonon(DECP).The observation of a strong probe-wavelength dependent COP amplitude reveals the resonant feature of the optical excitation-induced atomic displacement in PtTe2.Our findings provide deep insight into the ex-citation and dynamics of CAP and COP as well as the photocarriers'recovery pathway and lifetimes in PtTe2.Our study also demonstrates that the COP spectroscopy is a powerful tool to reveal the modulation of frequency-dependent optical constants induced by atomic vibrations,which may find applications in the fields of optoelec-tronics and ultrafast photonics.

PENG SUO;SHENGNAN YAN;RUIHUA PU;WENJIE ZHANG;DI LI;JIAMING CHEN;JIBO FU;XIAN LIN;FENG MIAO;SHI-JUN LIANG;WEIMIN LIU;GUOHONG MA

Department of Physics,Shanghai University,Shanghai 200444,China;National Laboratory of Solid State Microstructures,School of Physics,Collaborative Innovation Center of Advanced Microstructures,Nanjing University,Nanjing 210093,China;School of Physical Science and Technology,ShanghaiTech University,Shanghai 201210,China

光子学研究（英文）

[1]PENG SUO;SHENGNAN YAN;RUIHUA PU;WENJIE ZHANG;DI LI;JIAMING CHEN;JIBO FU;XIAN LIN;FENG MIAO;SHI-JUN LIANG;WEIMIN LIU;GUOHONG MA-.Ultrafast photocarrier and coherent phonon dynamics in type-Ⅱ Dirac semimetal PtTe2 thin films probed by optical spectroscopy)[J].光子学研究（英文）,2022(03):653-661

PtTe2,plati,ditelluride,subpico

Ultrafast,phonon,dynamics,type,Dirac,semimetal,thin,films,probed,by,optical,spectroscopy,We,report,ultrafast,excitation,num,via,femtosecond,fs,pump,room,temperature,Quantitative,analysis,revealed,that,incoherent,electronic,relaxation,consists,two,components,process,slow,hundreds,picoseconds,ps,Furthermore,launch,acoustic,CAP,but,absence,uncovers,dominant,role,gradient,producing,strain,sound,velocity,Young,modulus,thick,are,determined,GPa,respectively,In,addition,COP,frequency,THz,corresponding,atoms,out,plane,A1g,has,been,well,resolved,all,which,ascribed,displacive,DECP,observation,strong,wavelength,dependent,amplitude,reveals,resonant,feature,induced,atomic,displacement,Our,findings,provide,deep,insight,into,photocarriers,recovery,pathway,lifetimes,study,also,demonstrates,powerful,tool,modulation,constants,vibrations,may,applications,fields,optoelec,tronics,photonics

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