Ultrafast photocarrier and coherent phonon dynamics in type-Ⅱ Dirac semimetal PtTe2 thin films probed by optical spectroscopy

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.