Coherent Acoustic Phonon and Its Chirping in Dirac Semimetal Cd_3As_2

Ultrafast optical spectroscopy of a single crystal of a Dirac semimetal Cd_3As_(2 )is carried out.An acoustic phonon(AP)mode with central frequency f=0.037 THz(i.e.,1.23 cm~(-1)or 0.153 meV)is unambiguously generated and detected,which we attribute to laser-induced thermal strain.An AP chirping(i.e.,variation of the phonon frequency)is clearly detected,which is ascribed to heat capacity variation with time.By comparing our experimental results and the theoretical model,we obtain a chirping time constant,which is 31.2 ps at 6 K and 19.8 ps at 300 K,respectively.Significantly,we identify an asymmetry in the AP frequency domain peak and find that it is caused by the chirping,instead of a Fano resonance.Moreover,we experimentally demonstrate that the central frequency of AP is extremely stable with varying laser fluence,as well as temperature,which endows Cd_3As_2application potentials in thermoelectric devices.