  The relaxation dynamics of photoexcited quasiparticles of three-dimensional
(3D) Dirac semimetals are vital towards their application in high performance
electronic and optoelectronic devices. In this work, the relaxation dynamics of
photoexcited carriers of 3D Dirac semimetal Cd3As2 are investigated by
transient terahertz spectroscopy. The visible pump-THz probe spectroscopy
measurement shows clear biexponential decays with two characteristic time
constants. According to the pump-power and temperature dependence, these two
characteristic time constants are attributed to the electron phonon coupling
(1-4 ps) and anharmonic decay of hot coupled phonons to electronic uncoupled
phonons (2-9 ps), respectively. An anomalous electron-optical phonon coupling
reduction and a bottleneck slowing of hot optical phonons relaxation are
observed with higher excitation intensities similar to that in graphene. On the
other hand, the electron-optical phonon coupling can be enhanced due to the
phonon frequency broadening and softening at elevated lattice temperature.
Furthermore, the transient THz spectrum response is strongly modified by the
phonon assisted intraband absorption of hot carriers from a pure electronic
Drude model, which is evidenced by a characteristic THz absorption dip in the
transient THz absorption spectrum. This absorption dip is pinned by the
discrete optical phonon energy that assists the intraband transition enabled by
photoexcitation of hot carriers.
