Dynamic Terahertz Response in the Three-Dimensional Dirac Semimetal Cd₃As₂ by Ultrafast Optical Excitation

Three-dimensional (3D) Dirac semimetal, as 3D analogues of graphene, has great potential toward electronic and optoelectronic device applications with extreme performance benefiting from its topological protected state of Dirac Fermions. It is crucial to understand the behaviors of excited Dirac Fermions. Here, by using the low-photon-energy terahertz (THz) probe, we have measured the transient transmission spectra and dynamics of the photoexcited carriers that are Dirac-Fermi-like approaching the Dirac point, on a nano-film sample of 3D Dirac semimetal Cd₃As₂. Negative pump induced transmission is observed which is mainly attributed to pump excited photocarrier absorption of THz. This response is very sensitive and nonlinear increasing with the pump power. The relaxation dynamics can be illustrated by two time constants both of which are in picosecond time scale, and increasing with the pump power and lattice temperature, while the characteristics of THz transmission are decreasing with the temperature. This can be explained by two-temperature model. The Drude model has been used to get the kinetic parameters of Dirac Fermions in Cd₃As₂.