By using both linear and nonlinear terahertz spectroscopy on epitaxial Bi and Bi1-xSbx
thin films, we systematically investigated the linear and nonlinear terahertz dynamics
of Dirac electrons. The linear terahertz transmittance was analyzed by the Drude model
up to 50 THz, and then the plasma frequency and the damping constant were evaluated
as functions of the film thickness and Sb-concentration. We found surface metallic
state for Bi ultra-thin films, while semimetal to semiconductor crossover for Bi1-xSbx
thin films. In the nonlinear terahertz spectroscopy, the terahertz transmittance increases
with increasing the field strength, which could be assigned to the carrier acceleration
along the Dirac-like band dispersion at the L point in the Brillouin zone. In addition,
we observed the terahertz-induced absorption in terahertz-pump and terahertz-probe
spectroscopy, which could be assigned to carrier generation due to Zener tunneling
in Dirac band structure. The results demonstrate that Bi-related materials are promising
candidates for future nonlinear terahertz devices.
