THz generation and the detection on the Dirac-cone surface states in topological insulator Bi$_2$Se$_3$

A terahertz (THz) wave is generated from the (001) surface of cleaved Bi$_2$Se$_3$ and Cu-doped Bi$_2$Se$_3$ single crystals, using femtosecond pulses of 800 nm. The generated THz power is strongly dependent on the carrier concentration of the crystals, which can be explained by considering the absorption of both surface and bulk states altogether. In particular, the Dirac-cone surface states in Bi$_2$Se$_3$ significantly affect the THz emission efficiency. Therefore, the THz radiation from topological insulators can be used to ascertain the existence and characteristics of the Dirac-cone surface states.