Ultrafast bipolar conductivity driven by intense single-cycle terahertz pulses in a topological insulator Bi₂Se₃

We present the measurement of ultrafast terahertz (THz) conductivity of a 50 nm thick topological insulator Bi₂Se₃ at low temperature 5 K by using a THz pump and THz probe spectroscopy scheme. The THz conductivity driven by a single-cycle intense THz pump exhibits a bipolar lineshape and frequency-dependent relaxation dynamics due to the separated surface and bulk charge carriers with distinctly different scattering rates and relaxation times. We also demonstrate THz pump field-dependent excitation of the surface and bulk spectral weights, which suggests one of the optimal THz excitation conditions for generating the most charge carriers from the topology-protected surface state relative to the bulk. This also allows THz control of surface and bulk transport channels in a selective way. (paper)