Time-resolved THz dynamics in thin films of Bi₂Se₃

Topological insulators represent a new state of matter where bulk insulators exhibit metallic surfaces that are protected by time reversal symmetry. However, in common transport measurements residual bulk conductivity hinders the properties of the topologically protected surfaces. Alternatively, terahertz spectroscopy has recently been shown to be capable of discerning the bulk and surface carrier conductivity. Here we use time-resolved optical pump-THz probe spectroscopy at low temperatures to study the hot carrier response of thin films of Bi₂Se₃ of several thicknesses and separate the bulk from the surface response. We find that for thinner films the photo excitation changes the transport scattering rate and reduces the surface conductivity. For thicker films, this process competes with the photoinduced increase in bulk conductivity, which occurs on shorter timescales and scales with the increase in both the film thickness and optical excitation fluence. These different dynamics of the surface and bulk electrons indicate a decoupling of surface and bulk carriers, and present the possibility of accessing long-lived surface photo-carriers for optoelectronic applications.