Relaxation Dynamics Of The Optically Driven Nonequilibrium States In The Electron- And Hole-Doped Topological-Insulator Materials (Bi1-Xsbx)(2)Te-3

We report on time-resolved mid-infrared-pump terahertz-transmission-probe studies of the topological insulator materials (Bi1-xSbx)(2)Te-3, in which by varying x charge carriers are chemically tuned to be of n-type or p-type. Relaxation dynamics is found to be different in various aspects for transitions below or above the band gap, which are selectively excited by changing the pump-pulse energy. For the below-band-gap excitation, an exponential decay of the pump-probe signals is observed, which exhibits linear dependence on the pump-pulse fluence. In contrast, the relaxation dynamics for the above-band-gap excitation is characterized by a compressed exponential decay and nonlinear fluence dependence at high pump flunences, which reflects interaction of the excited nonequilibrium states.