Relaxation dynamics of the optically driven nonequilibrium states in the electron- and hole-doped topological-insulator materials $(Bi_{1-x}Sb_{x})_{2}Te_{3}$

TL;DR

This study investigates the ultrafast relaxation dynamics in topological insulators $(Bi_{1-x}Sb_{x})_{2}Te_{3}$, revealing distinct behaviors for below- and above-bandgap excitations and how doping influences these processes.

Contribution

It provides new insights into the charge carrier relaxation mechanisms in doped topological insulators under different optical excitations.

Findings

Below-bandgap excitation shows exponential decay with linear fluence dependence.

Above-bandgap excitation exhibits compressed exponential decay and nonlinear fluence dependence.

Relaxation dynamics vary significantly with doping and excitation energy.

Abstract

We report on time-resolved mid-infrared-pump terahertz-transmission-probe studies of the topological-insulator materials $(Bi_{1-x}Sb_{x})_{2}Te_{3}$, 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 bandgap, which are selectively excited by changing the pump-pulse energy. For the below-bandgap 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-bandgap excitation is characterized by a compressed exponential decay and nonlinear fluence dependence at high pump flunences, which reflects interaction of the excited nonequilibrium states.