Generalized framework for the step response of semiconducting materials to optically switched electrical bias input in terahertz emission spectroscopy

Abstract

In this paper, a generalized framework for the step response of semiconducting materials to optically switched electrical bias input in terahertz emission spectroscopy was developed, introducing a complex response function into the frequency domain. A comprehensive formula was obtained for calculating the transient current from the poles of the response function under bias field. This formula was found to give transient currents of an exponentially saturated, overshooting, or oscillating nature for three different regimes of charge transport in an isolated energy band. Furthermore, it was clarified how the transient terahertz emission is linked to the response function and its retarded contribution. When either true current or polarization current is linear with respect to bias field and dominates the transient current, the retarded contribution is identified as the small-signal ac complex conductivity. Two different spectral examples of the retarded contribution were given for actual terahertz Bloch oscillations in biased semiconductor superlattices, describing the small-signal response of polarization current under the Wannier–Stark localization and the large-signal response of true current under interminiband mixing.