Excitonic Bloch oscillations in a terahertz field

Abstract

We present a formalism for calculating the coherent response of a semiconductor superlattice in combined static and terahertz along-axis electric fields. The method is based upon the semiconductor Bloch equations in a basis of single-particle Wannier-Stark ladder states. We employ this formalism to calculate, for the first time, the short-pulse absorption spectrum of excitons in a superlattice in combined static and terahertz electric fields. We find that for terahertz fields with frequencies much less than the Bloch oscillation frequency, the absorption peaks are shifted by approximately ${eF(t}_{o})nd,$ where d is the superlattice period, n is the Wannier-Stark ladder index of the peak, and ${F(t}_{o})$ is the value of the terahertz field at the time when the optical pulse reaches the sample. For terahertz fields with frequencies near the Bloch oscillation frequency, the shifts are qualitatively similar, but of the opposite sign and are generated via mixing of the optical and terahertz frequencies.