Excitonic Bloch oscillations in a terahertz field

We present a formalism for calculating the coherent response of a semicondu ctor superlattice in combined static and terahertz along-axis electric fiel ds. The method is based upon the semiconductor Bloch equations in a basis o f single-particle Wannier-Stark ladder states. We employ this formalism to calculate, for the first time, the short-pulse absorption spectrum of excit ons in a superlattice in combined static and terahertz electric fields. We find that for terahertz fields with frequencies much less than the Bloch os cillation frequency, the absorption peaks arts Shifted by approximately eF( t(0))nd, where dis the superlattice period, n is the Wannier-Stark ladder i ndex of the peak, and F(t(0)) is, the value of the terahertz field at the t ime when the optical pulse reaches the sample, For terahertz fields with fr equencies near the Bloch oscillation frequency, the shifts are qualitativel y similar, but of the opposite sign and are generated via mixing of the opt ical and terahertz frequencies. [S0163-1829(99)08807-4].