METHOD FOR OBSERVING BLOCH OSCILLATIONS IN THE TIME DOMAIN

We suggest an experimental method which should, at least in principle, be capable of measuring Bloch oscillations directly in the time domain. The method consists of measuring the spontaneous photon-echo signal in a time-resolved four-wave-mixing experiment on a semiconductor superlattice. We calculate the third-order nonlinear polarization of an idealized superlattice in the presence of a homogeneous electric field applied along the growth direction. In the limits of both vanishing and infinite fields, the echo signal produced by the nonlinear polarization should be independent of the delay time tau between the exciting pulses, if irreversible dephasing processes are disregarded. For finite fields, the echo signal should exhibit a modulation periodic in tau, with the periodicity of the Bloch oscillations. If observed experimentally, this would represent a direct manifestation of Bloch oscillations in the time domain.