Asymmetry in the excitonic Wannier-Stark ladder: A mechanism for the stimulated emission of terahertz radiation

Using an excitonic basis, we calculate the coherent intraband response of a photoexcited undoped semiconductor superlattice in combined along-axis static and terahertz electric fields. We find that the terahertz field drives the amplitude of the excitonic Bloch oscillations, yielding an intraband polarization that continues to oscillate at times much longer than the intraband dephasing time---an effect which is totally absent for noninteracting electron-hole pairs. Furthermore, these driven, Bloch-oscillating excitons are found to produce stimulated emission at terahertz frequencies as long as the Bloch oscillation amplitude is driven.