The Coulomb coupling between Wannier-Stark excitons and the continuum of lower-lying states in an electrically biased superlattice leads not only to pronounced Fano resonances, but has also a heavy impact on the nonlinear polarization dynamics. The strong variation of line broadening in linear spectra with the electric field, however, is not reflected in the dephasing rate. Moreover, increasing the excitation density leads to a linear growth of the time-resolved four-wave-mixing signal instead of a cubic dependence. We present a theory based upon the Born-Markov equations of a superlattice. The unusual polarization dynamics can be traced back to the interference resulting from the simultaneous occupation of different Wannier-Stark subbands and the relaxation of these nonequilibrium distributions.

• Received 30 May 2003

DOI:https://doi.org/10.1103/PhysRevB.68.125325