Exceptionally slow dephasing of electronic continuum states in a semiconductor.

The coherent excitation of excitonic and continuum Wannier-Stark states in a semiconductor superlattice with femtosecond laser pulses leads to a modulation of both the interband and the intraband polarization. The field dependence of the Bloch oscillation frequency of the nonlinear interband polarization is shown to differ strongly from that of the intraband polarization. We find that the observed modulation of the intraband polarization results from the superposition of electronic continuum states. This implies that the intraband dephasing time of these continuum states is exceptionally longe