v 1 2 3 Ju n 19 99 Femtosecond Coherent Dynamics of the Fermi Edge Singularity – Exciton Hybrid

We study theoretically the coherent nonlinear optical response of doped semiconductors with partially occupied subbands. When the Fermi energy of an occupied subband approaches the exciton level of an upper subband, the absorption spectrum acquires a characteristic double–peak shape originating from the interference between the Fermi–edge singularity and the exciton resonance. We demonstrate that, for the off–resonant pump excitation, the pump/probe spectrum undergoes a dramatic transformation in the coherent regime, with a strong time–dependent exchange of oscillator strength between Fermi edge singularity and exciton peaks. We show that this effect originates from the many–body electron–hole correlations which determine the dynami-cal response of the Fermi see. To calculate the nonlinear absorption spectrum, we use the coupled cluster method combined with time–dependent Schrieffer– Wolff transformation. Possible experimental applications are discussed.