LINE-SHAPE ANALYSIS OF DIFFERENTIAL TRANSMISSION SPECTRA IN THE COHERENT REGIME

We show that for a resonant optical excitation of a semiconductor with laser pulses shorter than the intrinsic dephasing time, the shape of the differential transmission spectrum can deviate strongly from the d istribution function of the nonequilibrium electron-hole pairs. The sp ectrum shows bleaching below and induced absorption above the central laser frequency. For pulses with durations greater than or similar to 100 fs, this effect has been explained previously as a dynamic Fermi e dge singularity. It is shown here that one can understand this effect as a nonequilibrium Coulomb enhancement. This explanation predicts a s imilar line shape for even shorter pulses (e.g., 30-fs duration), wher e one would not expect to see a dynamic Fermi edge singularity due to the of the induced carrier distribution.