Line-shape analysis of differential transmission spectra in the coherent regime

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Abstract
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 distribution function of the nonequilibrium electron-hole pairs. The spectrum shows bleaching below and induced absorption above the central laser frequency. For pulses with durations ≳ 100 fs, this effect has been explained previously as a dynamic Fermi edge singularity. It is shown here that one can understand this effect as a nonequilibrium Coulomb enhancement. This explanation predicts a similar line shape for even shorter pulses (e.g., 30-fs duration), where one would not expect to see a dynamic Fermi edge singularity due to the smoothness of the induced carrier distribution.