Detection of light-induced band gaps by ultrafast femtosecond pump and probe spectroscopy

We use the semiconductor Bloch equations formulated for full bands with a semiempirical description of the density- and energy-dependent dephasing and relaxation in order to analyze the proposed resonant 5 fs pump-probe experiments. For slightly negative delay times a modulation of the probe beam absorption or the differential transmission spectrum is obtained due to the band Mollow triplet caused by coherent band mixing and the resulting light-induced gaps both in the valence and conduction bands. This ultrafast femtosecond experiment with presently available 5 fs pulses would provide a rather direct observation of a basic prediction for the interaction of a strong coherent light beam with the band electrons in semiconductors and would complement recent observations of light-induced gaps in nonlinear spectroscopy.