Ultrafast electroabsorption at the transition between classical and quantum response.

We report the first observation of unusually large induced absorption in semiconductors strongly driven by intense ultrashort midinfrared laser fields. This ultrafast electroabsorption has the largest extent below the band edge ( approximately 1 eV) ever observed, to our knowledge, which we interpret as being a manifestation of the dynamical Franz-Keldysh effect. The electroabsorption is observable when the ponderomotive potential is comparable to the photon energy of the applied field, i.e., when the applied field is at the transition between the classical and quantum regimes.