Ultrafast electroabsorption at the transition between classical and quantum response

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