Figure 1.

Upper panel represents the photoexcitation step, taking place on the femtosecond time scale (for details see text) by (a) single-photon and (b) multiphoton absorption of quanta with the energy hν, or (c) tunneling of carriers from the valence band (VB) to the conduction band (CB) through a barrier formed by an intense light-field F_el. Here, the potentials from two neighboring atoms/ions in a lattice, separated by a lattice period d, are schematically depicted as a function of the spatial coordinate x. The lower panel depicts the intraband carrier relaxation (red arrow) and photoemission (magenta arrow) proceeding on picosecond time scale, which is independent of the pumping process.