Fig. 6: Momentum-space interpretation for HHG in solid materials.
From: Attosecond science based on high harmonic generation from gases and solids
The microscopic mechanism for solid-state HHG showing two major channels semiclassically. The laser field promotes an electron from valence to conduction band leaving behind a hole in the valence band, as shown by the vertical solid line. Both the electron and hole are accelerated in their respective bands. Channel 1: the nonlinear intraband current in the conduction band can radiate. Channel 2: the electron can recombine with the hole at a later time, as shown by the dashed line, and emit a high-energy photon. Adapted with permission from ref. 14, Copyright 2019, Springer Nature.
