Wannier Representation of Intraband High-Order Harmonic Generation

We investigate the harmonic generation induced by the interaction of a laser field with a solid target. The harmonic spectra is composed of the contribution of two processes interpreted as interband and intraband transitions. The interband process corresponds to the recombination from an upper band, populated during the laser interaction, to a lower band. The intraband process originates from nonlinear processes of the current in individual bands. In this Letter, we develop a theory based on Wannier states and reveal in depth the underlying physics of intraband dynamics. In particular, this approach highlights the determinant role of transitions between different lattice wells. Furthermore, our approach provides quantitative predictions concerning high-order harmonic energy cutoffs, harmonic yields, and emission times.