Intra- and intercycle analysis of intraband high-order harmonic generation

We study intraband high-order harmonic generation arising from a band-gap material driven by a linearly polarized laser field. We factorize the intraband high-order harmonic-generation signal into intracycle and intercycle terms. The intracycle term uniquely determines the spectral characteristics whereas the intercycle term merely modulates the spectral features by imposing energy conservation in the long-pulse limit. Through analysis of the intracycle interference, the cutoff is identified, and the origin of the harmonic selection rules is revealed. Further, it is outlined how different components of the bandstructure contribute to different regions of the harmonic spectrum, giving rise to non-trivial intensity scaling of individual harmonics in the plateau region.