Interference-Induced Phenomena in High-Order Harmonic Generation from Bulk Solids

We consider a quantum mechanical model for the high-order harmonic generation in bulk solids. The bandgap is assumed to be considerably larger than the exciting photon energy. Using dipole approximation, the dynamical equations for different initial Bloch states are decoupled in the velocity gauge. Although there is no quantum mechanical interference between the time evolution of different initial states, the complete harmonic radiation results from the interference of fields emitted by all the initial (valence band) states. In particular, the suppression of the even-order harmonics can also be viewed as a consequence of this interference. The number of the observable harmonics (essentially the cutoff) is also determined by interference phenomena.