Formation of the solid-state high-order harmonic generation plateau through destructive interference

In frequently studied two-band models for solid-state high-harmonic generation, interband harmonics in principle can range from the minimum to the maximum bandgap. However, it is known that a laser-intensity dependent cutoff exists that may be well below the maximum bandgap unless the laser intensity is so high that the electrons explore the entire Brillouin zone. We show that this laser-intensity dependent cutoff is formed by destructive interference of the emission of electrons starting at different initial states in the Brillouin zone. The calculations in this work are for Su-Schrieffer-Heeger chains but our findings apply to other two-band systems as well. Only when the sampling of the Brillouin zone is fine enough or, equivalently, a finite chain is long enough in position space, the destructive interference is complete and forms the cutoff. For coarser sampling and shorter chains all harmonics between minimum and maximum bandgap are emitted. A time-frequency analysis shows how certain trajectories are responsible for the formation of the cutoff.