Momentum-resolved contributions to High Harmonic Generation

Solid-state High Harmonic Generation (HHG) is governed by quantum interference between emission pathways across the Brillouin zone. The three-step model captures this through a dominant saddle-point trajectory, offering an intuitive picture where HHG emission is localized in crystal momentum. To better understand the contributions to each harmonic across the reciprocal space, we computed the momentum-resolved HHG yield across the full Brillouin zone using semiconductor Bloch equations for a hexagonal Boron Nitride monolayer. We then compared it to the dominant momentum predicted by the saddle point approximation. We find that the different contributions to each harmonic are spread across extended regions of the Brillouin zone, showing significant interference and revealing the rich k-space structure of solid-state HHG.