High harmonics from backscattering of delocalized electrons

We demonstrate with a chain of atoms [1] that backscattering of laser-driven electrons can extend the cutoff for high harmonic generation (HHG) in solid-state like samples with broken translational symmetry [2]. Matching the spatial scale (here the chain length) with the laser-driven electron excursion maximizes the high harmonic yield. Our quantum results are corroborated by a refined semiclassical trajectory model we formulate. In addition, we demonstrate that other situations of broken translational symmetry also lead to high harmonics enhanced by coherent backscattering. Contrary to atoms or molecules, where backscattering hardly contributes to HHG due to a lack of delocalized electrons, our findings provide a generic mechanism to enhance HHG in solid-state environments: through backscattering induced by suitable kinds of broken translational symmetry.