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In the framework of time-dependent density functional theory, we obtainhigh-order harmonics of photon energies up to 10 Up from bilayer crystals withan interlayer spacing d = 70 {\AA}. At grazing incidence, a cleardouble-plateau structure is observed in the harmonic spectrum. The photonenergy of the second plateau far beyond atomic-like harmonics can be wellexplained by the inclusion of backscattering of ionized electrons. Ab initiosimulations reveal that the cutoff of the second plateau is continuouslyextended with an increasing d. Our classical calculations predict that themaximum electronic kinetic energy is linearly dependent on d over a wide range.Moreover, the harmonic yield in the second plateau is significantly enhanced byincreases in the wavelength of the driving laser. Owing to the confinedspreading of the electronic wave packet, a beneficial wavelength scaling of{\lambda}2.85 is obtained. This study therefore establishes a novel andefficient way of producing high-energy light source based on layerednanostructures.

Electron-Backscattering-Assisted High Harmonic Generation from Bilayer Nanostructures