  In the framework of time-dependent density functional theory, we obtain
high-order harmonics of photon energies up to 10 Up from bilayer crystals with
an interlayer spacing d = 70 {\AA}. At grazing incidence, a clear
double-plateau structure is observed in the harmonic spectrum. The photon
energy of the second plateau far beyond atomic-like harmonics can be well
explained by the inclusion of backscattering of ionized electrons. Ab initio
simulations reveal that the cutoff of the second plateau is continuously
extended with an increasing d. Our classical calculations predict that the
maximum electronic kinetic energy is linearly dependent on d over a wide range.
Moreover, the harmonic yield in the second plateau is significantly enhanced by
increases in the wavelength of the driving laser. Owing to the confined
spreading of the electronic wave packet, a beneficial wavelength scaling of
{\lambda}2.85 is obtained. This study therefore establishes a novel and
efficient way of producing high-energy light source based on layered
nanostructures.
