  An accurate analytic model describing high-harmonic generation (HHG) in
solids is derived. Extensive first-principles simulations within a
time-dependent density-functional framework corroborate the conclusions of the
model. Our results reveal that: (i) the emitted HHG spectra are highly
anisotropic and laser-polarization dependent even for cubic crystals, (ii) the
harmonic emission is enhanced by the inhomogeneity of the electron-nuclei
potential, the yield is increased for heavier atoms, and (iii) the cutoff
photon energy is driver-wavelength independent. Moreover, we show that it is
possible to predict the laser polarization for optimal HHG in bulk crystals
solely from the knowledge of their electronic band structure. Our results pave
the way to better control and optimize HHG in solids by engineering their band
structure.
