  High-order harmonic generation (HHG) is a fundamental process which can be
simplified as the production of high energetic photons from a material
subjected to a strong driving laser field. This highly nonlinear optical
process contains rich information concerning the electron structure and
dynamics of matter, for instance, gases, solids and liquids. Moreover, the HHG
from solids has recently attracted the attention of both attosecond science and
condensed matter physicist, since the HHG spectra can carry information of
electron-hole dynamics in bands and inter- and intra-band current dynamics. In
this paper, we study the effect of interlayer coupling and symmetry in
two-dimensional (2D) material by analyzing high-order harmonic generation from
monolayer and two differently stacked bilayer hexagonal boron nitrides (hBNs).
These simulations reveal that high-order harmonic emission patterns strongly
depend on crystal inversion symmetry (IS), rotation symmetry and interlayer
coupling.
