  We theoretically investigate high-harmonic generation in hexagonal boron
nitride with linearly polarized laser pulses. We show that imperfect
recollisions between electron-hole pairs in the crystal give rise to an
electron-hole-pair polarization energy that leads to a double-peak structure in
the subcycle emission profiles. An extended recollision model (ERM) is
developed that allows for such imperfect recollisions, as well as effects
related to Berry connections, Berry curvatures, and transition-dipole phases.
The ERM illuminates the distinct spectrotemporal characteristics of harmonics
emitted parallel and perpendicularly to the laser polarization direction.
Imperfect recollisions are a general phenomenon and a manifestation of the
spatially delocalized nature of the real-space wave packet, they arise
naturally in systems with large Berry curvatures, or in any system driven by
elliptically polarized light.
