  High harmonic generation (HHG) from gas phase atoms (or molecules) has opened
up a new frontier in ultrafast optics, where attosecond time resolution and
Angstrom spatial resolution are accessible. The fundamental physical pictures
of HHG are always explained by the laser-induced recollision of particle-like
electron motion, which lay the foundation of attosecond spectroscopy. In recent
years, HHG has also been observed in solids. One can expect the extension of
attosecond spectroscopy to the condensed matter if a description capable of
resolving ultrafast dynamics is provided. Thus, a large number of theoretical
studies have been proposed to understand the underlying physics of HHG. Here,
we revisit the recollision picture in solid HHG and show some challenges of
current methods with particle perspective, and present the recently developed
wave perspective Huygens-Fresnel picture in understanding dynamical systems
within the ambit of strong-field physics.
