  The present work discusses the impact of vacancy defects in solid high-order
harmonic generation. The total energy cutoff of the high-order harmonic
spectrum increases as a function of concentration of vacancy defects, and the
total spectrum gradually turns into a single slanted spectrum without having an
abrupt transition between primary and secondary plateaus. The spectral
intensity of the below-band-gap harmonics in a solid with vacancy defects is
enhanced significantly in comparison to the harmonics in a pristine solid. The
changes in the harmonic spectra are understood in terms of their effective band
structures. The presence of vacancy defects breaks the translational symmetry
of the unit cell locally. As a consequence of this, new defect states appear,
which open additional paths for the electron dynamics. The ill-resolved
electron trajectories in the Gabor profile confirm the interference of
additional paths. Moreover, the single slanted high-order harmonic spectrum
carries a unique signature of vacancy defects in comparison to the high-order
harmonic spectrum corresponding to solids with defects such as underdoping or
overdoping.
