  We theoretically investigate the piezo-optic effect of high-harmonic
generation (HHG) in shear-strained semiconductors. By focusing on a typical
semiconductor, GaAs, we show that there is optical activity, meaning different
responses to right-handed and left-handed elliptically polarized electric
fields. We also show that this optical activity is more pronounced for higher
harmonics whose perturbative order exceeds the band-gap energy. These findings
point to a useful pathway for strain engineering of nonlinear optics to control
the reciprocity of HHG.
