  The dependence of high-harmonic generation (HHG) on laser ellipticity is
investigated using a modified ZnO model. In the driving of relatively weak
field, we reproduce qualitatively the ellipticity dependence as observed in the
HHG experiment of wurtzite ZnO. When increasing the field strength, the HHG
shows an anomalous ellipticity dependence, similar to that observed
experimentally in the single-crystal MgO. With the help of a semiclassical
analysis, it is found that the key mechanism inducing the change of ellipticity
dependence is the interplay between the dynamical Bloch oscillation and the
anisotropic band structure. The dynamical Bloch oscillation contributes
additional quantum paths, which are less sensitive to ellipticity. The
anisotropic band-structure make the driving pulse with finite ellipticity be
able to drive the pairs to the band positions with larger gap, which extends
the harmonic cutoff. The combination of these two effects leads to the
anomalous ellipticity dependence. The result reveals the importance of
dynamical Bloch oscillations for the ellipticity dependence of HHG from bulk
ZnO.
