  We theoretically investigated the dependence of higher-order harmonic
generation (HHG) in solid-state materials on the ellipticity of the electric
field. We found that in the multiphoton absorption and ac Zener regimes, the
intensity of HHG monotonically decreases with increasing ellipticity of the
incident electric field, while in the semimetal regime, the intensity reaches a
maximum for finite values of ellipticity. Moreover, the characteristics of the
polarization of the emitted HHG change depending on the field intensity; only
parallel emissions with respect to the major axis exist in the multiphoton
absorption and ac Zener regimes, while both parallel and perpendicular
emissions exist in the semimetal regime. These peculiar characteristics of the
semimetal regime can be understood on the basis of changes in the HHG mechanism
and may be able to be identified in experiments utilizing solid-state materials
such as narrow-gap semiconductors.
