  Recent studies have demonstrated that the polarization states of high
harmonics from solids can differ from those of the driving pulses. To gain
insights on the microscopic origin of this behavior, we perform one-particle
intraband-only calculations and reproduce some of the most striking
observations. For instance, our calculations yield circularly polarized
harmonics from elliptically polarized pulses that sensitively depend on the
driving conditions. Furthermore, we perform experiments on ZnS and find partly
similar characteristics as reported from silicon. Comparison to our
intraband-only calculations shows reasonable qualitative agreement for a
below-band-gap harmonic. We show that intraband dynamics predict depolarization
effects for higher field strengths. For harmonics above the band gap, interband
dynamics become important and the high-harmonic response to elliptical
excitation looks systematically different. Our work proposes a method to
distinguish between different high-harmonic generation mechanisms and it could
pave the way to compact solid-state high-harmonic sources with controllable
polarization states.
