Spectrum and Carrier Envelope Phase Dependence of High-order Harmonics in Bulk and Thin Film ZnO

High order harmonic generation(HHG) in solids has been proposed as a way to measure band structure, study electron dynamics in solids, and produce attosecond pulses. However, this requires a detailed understanding of the microscopic electron dynamics underlying HHG in solids, which can be obscured by nonlinear propagation effects due to propagation in the bulk crystal. Here, we compare the carrier envelope phase(CEP) dependence and harmonic spectra of HHG in bulk and thin film ZnO using few cycle, mid-infrared pulses. We demonstrate that for a beam incident on both the a-plane and c-plane of the bulk ZnO, the harmonics give a clear CEP dependence consistent with the crystal symmetry. The thin film however showed only a weak and qualitatively different CEP dependence which can be explained using perturbative nonlinear optics. These results are in agreement with previous studies of HHG in bulk crystals, and indicate that nonlinear propagation has a significant impact on the CEP dependence in HHG in addition to the spectral and polarization properties of the emitted harmonics.