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High harmonic generation (HHG) in solids has been identified as a promising mechanism for light source generation and for spectroscopy of materials. HHG from bulk solids, however, often suffers from nonlinear propagation effects, resulting in a loss of spectral coherence and the skewing of spectroscopic measurements. Here, we study HHG in epitaxial ZnO thin films grown on Al₂O₃substrates using atomic layer deposition. We find that the HHG emission consists of narrow spectral peaks, in contrast to those seen in bulk, and that the dependence of the harmonic yield on the film thickness differs for above-gap and below-gap harmonics, which can be understood from analytical models based on the linear and nonlinear response of the medium. The measured harmonic spectra depend qualitatively on the preparation of the films, with as-grown films generating even harmonic orders, which are absent in annealed films. The results are interpreted using transmission electron microscopy measurements, which indicate different morphologies for the as-grown and annealed films. 

We study high-order harmonic generation from epitaxial ZnO films grown on Al₂O₃substrate. We observe a saturation of the harmonic intensity for a film thickness of 30 nm. 

We study the carrier-envelope phase (CEP) dependence of high-order harmonics generated in bulk ZnO crystals. The CEP dependence measured for laser polarization oriented parallel to and perpendicular to the c-axis exhibit different periodicities.