This study presents a significant advancement in the field of photonics by exploring how the thickness and quality of epitaxially grown zinc oxide (ZnO) thin films impacts yield scaling of a solid-state high harmonic (sHHG) source.

It is well-established that high harmonics generated by a mid-infrared driving laser in bulk crystals can be reabsorbed and nonlinear effects can reshape the laser pulse. These propagation effects cause the yield of sHHG to not scale with material thickness, presenting a challenge for using sHHG as a light source in applications needing spectrally tunable, coherent light from visible to deep-UV. Addressing this issue, Journigan and coauthors have thoroughly investigated how the sHHG spectra and yield depend on the thickness of carefully prepared ZnO films. Besides finding an optimal thickness to optimize yield for their parameters and sample geometry, a key discovery of their research is that the harmonic spectra differ based on film preparation methods. Notably, as-grown films produce even harmonic orders, which are absent in annealed films. Transmission electron microscopy shows morphological differences between these films, indicating a transition from monocrystalline a-plane ZnO in as-grown films to a polycrystalline ZnAl₂O₄ spinel phase in annealed films. This indicates that sHHG can discern between polar and nonpolar phases, making it a promising in-situ method for characterizing epitaxial film growth. This advancement opens new avenues for using HHG in probing crystal growth and strain engineering, offering crucial insights for the future of photonics and materials science and guiding the optimization of sHHG-based light sources.

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