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High-order harmonic generation (HHG), resulting from the interaction of an intense laser field with an atomic or molecular gas, has been of great importance to the study of ultrafast dynamics for more than two decades. In the last several years, HHG has been observed in condensed matter systems driven by intense mid-infrared lasers. Investigations of HHG from solids can offer new capabilities for studying electronic structure and ultrafast carrier dynamics in photo-excited materials. However, HHG from solids is not yet well-understood, and even the generation mechanism cannot be uniquely determined in many systems. In this paper, we experimentally investigate HHG driven in solids by a high-power femtosecond optical parametric amplifier, producing mid-IR driving pulses with tunable central wavelength and >10 μJ pulse energy. We generate coherent high order harmonic radiation in ZnO and Si crystals, and characterize the dependence of the harmonic spectrum on the 3D crystal orientation. We further compress the driving pulse duration to below three optical cycles and investigate the resulting high-order harmonic spectrum. Moreover, we investigate the potential to generate harmonics in novel materials with the goal of probing the ultrafast dynamics arising from strong-field photo-excitation in such materials.
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