Cascade random-quasi-phase-matched harmonic generation in polycrystalline ZnSe

We experimentally and numerically investigated the cascade random-quasi-phase-matched harmonic generation in polycrystalline zinc selenide with tightly focused mid-infrared nanosecond laser pulses. We observed a sharp transition between two distinct intensity scaling regimes as a function of pump energy from sub IN to super IN, where I is the pump intensity and N is the harmonic order. To gain an understanding of the observations of this complex nonlinear process, we carried out simulations of the coupled three-wave mixing equations with random grain sizes and orientations. We found a grain size dependence of the effective phase-matching condition, suggesting that manipulation of grain size can be an effective pathway to phase-matching optimization. This approach may provide a framework for effectively enhancing high harmonic generation and other wave mixing effects in polycrystalline materials.