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

We experimentally studied cascade random-quasi-phase-matched harmonic generation in polycrystalline zinc selenide with a mid-infrared nanosecond laser. Observations show a sudden transition between two scaling regimes, below I\textasciicircum N and above I\textasciicircum N respectively, where I is the pump intensity and N is the harmonic order. To understand this phenomenon, we simulated the coupled three-wave mixing equations that govern the process on a domain of randomly-oriented, randomly-sized grains. We found that the phase-matching condition varies with grain size, indicating that grain size manipulation can be used as a pathway for phase-matching optimization of various wave mixing effects in polycrystalline materials.