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

Abstract

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.

Effects of Grain Morphology on Nonlinear Conversion Efficiency of Random Quasi-Phase Matching in Polycrystalline Materials

“…Theoretical simulations for RQPM have been challenging because of the random grain morphology, including the size, orientation and sphericity. Earlier methods considered the fluctuation of field phase and effective nonlinear coefficient (deff) in different grains, but the grain morphology was supposed to follow simple Gaussian distribution and the deff was simply averaged [1], [7]- [10], instead of modeling the actual situation of realistic polycrystalline samples. Some basic conclusions such as the peak efficiency should fulfill the condition that the average grain size is close to the coherence length (Lcoh), the output signal grows linearly with the sample length, and RQPM possesses a broad bandwidth as well, have been widely accepted [1], [11], [12].…”

“…Figure2demonstrates several polycrystalline ZnSe models directly generated by "Neper" based on the grain growth module. There are two key factors to characterize polycrystals, the grain size and sphericity, which are both better fitted by the lognormal function[12],[17] rather than the Gaussian function[1],[2],[7], and a complete description requires both the mean value (μ) and standard deviation (σ). The size (diameter) of a grain polyhedron is defined as the diameter of a sphere of equivalent volume, and the sphericity is the ratio of the surface area of a sphere to that of the polyhedron with equivalent volume.…”

Comprehensive Model for Randomly Phase-Matched Frequency Conversion in Zinc-Blende Polycrystals and Experimental Results for ZnSe

“…The first frequency conversion in a polycrystalline semiconductor where monocrystals are embedded with random orientations was demonstrated in 1966 and the variation of an output signal in terms of intensity and polarization has been qualitatively discussed [3]. Thanks to the fact that disordered polycrystalline materials have an extremely wide acceptance bandwidth they have been used as a nonlinear gain medium for a number of applications such as nonlinear optical microscopy, autocorrelation measurements, sum and difference frequency generation, and cascade harmonic generation [4][5][6][7][8].…”

“…Namely, (1) the nonlinear conversion yield (in intensity) grows linearly with the sample thickness, (2) the highest conversion is achieved when the average grain size is close to the coherence length L c , and (3) at a fixed sample length and optimized conditions, conversion efficiency is higher for larger coherence lengths. Although there were several attempts to analytically describe the frequency conversion process based on RPM [7,8,13,14], none of them rigorously derived the probability distribution of the effective susceptibility of a randomly rotated crystal; also polarization analysis in RPM has been dismissed in these works. In this Letter, we present a model for ultrafast nonlinear χ (2) interactions in an RPM material that includes random grain orientation, realistic grain size distribution, as well as variation of the two orthogonally polarized outputs due to the randomly transformed susceptibility tensor.…”