Abstract
Monte-Carlo simulations were performed on the efficiency of random quasi-phase matching (RQPM) nonlinear optical frequency conversion in polycrystalline materials to study the detailed effects of the statistical grain morphology. The second harmonic generation (SHG) process was taken for example based on random structures generated by a realistic grain growth model, and the effective nonlinear coefficient was rigorously derived by tensor transformation in randomly rotated coordinates. The effects of average grain size, standard deviation, and sphericity on SHG efficiency were figured out to conclude that the grain-size deviation plays the most important role in RQPM efficiency and the optimal grain size is smaller than the coherence length. The impacts of fundamental wavelength variation were also investigated. These results reveal how the RQPM process is affected by statistical and nonlinear optical laws, which enrich the RQPM theory and provide criteria for polycrystalline material processing in specific nonlinear applications.
Highlights
Random quasi-phase matching (RQPM) nonlinear optical frequency conversion that occurs in disordered polycrystalline nonlinear medium is under increasing attention, as it neither requires a specific material orientation nor depends on a certain polarization combination [1]–[6]
Monte-Carlo simulations were performed on the efficiency of random quasiphase matching (RQPM) nonlinear optical frequency conversion in polycrystalline materials to study the detailed effects of the statistical grain morphology
Rigorous and accurate simulations based on numerous realistic polycrystalline models with lognormal grain-size distribution were performed, for the first time as far as we know, to study the effects of grain morphology on RQPM second harmonic generation (SHG) conversion efficiency
Summary
Random quasi-phase matching (RQPM) nonlinear optical frequency conversion that occurs in disordered polycrystalline nonlinear medium is under increasing attention, as it neither requires a specific material orientation nor depends on a certain polarization combination [1]–[6]. 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 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]. Some conclusions that have taken for granted from stereotypes of QPM theory, e.g., the optimal grain size for a specific nonlinear interaction, is clarified in this paper, which should be helpful in designing efficient RQPM devices
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