Long Wavelength Infrared Acousto-optic Crystal Hg2Br2: Growth Optimization and Photosensitivity Investigation

ABSTRACT Hg2Br2 crystals have drawn much attention in recent years due to their excellent acousto-optic properties in the field of spectral imaging and optical processing. The growth of high-quality Hg2Br2 crystals is critical for the characterization of crystal properties and the fabrication of acousto-optic devices. Herein, high-quality Hg2Br2 crystals with the size of 35 mm in diameter and 40 mm in length were successfully grown by physical vapor transport method. The properties of Hg2Br2 crystals before and after irradiation were systematically investigated. The as-grown Hg2Br2 crystals possess ultra-large birefringence of 0.97 and high optical transmittance of 70% in the wide wavelength range from 470 nm to 30 μm. However, the transparent Hg2Br2 crystals will darken gradually under the irradiation of light shorter than 450 nm, which origins from the photochemical decomposition of halides. Compared with the as-grown Hg2Br2 crystals, the darkened crystals have relatively narrow-band transmission and lower transmittance. Besides, the optical bandgap and the birefringence of darkened crystals also decrease. However, these properties could be restored after annealing at 160 °C for 72 hours. Our study indicates that Hg2Br2 crystal is a potential medium for far-infrared acousto-optic devices, and the decrease in optical performance caused by photosensitivity could be restored by annealing.