Room Temperature Photodarkening and Recovery of BaGa₄Se₇Crystal

In this study, we investigated the photodarkening and recovery dynamics of a nonlinear optical crystal, barium gallium selenide (BaGa₄Se₇ or BGSe). We studied three different BGSe crystals of different orientations and stoichiometries, including a stoichiometric crystal with a (010) orientation, a randomly oriented stoichiometric crystal, and a Ba-poor randomly oriented crystal, using UV-Vis spectroscopy under monochromatic LED excitations. Upon exposure to LEDs of wavelengths ranging from 660 nm to 375 nm, all samples showed the strongest transmission loss at 475 nm, corresponding to the band-edge excitation (~2.6 eV). The randomly oriented samples, regardless of their stoichiometry, exhibited nearly four times the effect of photodarkening compared to the (010) oriented crystal, indicating optical anisotropy plays a significant role in determining the extent of light-induced defect creation. These results reveal a strong orientation dependence of photo-induced absorption and recovery dynamics in BGSe, providing insight into its optical stability under visible illumination.