Photodarkening in crystalline BaGa4Se7

Abstract

Nonlinear chalcogenide single crystals play a key role in mid-infrared photonics, enabling efficient frequency conversion and high-power laser operation. In this work, we report room-temperature photodarkening (photochromism) in the chalcogenide crystal BaGa4Se7. Optical transmission measurements with polarized light indicate that the band edge varies by 55–75 meV as the sample is rotated by 90°, due to the anisotropy of the monoclinic structure. When illuminated with a 475 nm LED, near the direct bandgap of ∼2.62 eV, the transmission drops by 5%–8%, depending on the crystallographic orientation. The crystals recover their transmission at room temperature with stretched-exponential kinetics and a time constant of 10–20 h, consistent with carriers escaping from traps with a range of activation energies rather than from a single well-defined barrier.