Angle-Dependent Photoinduced Changes of Near-Infrared Transmission in Amorphous Selenium Films

Amorphous selenium (a-Se) is a promising semiconductor for a variety of photoconductive applications, predominantly in X-ray detection. In addition, material properties introduce several other potential applications in nonlinear optics. Photodarkening (PD) presents an interesting area of study; when exposed to band-region light, metastable structural changes induce a shift in the band edge and increase tail absorption. In this work, we investigate these effects utilizing a near-infrared (NIR) probe to avoid generating any darkening during beam transmission. We observe an unexpected result; here, we present the effects of band region exposure in NIR transmission (1570 nm). We observe a shift from darkening to lightening as the angle of incidence changes and compare PD with a red probe (633 nm), which does not exhibit this same phenomenon. We discuss the possible mechanisms and the next steps in delving into the physics underlying this behavior. These effects have strong implications for applications in nonlinear optics, memory, and sensing in the NIR.