Nanophotonics relies on precise nanoscale structuring, yet conventional fabrication techniques remain complex and costly. Layered van der Waals (vdW) materials, with their intrinsic anisotropy and high refractive indices, offer a promising route toward simplified nanostructuring and tunable optical functionality. However, no vdW material has previously been shown to exhibit a strong photorefractive effect—a key requirement for light-based modulation. Here, we report a giant photorefractive response (Δ n up to 0.3) in crystalline arsenic trisulfide (As 2 S 3 ), observed at low optical intensities. In addition to refractive-index modulation, light exposure enables controlled thickness tuning of As 2 S 3 . The material exhibits a giant photoexpansion of up to 7%, depending on the illumination intensity, which may originate from light-induced generation of point defects, consistent with molecular-dynamics modeling. Building on this photoexpansion effect, we introduce a maskless nanopatterning technique based on continuous-wave laser writing, achieving ~500 nm pitch (~50,000 dpi) without the need for ultrafast lasers. The combination of high photosensitivity, anisotropy, ease of exfoliation and transfer, and optical transparency positions vdW As 2 S 3 as a practical platform for integrated photonics, adaptive optics, reconfigurable photonic elements, and dense optical encoding.
Minnekhanov et al. (2026) studied this question.