Giant photorefractive and photoexpansion effects in a van der Waals semiconductor

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  ≈ 0.1) in crystalline arsenic trisulfide (As ₂ S ₃ ), observed even under low-intensity illumination. 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 5%, depending on the illumination intensity. Building on this photoexpansion effect, we introduce a maskless, low-cost nanopatterning technique based on continuous-wave laser writing, achieving resolutions up to 50,000 dots per inch without the need for ultrafast lasers. The combination of high photosensitivity, optical anisotropy, and transparency positions As ₂ S ₃ as a versatile platform for integrated photonics, adaptive optics, data storage, biomedical imaging, and nanoscale sensing.