Giant photorefractive and photoexpansion effects in a van der Waals semiconductor

<title>Abstract</title> 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<italic>n</italic> ≈ 0.1) in crystalline arsenic trisulfide (As₂S₃), observed even under low-intensity illumination. In addition to refractive index modulation, light exposure enables controlled thickness tuning of As₂S₃. 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 As₂S₃ as a versatile platform for integrated photonics, adaptive optics, data storage, biomedical imaging, and nanoscale sensing.