Scientists from the XPANCEO Emerging Technologies Research Center, in collaboration with Nobel Laureate Prof. Konstantin Novoselov (University of Manchester and the National University of Singapore), have discovered novel optical properties in arsenic trisulfide (As2S3), a crystalline van der Waals semiconductor. The results of the research reveal that this material can be permanently modified by light and physically sculpted at the nanoscale level using simple continuous-wave (CW) light, entirely bypassing the need for complex, multi-million dollar cleanroom lithography or expensive femtosecond pulsed lasers.
To connect this capability to a familiar concept, consider the refractive index, a key property that measures how much a material causes light to bend or slow down. The higher the index, the better a material is at trapping and guiding light through a device. Photorefractivity refers to a change in refractive index when light interacts with a material, and this response can be triggered in crystalline As2S3 even under low-intensity UV illumination. In the reported study, crystalline As2S3 shows an unusually large light-induced refractive-index change (up to Δn ≈ 0.3), which is higher than values typically cited for classic photorefractive crystals such as BaTiO3 or LiNbO3.
Materials with a strong photorefractive effect are valuable in applications where light can directly "set" an optical function inside a material, rather than manufacturing it through many mechanical steps. In real-world terms, this mechanism enables components that shape and steer light in everyday technology: the tiny optical structures that help route light through telecom hardware, the diffractive elements used in compact sensors and imaging systems, and the hologram-like optics used for security features on products and documents where the optical pattern itself becomes the identifier.
In As2S3, this approach extends to a much finer scale. The unusually strong refractive-index modulation helps explain why the material can support extremely fine "optical fingerprints" in a transparent format. Such patterns are difficult to reproduce and can act as identifiers for anti-counterfeiting and traceability, from high-value goods to critical components. To demonstrate this precision, the scientific team used a standard laser to "sculpt" a microscopic monochromatic portrait of Albert Einstein onto a flake of the material using a 700-nanometer spacing between points. In separate tests, the researchers showed that the technique can go even finer (to ~50,000 dots per inch, which corresponds to 500 nanometers between points), with strong optical contrast coming from the light-driven change in refractive index, making the written pattern stand out clearly under optical readout.
"The discovery of new functional materials, particularly within the unique family of van der Waals crystals, is the fundamental engine for moving the entire field of photonics forward. Developing sophisticated optical devices, such as advanced smart contact lenses, is a deeply complex challenge that requires a solid foundation in fundamental materials science. In these systems, the material itself is the key component that determines what is physically possible. By identifying natural crystals with this level of sensitivity, we are effectively providing the essential building blocks for a new generation of technology that is driven entirely by light rather than electricity." – Valentyn Volkov, Founder and Chief Technology Officer at the XPANCEO Emerging Technologies Research Center
The true potential of As2S3 lies in its multifunctionality for broader optical hardware. Its ability to physically expand by up to 5% under exposure to light allows researchers to "sculpt" optical elements, such as microlenses and gratings, directly into the surface of the material. These properties are fundamental to the creation of the ultra-wide field-of-view waveguides required for immersive augmented reality glasses and smart contact lenses. Beyond wearables, the sensitivity of the material positions it as a candidate for photonic circuits and nanoscale sensors, marking a significant leap in our ability to guide and manipulate light.
