Researchers at the XPANCEO Emerging Technologies Research Center, in collaboration with Nobel Prize winner Professor Konstantin Novoselov (University of Manchester and National University of Singapore), have discovered an unusual optical behavior in the van der Waals semiconductor crystal arsenic trisulfide (As2S3). Their findings show that this material can be permanently changed by light and shaped even at the nanoscale using simple continuous wave (CW) light. This approach eliminates the need for costly cleanroom manufacturing and sophisticated femtosecond laser systems.
The key concept behind this discovery is the refractive index, which describes how much a material bends or slows light. Materials with a high refractive index are better at confining and directing light within a device. Light can also change this property of certain materials. This effect, known as photorefraction, occurs when the refractive index changes when exposed to light.
In crystalline As2S3, this reaction occurs even under low-intensity ultraviolet light. In this study, very large refractive index changes (up to Δn ≈ 0.3) are reported, exceeding those typically observed in well-known photorefractive materials such as BaTiO3 and LiNbO3.
Why strong photorefractivity is important for technology
Materials that respond strongly to light in this way are extremely useful because optical functionality can be written directly into the material. Rather than relying on multiple mechanical or manufacturing steps, the light itself can define how the device processes and directs the light.
This feature is important for many everyday technologies. This supports the creation of small structures to guide signals in telecommunications systems, enables compact optical components used in sensors and imaging devices, and enables the formation of hologram-like features used for product authentication and security.
Nanoscale optical patterns and “optical fingerprints”
For As2S3, this effect is particularly strong at very small scales. The large change in refractive index allows the formation of extremely fine patterns that remain embedded in the transparent material. These patterns serve as unique optical identifiers that are difficult to duplicate, making them useful for anti-counterfeiting and traceability applications.
To demonstrate this precision, the researchers used a standard laser to create a microscopic black-and-white portrait of Albert Einstein in a thin section of material, with dots spaced just 700 nanometers apart. Further experiments showed that even finer resolution (approximately 50,000 dots per inch, equivalent to 500 nanometers between points) can be achieved with this technique. The resulting pattern exhibits strong optical contrast due to the light-induced refractive index change, making it easy to detect with optical methods.
The future of light-driven materials and photonics
“The discovery of new functional materials, especially within unique van der Waals crystal families, is a fundamental engine for advancing the entire field of photonics. The development of advanced optical devices, such as advanced smart contact lenses, requires a very strong foundation in fundamental materials science. It’s a complex challenge. In these systems, the material itself is the key element 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 technologies that are powered entirely by light rather than electricity.” said Valentin Volkov, founder and chief technology officer of the XPANCEO Emerging Technology Research Center.
Crystal expansion enables new optical devices
In addition to patterning, As2S3 physically changes when exposed to light. The material expands by up to 5%, allowing researchers to form optical structures such as microlenses and diffraction gratings directly on its surface. These features are important for building wide-field waveguides used in augmented reality glasses and smart contact lenses.
Due to the excellent responsiveness of this material, it is also expected to be used in optical circuits and nanoscale sensors. Taken together, these properties represent a significant advance in light control and manipulation for next-generation technologies.
