A team of scientists has uncovered a crystal that can be reshaped and programmed using ordinary light, opening a new path for building optical technology.
Researchers at the XPANCEO Emerging Technologies Research Center, working alongside Nobel Laureate Prof. Konstantin Novoselov (University of Manchester and the National University of Singapore), have identified unusual optical behavior in arsenic trisulfide (As2S3), a crystalline van der Waals semiconductor. Their work shows that this material can be permanently altered by light and even shaped at the nanoscale using simple continuous-wave (CW) light. This approach eliminates the need for expensive cleanroom lithography or advanced femtosecond laser systems.
Understanding Refractive Index and Photorefractivity
A key property behind this discovery is the refractive index, which determines how much a material bends or slows light. Materials with higher refractive indices are better at guiding light through optical systems. In some cases, light itself can change this property. This effect is called photorefractivity, where exposure to light modifies the refractive index.
Crystalline As2S3 shows this effect even under low-intensity ultraviolet light. In the study, the material exhibited a very large light-induced refractive index change (up to Δn ≈ 0.3). This is significantly higher than what is typically reported for well-known photorefractive materials such as BaTiO3 or LiNbO3.
Writing Optical Functions Directly With Light
Materials with strong photorefractive responses allow optical functions to be created directly inside the material using light. Instead of relying on complex manufacturing steps, light can define how the material interacts with optical signals.
This capability supports a wide range of applications. It can be used to form tiny structures that guide light in telecommunications equipment, create diffractive elements for sensors and imaging systems, and produce hologram-like features for security and authentication, where the optical pattern itself serves as a unique identifier.
Nanoscale Patterns and Optical Fingerprints
In As2S3, these effects operate at extremely fine scales. The strong change in refractive index enables the formation of highly detailed patterns embedded within the transparent material. These patterns can act as “optical fingerprints” that are difficult to replicate, making them useful for anti-counterfeiting and traceability.
To demonstrate this level of control, researchers used a standard laser to “sculpt” a microscopic monochromatic portrait of Albert Einstein on a thin flake of the material, with points spaced 700 nanometers apart. Additional experiments showed even higher resolution (to ~50,000 dots per inch, which corresponds to 500 nanometers between points). The patterns show strong contrast because of the light-induced refractive index change, allowing them to be clearly read using optical methods.
Light-Driven Materials and Future Photonics
“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,” said Valentyn Volkov, Founder and Chief Technology Officer at the XPANCEO Emerging Technologies Research Center
Expanding Crystals Enable New Optical Devices
Beyond changing its optical properties, As2S3 also undergoes physical expansion when exposed to light. The material can expand by up to 5%, which allows researchers to directly form structures such as microlenses and gratings on its surface.
These capabilities are important for developing wide field-of-view waveguides used in augmented reality glasses and smart contact lenses. In addition, the material’s sensitivity makes it a strong candidate for photonic circuits and nanoscale sensors. Together, these properties represent a significant advance in the ability to control and manipulate light for future technologies.
Reference: “Giant photorefractive and photoexpansion effects in a van der Waals semiconductor” by Anton A. Minnekhanov, Georgy A. Ermolaev, Alexey P. Tsapenko, Ilia M. Fradkin, Gleb I. Tselikov, Adilet N. Toksumakov, Aleksandr S. Slavich, Arslan B. Mazitov, Sergey A. Smirnov, Nikita D. Orekhov, Ivan A. Kruglov, Sergei A. Ivanov, Ilya P. Radko, Andrey A. Vyshnevyy, Aleksey V. Arsenin, Kostya S. Novoselov and Valentyn S. Volkov, 27 March 2026, Proceedings of the National Academy of Sciences.
DOI: 10.1073/pnas.2531552123
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