Scientists sculpt Einstein onto a crystal using only light

A key concept behind this discovery is the refractive index, which describes how much a material bends or slows light. Materials with higher refractive indices are better at confining and directing light within devices. In certain materials, light can also change this property. This effect, known as photorefractivity, occurs when exposure to light alters the refractive index.

In crystalline As₂S₃, this response happens even under low-intensity ultraviolet light. The study reports an exceptionally large change in refractive index (up to Δn ≈ 0.3), which exceeds the values typically observed in well-known photorefractive materials such as BaTiO₃ or LiNbO₃.

Why Strong Photorefractivity Matters for Technology

Materials that respond strongly to light in this way are highly useful because they allow optical functions to be directly written into the material. Instead of relying on multiple mechanical or manufacturing steps, light itself can define how a device handles and directs light.

This capability is important for many everyday technologies. It supports the creation of tiny structures that guide signals in telecommunications systems, enables compact optical components used in sensors and imaging devices, and allows the formation of hologram-like features used in product authentication and security.

Nanoscale Optical Patterns and "Optical Fingerprints"

In As₂S₃, the effect is especially powerful 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 act as unique optical identifiers that are difficult to replicate, making them useful for anti-counterfeiting and traceability applications.

To demonstrate this precision, the researchers used a standard laser to create a microscopic monochrome portrait of Albert Einstein on a thin piece of the material, with points spaced just 700 nanometers apart. Further experiments showed that the technique can achieve even finer resolution (to ~50,000 dots per inch, which corresponds to 500 nanometers between points). The resulting patterns show strong optical contrast because of the light-induced refractive index changes, making them easy to detect with optical methods.

Light-Driven Materials and the Future of 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 patterning, As₂S₃ also changes physically when exposed to light. The material can expand by as much as 5%, allowing researchers to directly form optical structures such as microlenses and diffraction gratings on its surface. These capabilities are important for building wide field-of-view waveguides used in augmented reality glasses and smart contact lenses.

The material's responsiveness also makes it promising for use in photonic circuits and nanoscale sensors. Together, these properties represent a significant step forward in controlling and manipulating light for next-generation technologies.