Arsenic trisulfide (As₂S₃), a crystalline semiconductor, can literally be sculpted at the nanoscale by a simple continuous laser, without requiring clean rooms or complicated systems. This property opens the way to optical technologies that are very easy to manufacture, supplanting current electronic-based technologies.
It becomes possible to sculpt the material directly without elaborate fabrication steps. One can thus create structures that guide light for telecommunications, diffractive elements for sensors and imaging, or holographic patterns for security. An optical fingerprint can also serve as a unique identifier that is very difficult to reproduce.
A 532 nm continuous laser sculpted microscopic patterns on a nugget of As₂S₃, including a monochrome portrait of Albert Einstein (dots spaced 700 nm apart) and a pattern resembling a QR code (dots spaced 600 nm apart).
Credit: XPANCEO At the nanoscale, the details are remarkably fine. The researchers wrote an Einstein portrait with dots spaced 700 nanometers apart, even achieving a resolution of 50,000 dots per inch (500 nanometers between dots). These patterns, which exhibit high contrast due to the change in refractive index, are easily readable by conventional optical methods.
But light does not only alter the optical properties of the crystal; it also physically causes it to expand. As₂S₃ can swell by up to 5%, which allows for the direct formation of microlenses and gratings on its surface. This capability is valuable for developing wide-field-of-view waveguides, usable in augmented reality glasses and even smart contact lenses.
The key to this phenomenon lies in the refractive index, which measures a material's ability to bend light. Under the influence of light, this index can change. In As₂S₃, the change is enormous (up to 0.3), far surpassing that of other known materials. This result was achieved with low-intensity ultraviolet light, making the process very accessible.
These discoveries could well mark a turning point in photonics. As Valentyn Volkov, Chief Technology Officer of the XPANCEO Research Center, explains, the discovery of new functional materials is the engine of photonic innovation. These natural crystals, with their exceptional sensitivity, provide the building blocks for a technology driven by light rather than electricity.
Refringence and photorefractivity
The refractive index is a fundamental property of optical materials: it determines the speed of light in the medium and the deviation of light rays. The higher this index, the more light is slowed down and bent. In some materials, light itself can modify this index: this is the photorefractive effect. This effect is used to create holograms or optical storage devices.
Arsenic trisulfide (As₂S₃) exhibits a giant photorefractive effect: its index can change by up to 0.3 when exposed to ultraviolet light. That is ten times more than conventional photorefractive materials like lithium niobate. Such an amplitude allows for the engraving of highly contrasted patterns with simple continuous light, without expensive pulsed lasers.
This discovery opens paths for the design of optical components. By locally modifying the refractive index, one can create waveguides, diffraction gratings, or lenses directly within the material. Applications range from telecommunications to sensors, including augmented reality.