| Apr 20, 2026 |
Crystalline arsenic trisulfide can be sculpted into nano-optics with a standard laser
Crystalline arsenic trisulfide shows record photorefractive response and can be laser-patterned at 500-nm resolution without cleanroom lithography.
(Nanowerk News) Researchers have discovered that crystalline arsenic trisulfide (As2S3), a van der Waals semiconductor, can be permanently reshaped at the nanoscale using only a standard continuous-wave laser. The work, published in the Proceedings of the National Academy of Sciences ("Giant photorefractive and photoexpansion effects in a van der Waals semiconductor"), was carried out by scientists at the XPANCEO Emerging Technologies Research Center in collaboration with Nobel Laureate Prof. Konstantin Novoselov of the University of Manchester and the National University of Singapore. The finding bypasses the need for cleanroom lithography or expensive femtosecond pulsed lasers.
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Key Findings
- Crystalline As2S3 exhibits a light-induced refractive index change of up to Δn ≈ 0.3, exceeding values reported for classic photorefractive crystals such as BaTiO3 and LiNbO3.
- A 532-nm continuous-wave laser wrote patterns at resolutions reaching approximately 50,000 dots per inch, with 500-nanometer spacing between points.
- The material physically expands by up to 5% under light exposure, allowing direct sculpting of functional optical elements like microlenses and gratings.
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As2S3 responds to light in two distinct and practically useful ways. First, it displays a strong photorefractive effect, meaning its refractive index — the measure of how strongly a material bends and slows light — shifts substantially when exposed to even low-intensity ultraviolet illumination. The magnitude of this shift, up to Δn ≈ 0.3, surpasses values typically associated with well-known photorefractive crystals.
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| A 532-nm continuous-wave laser “sculpted” microscopic patterns onto a flake of As₂S₃, including a monochromatic portrait of Albert Einstein (700-nm point spacing) and a QR-code-like design (600-nm point spacing). (Image: XPANCEO)
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A refractive index change this large means optical functions can be written directly into the material with strong contrast, rather than fabricated through conventional mechanical or chemical processing. Second, the material physically expands by up to 5% under illumination. This photoexpansion allows researchers to sculpt three-dimensional surface features, such as microlenses and diffraction gratings, directly onto the crystal using light alone.
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"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.
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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
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To demonstrate the resolution achievable with this approach, the team used a standard 532-nm continuous-wave laser to write a microscopic monochromatic portrait of Albert Einstein onto a flake of As2S3, using a point spacing of 700 nanometers.
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They also produced a QR-code-like pattern at a finer 600-nanometer spacing. In further tests, the researchers pushed the resolution to approximately 500 nanometers between points, equivalent to roughly 50,000 dots per inch. The written patterns exhibited strong optical contrast driven by the refractive index change, making them clearly visible under optical readout.
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Because As2S3 supports extremely fine optical patterns in a transparent format, the resulting structures are difficult to replicate. They can function as embedded identifiers — essentially microscopic optical fingerprints — for high-value goods and critical components. Creating them requires no additional hardware beyond a standard laser.
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Materials with a strong photorefractive effect are valuable wherever optical functions need to be set directly within a material rather than built through multiple lithographic steps. Practical examples include the miniature optical structures used to route signals through telecommunications hardware, the diffractive elements found in compact sensors and imaging systems, and the holographic security features embedded in documents and products.
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The combined photorefractive and photoexpansion properties of As2S3 also make it relevant to the development of ultra-wide field-of-view waveguides needed for augmented reality glasses and smart contact lenses. The material's sensitivity to light makes it a potential platform for photonic circuits and nanoscale sensors, where the ability to guide and manipulate light at very fine scales is essential.
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