Novoselov Team Turns Arsenic Trisulfide Into a Photosensitive Clay, Sculpting 50,000-DPI Optics With a Single Continuous-Wave Laser
A PNAS paper from XPANCEO and Nobel laureate Konstantin Novoselov shows the van der Waals crystal As2S3 undergoes a refractive-index shift of 0.3 and expands up to 7% under ordinary light, enough to write 50,000-dpi patterns without lithography.
Overview
A team led by the XPANCEO Emerging Technologies Research Center and Nobel laureate Konstantin Novoselov has reported that arsenic trisulfide (As2S3), a long-studied van der Waals semiconductor, behaves like a photosensitive clay under ordinary light, undergoing a giant change in refractive index and physically expanding enough to let researchers sculpt nanoscale optics with nothing more than a standard continuous-wave laser. The paper, titled Giant photorefractive and photoexpansion effects in a van der Waals semiconductor, was published in the Proceedings of the National Academy of Sciences and is indexed by PNAS under DOI 10.1073/pnas.2531552123.
What We Know
According to the paper’s abstract on PNAS, crystalline As2S3 shows a light-induced refractive-index change of up to Delta-n approximately 0.3, together with controllable photoexpansion of up to 7 percent. The same abstract states that these effects occur under continuous-wave illumination and enable direct laser writing of nanoscale patterns at resolutions of up to 50,000 dots per inch without pulsed lasers or lithography. The PNAS listing notes that the refractive-index response exceeds values typically cited for classic photorefractive crystals such as barium titanate (BaTiO3) and lithium niobate (LiNbO3).
The work is a collaboration between XPANCEO Research on Natural Science LLC and Professor Novoselov, who holds appointments at the University of Manchester and the National University of Singapore, as detailed in the press release distributed through EurekAlert. That release identifies XPANCEO founder and chief technology officer Valentyn Volkov as a corresponding author and quotes him describing the discovery of new functional materials within the van der Waals family as a fundamental engine for the photonics field.
To demonstrate the technique, the team used a 532-nanometer continuous-wave laser to sculpt a monochromatic portrait of Albert Einstein onto a flake of As2S3, with 700-nanometer spacing between points, as reported by ScienceDaily. ScienceDaily also describes QR-code-like patterns written with 600-nanometer point spacing and a fine-resolution regime down to roughly 500-nanometer spacing, corresponding to the 50,000-dpi figure cited in the PNAS abstract.
The trade publication Compound Semiconductor summarizes the mechanism as photorefractivity — a light-driven shift in refractive index — that occurs even under low-intensity ultraviolet illumination and permanently modifies the crystal, as reported by Compound Semiconductor. The same article contrasts the approach with conventional nanofabrication, noting that it bypasses the need for multi-million-dollar cleanroom lithography or femtosecond pulsed laser systems.
Why It Matters
Photorefractive crystals are already workhorses of holography, beam steering and adaptive optics, but the two incumbents Compound Semiconductor benchmarks against — BaTiO3 and LiNbO3 — are grown as bulk inorganic crystals and typically require elaborate poling, doping or thin-film deposition before they can be patterned. The PNAS abstract’s claim of Delta-n approximately 0.3 in As2S3 is notable because it places a van der Waals material, which can be exfoliated into atomically thin flakes, in the same performance bracket as those established workhorses.
The 7 percent photoexpansion figure from the PNAS listing is the mechanical counterpart: the crystal physically swells where the laser writes, which the EurekAlert release says allows microlenses and gratings to be sculpted directly into the surface. In effect, a single beam can both imprint optical function (the index change) and physical structure (the expansion) in one step.
According to EurekAlert, the research team points to augmented-reality waveguides, smart contact lenses, diffractive sensors, telecom routing elements and anti-counterfeiting marks as potential downstream applications. XPANCEO itself is a Dubai-based research group whose stated goal is smart contact lenses, which puts a commercial motive behind the academic paper.
The result also fits within the broader van der Waals materials program that Novoselov has pursued since his Nobel-winning graphene work, and it adds As2S3 — a compound that chemists have known for more than a century — to the list of 2D-compatible crystals with unusually strong light-matter coupling.
What We Don’t Know
The press materials do not quantify how stable the written patterns are over long periods or under repeated illumination, nor how thermal cycling or humidity affect the photoinduced index change. Source coverage also does not specify the optical loss of the patterned regions, which will determine whether the material is suitable for low-loss photonic circuits or only for surface-relief optics and security features.
The reporting chain around the paper is unusual: ScienceDaily’s write-up cites 5 percent photoexpansion while the PNAS abstract states up to 7 percent. The discrepancy may reflect different measurement conditions or simply an editorial simplification in secondary coverage, and neither source explains the gap.
Scalability beyond laboratory flakes is another open question. None of the cited reports describe wafer-scale growth of crystalline As2S3 or yield data for patterned devices, so the distance between the Einstein-portrait demonstration and a manufacturable AR waveguide or photonic chip remains to be seen. Arsenic trisulfide is also a toxic compound, and the press materials do not address how that affects handling in consumer-facing products such as contact lenses.