To realize the vision of "invisible" wearable optoelectronics such as smart contact lenses and ultrathin AR glasses, traditional, bulky optical components must be reinvented at the atomic level. XPANCEO researchers, alongside collaborators from the National University of Singapore and the University of Chemistry and Technology, Prague, have taken a major step toward this goal by uncovering the extraordinary properties of the layered crystal molybdenum oxychloride MoOCl2.
Published in Nano Letters , the study provides the first experimental map of the material that possesses the strongest light-bending effect ever recorded in a natural substance, offering a direct path to miniaturizing high-performance optical hardware. The research reveals that MoOCl2 is a sort of "chameleon" in the world of physics. Due to its extreme optical anisotropy, this crystal's function depends entirely on its orientation. Oriented one way, the crystal reflects light like a metal; turned by 90 degrees, it becomes transparent like glass. With an in-plane birefringence value of about 2.2, the crystal can split and bend light with unprecedented efficiency. For XPANCEO's development, this means the complex light manipulation required for AR displays can now be achieved using materials thousands of times thinner than the diameter of a human hair.
The team also identified a rare epsilon-near-zero point at 512 nm (green light). At this threshold, the material's optical response drops to nearly zero, causing light to "slow down" and the electric field inside to strengthen. This phenomenon is a game-changer for integrated photonic chips, as it allows for faster light–matter interactions and high-speed data processing with significantly lower power consumption.
From observing to engineering
The physics community has been watching MoOCl2 closely for a reason. Classified as a "bad metal," the material contains one-dimensional chains of molybdenum atoms that let electrons move more freely in one direction than in the other. This results in a metallic response along one crystal axis and a dielectric response in the perpendicular direction, giving the material its unusually strong anisotropy.
Earlier studies, including research published in Science and Nature Communications , had already visualized tightly confined light waves known as hyperbolic plasmon polaritons moving through the crystal. Those experiments demonstrated that MoOCl2 could guide light in unexpected and highly directional ways. However, the field was still missing a more fundamental piece. Researchers could observe the effects, yet the exact optical constants behind them had not been directly measured, making design work much less certain.
Additionally, the new measurements revealed that near 512 nanometers, in the green part of the spectrum, one part of the material's optical response drops close to zero. In practice, that can strengthen the electric field inside the material and slow light down, compressing electromagnetic energy into a very small volume and increasing light–matter interactions. This threshold is known as a visible-light epsilon-near-zero, or ENZ, point. While many materials reach ENZ conditions in the deep ultraviolet or mid-infrared, MoOCl2 does so in the visible range, where many lasers, microscopes, cameras, and sensing systems already operate.
"Observing a phenomenon is the first step, but engineering requires precise numbers," said Dr. Valentyn Volkov, founder and CTO of XPANCEO and corresponding author of the study. "By rigorously measuring the complete dielectric tensor of MoOCl2, our work provides the experimental foundation needed to understand why this material behaves the way it does and to design around it with greater confidence. That makes it a valuable scientific result for the field, with possible relevance across compact polarization optics, nonlinear devices, and, in the longer term, highly miniaturized integrated systems including smart contact lenses."
Squeezing optics onto a chip
The precise mapping of these properties points to further miniaturization in optical hardware. Because of its extreme structural anisotropy, the material acts as a natural "hyperbolic" medium. In simple terms, this lets the crystal efficiently guide light in highly directional nanoscale rays without diffracting (or scattering): a hard requirement for shrinking optical circuits.
Furthermore, its operation in the visible range makes MoOCl2 a promising candidate for integrated photonic chips, where light must be routed, filtered, and concentrated in tiny spaces. The researchers outline possible applications. These include ultrathin broadband polarizers, which help to filter and control the direction of light in compact optical systems, and sub-diffractional waveguides, which can channel light through spaces smaller than those allowed by conventional optics. The work also points to enhanced nonlinear nanophotonics, where intense light–matter interactions can be used to generate new colors of light or process optical signals more efficiently.
