Creating nearly invisible wearable technologies, such as smart contact lenses and ultra-thin augmented reality (AR) glasses, requires a fundamental redesign of traditional optical components. Rather than relying on bulky lenses and hardware, researchers are exploring materials that can manipulate light at the atomic scale.
The XPANCEO team collaborated with scientists from the National University of Singapore and the Technical University of Chemistry in Prague and reported significant progress in their efforts. Their research focuses on a layered crystal called molybdenum oxychloride (MoOCl2). The crystal exhibits a set of unusual optical properties that could help significantly scale down future optical devices.
Published in nano letterthis study presents the first experimental mapping of the optical behavior of crystals. The results of this study show that MoOCl2 exhibits the strongest light-bending effect ever measured in a natural material, potentially paving the way for smaller and more capable optical technologies.
Crystals that act like metal and glass
Researchers describe MoOCl2 as a type of optical “chameleon.” The behavior changes depending on the orientation of the crystal.
When placed in one direction, it reflects light like metal. When rotated 90 degrees, it becomes transparent like glass. This unusual property stems from its extreme optical anisotropy, whose properties change dramatically depending on direction.
The crystal also has an in-plane birefringence value of approximately 2.2, allowing it to split and bend light with great efficiency. In the case of XPANCEO, this could potentially allow materials thousands of times thinner than human hair to perform the advanced light control needed for AR displays.
A rare optical slowing effect discovered in visible light
The researchers also identified a rare near-zero epsilon point at 512 nm (green light).
At this point, some of the optical response of the material is almost zero. As a result, the speed of light effectively slows down and the electric field within the crystal becomes stronger. This combination greatly enhances the interaction between light and matter.
For integrated photonic chips, this effect may be particularly valuable. Enhanced light-matter interactions could enable faster data processing while significantly reducing power consumption.
Why scientists are interested in MoOCl2
MoOCl2 has been studied by physicists for several years because of its unusual electronic structure.
This material is classified as a “bad metal” and contains one-dimensional chains of molybdenum atoms. These chains allow electrons to move more easily in one direction than another. As a result, the crystal behaves like a metal along one axis and like a dielectric along the vertical axis, giving rise to its very strong anisotropy.
Previous research published in science and nature communications The researchers had already observed tightly confined light waves called hyperbolic plasmon polaritons traveling through crystals. These experiments showed that MoOCl2 can guide light in a highly directional and unexpected manner.
But an important piece of the puzzle was still missing. Although scientists have been able to observe optical effects, they have never directly measured the complete optical constants of a material. Without these measurements, designing practical devices based on crystals remained much more difficult.
Mapping the optical properties of crystals
A new study provides the missing measurements.
The researchers found that one component of the crystal’s optical response approaches zero around 512 nanometers in the green region of the visible spectrum. In practice, this strengthens the electric field within the material, slowing down the speed of light, compressing electromagnetic energy into a very small volume, and facilitating light-matter interactions.
This phenomenon is known as the visible epsilon near zero (ENZ) point. While many materials exhibit ENZ behavior only in the deep UV or mid-infrared region, MoOCl2 reaches this state in the visible spectrum. This is particularly important since many existing technologies such as lasers, microscopes, cameras, and sensing systems already operate in this range.
“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 study provides the experimental foundation needed to understand why this material behaves the way it does and to design more confidently based on it. This could be a valuable scientific result for the field and has implications for highly miniaturized integrated systems, including compact polarization optics, nonlinear devices, and, in the long term, smart contact lenses.”
Future optical hardware shrinks
The detailed optical map also highlights the material’s potential for further miniaturization of optical technology.
MoOCl2 acts as a natural hyperbolic medium due to its strong structural anisotropy. Simply put, this allows light to pass through the crystal in highly directional nanoscale paths without being diffracted (or scattered). This is a key requirement for building smaller optical circuits.
Its ability to operate in the visible spectrum further strengthens its appeal for integrated photonic chips that need to route, filter, and focus light within very small spaces.
The researchers point to several potential applications. These include ultra-thin broadband polarizers that control the direction of light in compact optical systems and subdiffractive waveguides that can direct light into smaller spaces than traditional optics.
The discovery also suggests potential in nonlinear nanophotonics, where strong light-matter interactions can be used to create new colors of light or process optical signals more efficiently.
