Researchers break light symmetry with simple materials

Light typically interacts with a material the same way whether it enters through the front or the back - like polarized sunglasses that work the same from either side. Cornell researchers have demonstrated a simple route to breaking that symmetry, opening new possibilities for photonics and quantum information processing.

Optical reciprocity, the principle that a system responds identically regardless of which side faces the light, underlies most lenses, mirrors and other optical devices. A new study published July 27 in Nature Materials demonstrates that some materials can be engineered to exhibit nonreciprocal absorption and emission of linearly polarized light.

"Imagine window blinds with sunlight coming through its horizontal slats, but from the opposite side, the same blinds let light through as if the slats were vertical, completely inverted," said corresponding author Richard Robinson, professor of materials science and engineering in the Cornell Duffield College of Engineering. "To get this type of behavior, you typically need complex metamaterials or external magnetic fields, but we show that it can be achieved in simple, solution-processed semiconductor nanoclusters."

The discovery emerged from years of work in Robinson's lab developing "magic-size clusters" - nanomaterials that self-assemble into precisely organized spiral structures and can produce thin films with light-bending properties.

Some light waves oscillate in a straight line, known as linear polarization, while others rotate like a corkscrew, creating circular polarization. When carefully processed into thin films, the magic-size clusters interact unusually strongly with both forms of polarized light, simultaneously exhibiting strong linear and chiral dichroism.

That combination allowed Thomas Ugras, lead author and doctoral student, to identify a previously overlooked interaction between the two optical effects. By studying the mathematics describing how polarized light interacts with matter, he realized that materials with both interactions of comparable magnitude should exhibit a directional asymmetry.

The idea had been overlooked because circular optical effects are usually much weaker than linear ones, leading researchers to assume that any directional asymmetry would be too small to observe. The Cornell researchers showed that this directional asymmetry phenomenon is possible, demonstrating the behavior in films made from cadmium sulfide, cadmium selenide and cadmium telluride, implying the effect may be broadly accessible across a range of materials.

Ugras said the directional dependence could be used for applications such as nonreciprocal image generation.

"If you control and pattern a material onto a substrate, varying its chiral handedness and orientation spatially, it's possible to create a film that appears unique depending on if viewed from the front or back," Ugras said. "To illustrate this, we show a film that would read 'Y' from the front and 'N' from the back. This concept could easily be expanded with spectral variation or with emission, for instance to create intricate holograms that have unique appearances."

Beyond image generation, the discovery could enable compact optical components that route information differently depending on direction, as well as new approaches to optical encryption and polarization-based quantum technologies.

"What's really exciting here is that the same functionality can emerge from the mixing of optical effects in comparatively simple materials," Robinson said. "The surprising part is not that we discovered a new material with a new property, but that this novel optical response can emerge in materials that researchers already know how to make. That realization opens a much broader landscape for designing nonreciprocal photonic systems."

The research was funded by the U.S. National Science Foundation, the Spanish Ministry of Science and the European Regional Development Fund. A portion of the work was performed in the Cornell Center for Materials Research.

Syl Kacapyr is associate director of marketing and communications for Duffield Engineering.