Researchers visualise directional coupling between light and organic molecules at the nanoscale

Nano-optics studies how light behaves when compressed to extremely small dimensions, comparable to a ping pong ball next to the size of the Earth. In this tiny universe, the rules change and the properties of light and its interaction with matter are significantly altered, allowing scientists to explore phenomena that cannot be observed at larger, everyday scales.

Researchers from the Quantum Nano-Optics group at the University of Oviedo and the Center for Research on Nanomaterials and Nanotechnology (CINN-CSIC), in collaboration with the 2D Nanophotonics group at Donostia International Physics Center (DIPC), have observed for the first time how light confined at the nanometer scale can interact directionally with the vibrations of organic molecules. The study, published in Nature Photonics, opens possibilities for manipulating chemical properties of matter (nano-chemistry), as well as for developing novel sensors capable of recognizing extremely small amounts of molecules.

The researchers from the Quantum Nano-Optics group at University of Oviedo took advantage of the unique properties of molybdenum trioxide (MoO₃), a material that can be separated into sheets only a few atoms thick. For this reason, it is known as a two-dimensional material, and it exhibits extraordinary characteristics, such as allowing light at the nanoscale to travel differently depending on the spatial direction. The same group had already shown that stacking several sheets of this material allows to control the propagation direction of light at the nanoscale. However, in this new study, the team has gone a step further, demonstrating how this nanolight can strongly interact with organic molecules (such as those found in our bodies) in a directional manner, that is, with greater intensity in some directions than in others.

This interaction occurs between the nanolight and molecular vibrations. Each molecule vibrates in a unique way, as if it had its own fingerprint, allowing for highly accurate identification. Detecting these fingerprints is the basis of many technologies, from medical diagnostics to environmental sensors.

“What we have seen is that this interaction between nanolight and molecular vibrations can be guided and intensified in certain spatial directions, and that is completely new,” explains Ana Isabel Fernández-Tresguerres Mata, co-first author of the study and recent PhD graduate in Physics from the University of Oviedo. “MoO₃ acts as an excellent platform to enhance coupling in specific directions, opening new possibilities for the development of advanced detection systems,” adds José Álvarez Cuervo, co-author of the study and predoctoral researcher in the group.

Moreover, this type of intense interaction is known as strong coupling, and it allows light and matter to exchange energy very efficiently. In this regime, extraordinary phenomena can occur, such as alterations in chemical properties. “This discovery has profound implications for chemical reactivity and molecular detection. We are taking another step toward the selective manipulation of matter at the nanometric scale,” adds Christian Lanza, predoctoral researcher and also co-first author of the article.

“Strong coupling is one of the most significant phenomena in quantum optics. Under such a strong interaction, light and matter can no longer be considered separate entities but merge to create a fundamentally new state. Our work broadens the understanding of this remarkable effect,” emphasizes Kirill Voronin, researcher at DIPC and recent PhD graduate in Physics from the University of the Basque Country.

“In the future, we want to understand better this directional strong coupling. The ultimate goal is to control individual chemical bonds using light — a form of optical nanochemistry” says Pablo Alonso González, leader of the Quantum Nano-Optics group at the University of Oviedo.