The proposal is based on 'crystalline polaritons,' hybrid excitations that combine properties of light and matter. The study, published in Physical Review Letters, is led by Johannes Feist and Francisco José García Vidal, from the Department of Theoretical Condensed Matter Physics and the Center for Physics of Condensed Matter (IFIMAC) at UAM. The team also includes Carlos José Sánchez Martínez and Frieder Lindel, the paper's first author (currently at ETH Zurich, Switzerland).
The phenomenon of strong coupling, where light and matter rapidly exchange energy, leads to hybrid light-matter states. The new work extends this principle to two periodic lattices spread across a surface: a metasurface with repeating nanostructures and another composed of quantum emitters (atoms, molecules, or defects in materials). When both lattices share the same periodicity and their excitations are in resonance, crystalline polaritons emerge.
The authors developed a theoretical method to describe this system, translating the metasurface's electromagnetic behavior into a quantum model. Applied to lattices of silver and silicon nanospheres, calculations suggest that one emitter per unit cell is sufficient in the silicon configuration to achieve strong coupling, using elements available with current technology, although precise emitter placement presents an experimental challenge.
Crystalline polaritons exhibit a nonlinear optical response, enabling quantum effects even with few excitations. The efficiency for generating entangled photon pairs is estimated to be at least fourteen orders of magnitude higher than in conventional nonlinear metasurfaces, requiring significantly less intense lasers.
"Our results show that metasurfaces, previously seen as simple elements for shaping classical light, can become a versatile platform for studying and controlling periodic quantum matter," explains Johannes Feist. "Treating light and matter as equal partners allows us to design very different crystalline polaritons simply by changing the lattice geometry or the emitter's position within it."
Future possibilities include extending the model to denser lattices and exploring chiral light modes and topological states, with potential applications in sensors and quantum devices.
