Exciton-polaritons — the hybrid quasiparticles of excitons and photons — have the potential to serve as a tunable, nonlinear platform for studying topological phenomena. However, due to limitations in materials, experimental observations using an exciton-polariton platform have so far been unable to enter the nonlinear condensation regime. A photonic topological insulator can guide photons to interfaces designed within the material while preventing light from scattering through the material. Using this property, topological insulators can compel many photons to behave coherently, as if they were a single photon. A polariton lattice that behaves like a photonic topical insulator, developed by a research team at Rensselaer Polytechnic Institute (RPI), could make it easier for scientists to study the topological effects of nonlinear behavior in light-matter interactions. A graphical rendering of the photonic topological insulator developed in the study. Courtesy of Rensselaer Polytechnic Institute. The researchers used halide perovskites with a valley Hall lattice design to create a polariton lattice that operates in a strong light-matter interaction regime. The lattice-based topological insulator can operate at room temperature without any external magnetic field and can be used to explore topological phenomena without the need for bulky, expensive equipment. “The photonic topological insulator we created is unique,” professor Wei Bao said. “It works at room temperature. This is a major advance. Previously, one could only investigate this regime using big, expensive equipment that supercools matter in a vacuum. Many research labs do not have access to this kind of equipment, so our device could allow more people to pursue this kind of basic physics research in the lab.” To fabricate the device, the researchers followed the technique that is used to build semiconductor chips, layering different kinds of materials to form a structure with the desired properties. The researchers grew ultrathin plates of halide perovskites made of cesium, lead, and chlorine (CsPbCl3) and etched a polymer with a pattern on top of the perovskite layer. They sandwiched the perovskite and polymer layers between sheets of oxide materials, forming a lattice about 2 μm thick and 100 μm in length and width. The polariton lattice features a large bandgap of 18.8 million electron volts and exhibits strong nonlinear behavior, with clear long-range spatial coherence across the critical pumping density. When the researchers shined a laser on the device, a triangular pattern appeared at the interfaces designed in the material, indicating the topological characteristic of the laser. The photonic topological insulator can be used as a quantum simulator to study quantum effects. “Being able to study quantum phenomena at room temperature is an exciting prospect,” Shekhar Garde, dean of the RPI School of Engineering, said. “Professor Bao’s innovative work shows how materials engineering can help us answer some of science’s biggest questions.” Additionally, the photonic topical insulator may help improve efficiency in lasers. “It is also a promising step forward in the development of lasers that require less energy to operate, as our room-temperature device threshold — the amount of energy needed to make it work — is seven times lower than previously developed low-temperature devices,” Bao said. The parameters and material composition of the photonic topical insulator can potentially be tailored to study topological phenomena related to other interquasiparticle interactions. The research was published in Nature Nanotechnology (www.doi.org/10.1038/s41565-024-01674-6).
