Ultrastrong light–matter coupling in two-dimensional metal–organic chalcogenolates

Abstract

Hybridization of excitons with photons to form hybrid quasiparticles—exciton–polaritons (EPs)—has been widely investigated in a range of semiconductor material systems coupled to photonic cavities. Self-hybridization occurs when the semiconductor itself can serve as the photonic cavity medium, resulting in strongly coupled EPs with Rabi splitting energies (ħΩ) of >200 meV at room temperature, which were recently observed in layered two-dimensional excitonic materials. Here we report an extreme version of this phenomenon—an ultrastrong EP coupling—in a nascent, two-dimensional excitonic system, namely, the metal–organic chalcogenolate compound called mithrene. The resulting self-hybridized EPs in mithrene crystals placed on Au substrates show Rabi splitting in the ultrastrong-coupling range (ħΩ > 600 meV) due to the strong oscillator strength of the excitons concurrent with the large refractive indices of mithrene. We further show that bright EP emission occurs at room temperature as well as EP dispersions at low temperatures. Importantly, we find lower EP emission linewidth narrowing to ~1 nm when mithrene crystals are placed in closed Fabry–Pérot cavities. Our results suggest that metal–organic chalcogenolate materials are ideal for polaritonics in the deep green-blue part of the spectrum in which strong excitonic materials with large optical constants are particularly scarce.