Ultrastrong Coupling of Band-Nested Excitons in Few-Layer Molybdenum Disulphide

The 2D transition-metal dichalcogenides (2D TMDCs) are an intriguing platform for studying strong light-matter interactions because they combine the electronic properties of conventional semiconductors with the optical resonances found in organic systems. However, the coupling strengths demonstrated in strong exciton-polariton coupling in the 2D TMDCs remain much lower than those found in organic systems. In this paper, a new approach is taken by utilizing the large oscillator strength of the above-band gap C exciton in few-layer molybdenum disulphide (FL-MoS2). A k-space Rabi splitting of 293 meV is shown when coupling FL-MoS2 C excitons to surface plasmon polaritons at room temperature. This value is 11% of the uncoupled exciton energy (2.67 eV or 464 nm), approximate to 2x what is typically seen in the TMDCs, placing the system in the ultrastrong coupling regime. The results take a step toward finally achieving the efficient quantum coherent processes of ultrastrong coupling in a CMOS-compatible system-the 2D TMDCs-in the visible spectrum.