Room-temperature ultrastrong exciton-plasmon coupling in WS2 monolayer synthesized with a multi-singular metasurface

Abstract Monolayer semiconductors exemplified by two-dimensional transition-metal dichalcogenides (TMDs) have promised next-generation atomically thin optoelectronics. Boosting their interactions with light is vital for practical applications, especially in the quantum regime where ultrastrong interaction is highly demanded but not yet realized. Here we report ultrastrong exciton-plasmon coupling in a tungsten disulfide (WS2) monolayer loaded with a multi-singular plasmonic metasurface at room temperature. Different from seeking perfect metals or high-quality resonators, we create a unique type of cold-etched metasurface with multiple singularities. Multiple plasmonic hotspots that support tightly confined gap plasmons fit into the coherent area of WS2 excitons and thus trigger the ultrastrong exciton-plasmon coupling with a normalized coupling strength of 0.104. As an application demonstration, we showed dispersive polariton-enhanced and tunable second-order nonlinearities in the WS2 monolayer. Our findings reveal room-temperature extreme light-matter interactions in TMD monolayers for future applications in nonlinear optics, quantum information processing, laser physics and others.