Enhancement of Second-Harmonic Generation and Modulation of Excitonic Emission from Monolayer WS2 in Au Nanobipyramid Plasmonic Nanocavities

Plasmonic nanocavities provide an effective platform for enhancing nonlinear light−matter interactions in two-dimensional transition metal dichalcogenides (TMDs), with potential applications in compact on-chip nonlinear light sources. However, the simultaneous enhancement of nonlinear optical conversion and modulation of excitonic emission within a single hybrid structure remains insufficiently understood. Here, we report an Au nanobipyramid-on-mirror nanocavity integrated with monolayer WS2. The sharp nanobipyramid tips and nanoparticle−mirror plasmonic coupling strongly confine the fundamental field within the nanogap, producing an area-normalized second-harmonic generation (SHG) enhancement factor of up to ∼7522. Polarization-resolved measurements further reveal an anisotropic SHG response arising from the interplay between the intrinsic crystalline symmetry of monolayer WS2 and the polarization-sensitive field confinement of the nanocavity. More importantly, excitation-power-dependent measurements reveal a transition from trion-dominated to exciton-dominated emission. This spectral redistribution likely results from the combined effects of localized electromagnetic-field confinement, excitation-induced carrier-density variations, and local photothermal heating at the metal−semiconductor interface, which together modify exciton−trion interconversion and shift the emission balance toward neutral excitons. These combined nonlinear and excitonic responses make the Au NBP/WS2/Ag nanocavity particularly promising as a hybrid nanomaterial platform for compact on-chip frequency-doubling light sources with excitation-tunable excitonic output.