A large Rabi splitting (∼145 meV) is demonstrated in a plasmonic nanocavity coupled to a WS2 monolayer at room temperature. The nanocavity is composed of a silver nanocube and a silver film with an Al2O3 spacer of a few nanometers, which belongs to a nanoparticle on mirror (NPoM) type. The surface plasmon resonance (SPR) of the nanocavity can be tuned by controlling the thickness of nanogap and the size of silver nanocubes, which allows to successively adjust the SPR to accurately match the exciton energy of WS2 monolayers (2.02 eV). A mode splitting can be clearly observed from the dark-field scattering spectrum of the single hybrid nanocavity, which is ascribed to a strong coupling between the nanocavity mode and the excitonic mode. Furthermore, the anticrossing curves of the hybrid system are obtained by recording the scattering spectra with varied sizes of silver nanocubes, which further validate the interaction regime. It presents a strong coupling platform for two-dimensional monolayers, which is of potential applications of the development of hybrid nanostructure devices.

The enhanced second-harmonic generation (SHG) from a monolayer WS2 coupled to a plasmonic nanocavity is experimentally and theoretically investigated. The nanocavity is comprised of monodispersed Ag nanocubes separated from an Ag film by a spacer Al2O3, namely, the nanoparticle on mirror (NPoM) system. When the surface plasmon polariton resonance (SPPR) wavelength of NPoM nanocavity overlaps well with the SHG wavelength of the monolayer WS2 (namely, harmonic resonance), a ∼300-fold SHG enhancement is achieved in experiment. For theoretical understanding, the quantum mechanical density matrix method has been used to develop a theory for SHG. It is found that the SHG intensity of nanohybrid is proportional to the square of the local-field intensity in NPoM nanocavity at SHG wavelength, which is ascribed to the dipole–quadrupole interaction between dipole, P SHG, in the monolayer WS2 and quadrupole, Q Ag, in Ag nanocavity. It is significantly different from that in metal nanoparticles under harmonic resonance, which is proportional to the local-field intensity. Therefore, it provides a novel mechanism for enhancing SHG signals from metal–semiconductor nanohybrids, which has potential applications in nonlinear devices and hybrid nonlinear metasurfaces.

Herein, we demonstrate a highly tunable enhancement and switching of nonlinear emission from all-inorganic metal halide perovskites based on an asymmetrically biased metal− insulator−semiconductor (MIS) structure. We achieve 2 orders of magnitude enhancement of the two-photon-pumped photoluminescence (TPL) from CsPbBr 3 microplates with the MIS structure, due to comprehensive effects including localized field effect, trapfilling effect, and collection enhancement. In particular, taking advantage of electric-field-induced passivation/activation of Br vacancies, we realize highly tunable TPL enhancement, ranging from ∼61.2-fold to ∼370.3-fold. Moreover, we demonstrate an efficient modulation of the two-photon-pumped lasing from the MIS structure, which exhibits electric field induced switching with a high on/off ratio of 67:1. This work has opened new avenues for steering carrier transport and nonlinear emission in lead halide perovskites, which shows great promise for realizing high-efficiency and tunable nonlinear nanophotonic devices.