Fig. 1. Schematic diagrams of localized surface plasmonic nanocavities. (a) Comparison between plasmonic nanocavities and traditional optical microcavities^[15]; (b) configurations of common plasmonic nanocavities^[16]

Fig. 2. Single-layer transition metal dichalcogenide materials. (a) Crystal structure and the band structure of the MoS₂ monolayer^[25]; (b) photoluminescence spectra of MoS₂ with different number of layers^[26]

Fig. 3. Coupling of cavity-exciton system. (a) Schematic diagrams illustration of weak and strong coupling between microcavity and excitons^[44]; (b) coupling between an exciton and a nanocavity^[7]

Fig. 4. Fluorescence modulation of excitons by plasmonic nanocavities. (a) Dark-field image of 1L-MoS₂ surface deposited with both single Ag NC and OCT, and modulated photoluminescence signals^[70]; (b) schematic diagram of the coupling of Ag NC and monolayer MoS₂,_{and circular polarization degree of the exciton PL under the excitation of different circular polarized lasers^[71]; (c) schematic diagram of the experimental setup based on BFP imaging for the investigation of the coupling between gold nanorod and monolayer MoS2, and valley polarization measurements of the coupled excitons under σ⁺ and σ^- polarization excitations^[50]}

Fig. 5. Valley polarization modulation of excitons by plasmonic nanocavities. (a) Exciton multiplexer device realized by the coupling between the propagated plasmon mode in silver nanowire and the excitons in different TMD monolayers, and PL spectra for on (-100 V) and off (100 V) states in a long channel exciton transistor^[75]; (b) conceptual illustration of directional emission of the valley-polarized exciton in WS₂^[76]; (c) directional coupling efficiency of valley exciton by scanning along the y direction of nanowire with different circularly polarized lasers^[76]

Fig. 6. Strong coupling between plasmonic nanocavity and semiconductor excitons with small exciton numbers. (a) Strong plasmonic-exciton coupling between single silver nanorods and monolayer WSe₂^[85]; (b) strong coupling of gold dimer nanocavity on monolayer WS₂ with small exciton number^[66]; (c) the field distribution of in-plane x component of electric fields of bowtie nanocavity and the field distribution of out-of-plane z component of electric fields^[80]; (d) the mode volumes, ratios of in-plane field, and the effective exciton number of the coupled system as a function of the number of MoS₂ layers^[80]

Fig. 7. Researches on further compression of the exciton number (N<10). (a1) Schematic diagram of the heterostructure composed of an individual Au@Ag nanocuboids coupled to the WS₂^[67]; (a2) contour map of the multiplication factors' peak values with the point dipole excitation source placed at different positions within the middle cross-section of the monolayer WS₂^[67]; (a3) functional relationship graph of relative deviation and coupling strength^[67]; (b1) schematic diagram of the hybrid cavity structure^[69]; (b2) electric field distribution of gold bowtie structures^[69]; (b3) functional relationship graph of the volume of the nanostructure mode and the resonance energy of the dielectric layer^[69]

Fig. 8. Plasmonic nanocavity enhanced 2D quantum emitters. (a1) Low-temperature PL spectra of TMD monolayer exhibiting sharp emission lines at low laser power^[90]; (a2) second-order correlation measurement of single quantum emitter^[90]; (b1) schematic diagram of monolayer WSe₂ coupled to plasmonic Au nanocube cavity^[91]; (b2) simulation results of the cavity mode profile with four plasmonic hot spots at nanocube corners as well as the strain profile induced in the WSe₂ layer^[91]; (b3) plasmon resonance spectra together with the localized exciton spectrum of the typical quantum emitter^[91]; (b4) PL intensity of the quantum emitter and measured spontaneous emission lifetime curves^[91]; (c1) schematic diagram of coupling between the silver nanowire and quantum emitters in WSe₂ monolayer^[92]; (c2) PL intensity of the quantum emitter and measured spontaneous emission lifetime curves^[92];

Fig. 9. Chiral coupling between quantum emitter and plasmonic nanocavity. (a1) Schematic diagram of circularly polarized single-photon emission using chiral nanostructure^[96]; (a2) PL spectra of right-handed and left-handed circularly polarized single-photon emissions^[96]; (b1) polar plots of the integrated intensity of the emission measured with a half-wave plate and a linear polarizer^[96]; (b2) polar plots of the integrated intensity of the emission measured with a quarter-wave plate, a half-wave plate, and a linear polarizer^[96]; (c) schematic diagram of engineering of polarization states of intervalley defect exciton by chiral plasmonic nanocavities^[97]; (d1) polarization-resolved magneto-PL spectroscopy of quantum emitters^[97]; (d2) degree of circular polarization of quantum emitter extracted from the experimental results^[97]