# Principle and Applications of the Coupling of Surface Plasmons and Excitons
## 3 Recent Progress in Coupling between Surface Plasmons and Excitons
### Strong Coupling between Surface Plasmons and Transition Metal Dichalcogenides  
Recently, transition metal dichalcogenides (TMDs), with their excellent electrical and optical properties, were regarded as ideal candidate materials for realization of a strong coupling regime because of their high absorptivity. The strong coupling phenomenon based on use of monolayer TMD materials and single-particle plasmon nanoresonators has only been discovered gradually. [PERSON] et al. [94] demonstrate the evidence of strong coupling between surface plasmon polaritons and MoS\({}_{2}\) excitons involving three types of resonances for the first time. In the same year, [PERSON] et al. [95] provide the unambiguous signature of WS\({}_{2}\) monolayer strong coupling in easily fabricated metallic resonators at room temperature, which suggests many possibilities for combining light-matter hybridization with spin and valleytronics. In addition, the different sizes and shapes of the NPs and the TMD states caused different fluorescence characteristics to appear, including fluorescence enhancement, unchanged fluorescence, and fluorescence quenching. Rabi splitting based on TMD materials [96] has also been reported in a series of optical microcavity systems. [PERSON] et al. [97] proposed a continuous transition from weak coupling to strong coupling between the excitons in a molybdenum disulfide (MoS\({}_{2}\)) monolayer and highly localized plasmons in an ultracompact nanoantenna at room temperature. The interactions between the excitons and plasmons were then directly prompted toward the strong coupling regime by reducing the thickness of the spacer layer, and 190 meV Rabi splitting was observed. The monolayer characteristics of the MoS\({}_{2}\) was confirmed by atomic force microscopy (AFM), as shown in Figure 17a, while Figure 17b shows the individual NCoM constructs, which are observed as diffraction-limited, bright and colorful point scatterers in the dark field images under white lamp illumination. Figure 17: (**a**) 3D illustration of NCoM cavity encapsulating monolayer MoS\({}_{2}\) flakes. (**b**) Tunability of plasmonic cavity resonance in NCoM systems [97]. [PERSON] et al. [98] addressed the issue of resonance coupling in hybrid exciton-polariton structures based on single Si NPs coupled to monolayer (1L)-WS\({}_{2}\). They found that when monolayer tungsten disulfide was added to the silicon NPs, strong coupling was generated at the magnetic optical Mie resonance and the Rabi splitting energy exceeded 110 meV. Additionally, when they changed the surrounding dielectric material from air to water, this led to a Rabi splitting energy of up to 208 meV. This demonstrated that optical resonance sphere coupling tunability could be achieved for the silicon nanoparticles with the addition of the monolayer tungsten disulfide. [PERSON] et al. [99] investigated the interference between the plasmonic and excitonic resonances in hybrid MoSe\({}_{2}\)@Au nanostructures composed of monolayer MoSe\({}_{2}\) supported by Au nanodisks, with results as shown in Figure 18. They used a coupled oscillator model to extract the relationship between the interaction energy and the temperature and obtained a plasmonic-excitonic interaction energy of \(42\pm 8\) meV in the Fano-type regime. [PERSON] et al. [100] reported on the strong interactions between localized surface plasmons in silver nanoprisms and the excitons and trions in monolayer tungsten disulfide (WS\({}_{2}\)). Their temperature-dependent single-particle scattering spectrum test demonstrated the strong coupling behavior of the neutral exciton and the charged three-particle states, trions, and the silver nanotriangular plasmon in the system.