# 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
In particular, they demonstrated that a stable trion state emerged and then coupled efficiently to the plasmon resonance at low temperatures by forming three bright intermixed plasmon-exciton-trion polariton states, as depicted in Figure 19.  
It is possible to reduce the number of excitons further to a few or even to one by using TMDs as the coupling system for the exciton materials [101], and this truly reaches the scope of quantum optical research. Therefore, studies of isolated single exciton layered materials have also attracted research attention. Figure 18: Typical MoSe\({}_{2}\) flake transferred to Au nanodisk arrays. (**a**) Schematic showing the MoSe\({}_{2}\) monolayer coated on top of the Au nanodisk array on a SiO\({}_{2}\) substrate. (**b**) Bright field optical microscopy image (50\(\times\) magnification) of MoSe\({}_{2}\) monolayer transferred to Au nanodisk array and fabricated using the shaped nanosphere lithography process [99]. [PERSON] et al. [102] constructed a single-atomic-layer tungsten diselenide hybrid system with a high length-diameter ratio for a silver bar with a low-loss third-order mode and a high vibronic strength exciton using a single particle scattering spectrum, and room-temperature formation of plexcitons with Rabi splitting energies as large as 49.5 meV was observed, as shown in Figure 20. This was also the first time that the strong interaction between the surface plasmon and the monolayer tungsten diselenide exciton was achieved at room temperature in this complex system. In addition, [PERSON] et al. [103] demonstrate the valley-dependent directional coupling of light using a plasmonic nanowire-tungsten disulfide (WS2) layers system and [PERSON] et al. [104] demonstrate valley contrasting spin-momentum locked chiralitonic states in an atomically thin TMD semiconductor coupled to a plasmonic optical spin-orbit resonator at the same time in 2018, which both represent an important new research to the potential applications of TMDs at the nanoscale. The interest in single layers of TMDs is focused on their exceptional optical properties: the strong exciton binding energy and transition dipole momentum, large spin-orbit interaction, efficient  
Figure 19: (**a**) Photoluminescence (PL) spectra of WS2 monolayer as a function of temperature. (**b**) A high density of photonic states (hot spots) is shown at the corners of the nanoprism, which overlaps with the WS2 monolayer for efficient plasmon-exciton interactions [100]. Figure 20: (**a**) Schematic of system with a single silver nanorod on a WSe2 monolayer. (**b**) Energies of the upper plexiton branch (UPB) (E+, blue square) and the lower plexciton branch(LPB) (E–, magenta triangle) as a function of the detuning [102]. valley\(-\)spin coupling, and strong nonlinear optical responses of these materials mean that strong coupling systems based on TMDs and their related properties are now sought after by researchers. In all, different coupling principles with their applications can be shown in Table 1.