# Principle and Applications of the Coupling of Surface Plasmons and Excitons
## 3 Recent Progress in Coupling between Surface Plasmons and Excitons
### Strong Coupling between Plasmons and Excitons on Localized Surfaces  
While strong coupling between inorganic semiconductor materials and light at room temperature is possible, fine-grained microcavities are still required. When the surface isoionization excimer is introduced, the electric field intensity around it is greatly enhanced because the mode volume is very small and the light is localized within the subwavelength range. In addition, use of high concentrations of molecules or quantum dots can increase the effective dipole moment. Therefore, strong coupling between matter and localized surfaces has become a hot research topic. In 2009 and 2011, the Rabi splitting value reached 450 meV [69] and 650 meV [70], respectively, which already represented the super-strong coupling zone. After the introduction of surface plasmons, Rabi splitting could be observed without use of a closed cavity at room temperature. Figure 9 compares  
Figure 7: (**a**) 3-dimensional schematic of the G-NFG system. (**b**) Cross-sectional schematic of the G-NFG system. (**c**) Strong near-field due to NPs-film coupling. (**d**) Chemical mechanism enhancement. (**e**) Schematic of the fabrication process of the graphene separated G–NFG system. [66]. the weak and strong coupling between an optical cavity with resonant LSP modes supported by metal nanoparticles [71]. Figure 8: Illustration of proposed textured Al nanoparticle/graphene-enhanced screen-printed solar cell [68]. Figure 9: Weak and strong coupling. Adapted from Ref. [71]. Strong coupling is more easily realized in molecules with narrow absorption spectra such as J-aggregates, depending on the coupling conditions. However, strong coupling could also be obtained in SP nanostructures and molecules that have broad absorption spectra. In 2009, [PERSON] et al. [72] reported two types of experiments. Initially, the surface of a Ag film was covered with a poly (methyl methacrylate) (PMMA) film containing Rhodamine 6G, and the reflection spectrum of the structure was measured using a Kretschamann-Raether (K-R) structure. The resulting curve showed a significant anticrossing, and the size of the Rabi split increased linearly as the square root of the molecule concentration increased, resulting in a Rabi splitting energy of up to 230 meV. Then, by varying the thickness of the PMMA layer, which affects the coupling time of the SPPs and the molecules directly, increased coupling strength was realized. [PERSON] et al. [73] performed polarized Raman scattering measurements using a hybridized system composed of Ag nanodimer structures and organic dye molecules. The energy of the hybrid polaron in this system could be regulated by adjusting the LSP energy. The energy of the surface plasmon could also be adjusted by varying the spacing of the paired Ag particles. The anticrossing behavior of the polariton energy indicated a strong coupling regime and a maximum Rabi splitting energy of 39 meV. Most importantly, effective Raman enhancement could be obtained by exciting the high and low levels of the hybrid state within the gap between the paired metal particles, and the maximum Raman enhancement was located at an optimized resonant energy between the hybrid states and the Raman excitation energy. This was the first study of Raman enhancement caused by LSPs coupled with dye molecules. In addition, to enhance the interactions between light and matter, growing numbers of research teams have begun to use the subwavelength confinement properties of metal NPs and nanocavity surface plasmons [74]. In the earliest work on strong coupling of local surface plasmons and excitons, [PERSON] et al. [75] reported the first observation of polaritonic spectral narrowing and strong coupling between localized plasmons and J-aggregate excitons with a Rabi splitting energy of 230 meV at room temperature, as illustrated in Figure 10.  
Since then, both theoretical and experimental work on LSP Resonances (LSPRs) and exciton coupling has continued to emerge.