# Principle and Applications of the Coupling of Surface Plasmons and Excitons
## 3 Recent Progress in Coupling between Surface Plasmons and Excitons
### Strong Coupling between Metal Periodic Structure Plasmons and Excitons  
At present, researchers are focusing on how to study the strong coupling effect in the quantum optics field using a single plasmon nanoscale resonator and a strong interaction system of excitons. However, because of the limitations of the manufacturing processes or the test methods, some of the strong coupling phenomena are difficult to observe experimentally. Therefore, periodic nanostructures, e.g., metal nanoparticle arrays or gratings, are used to study the strong interactions between light and matter because they offer unique advantages. A periodic array of metal nanostructures can form hybrid LSP resonance modes and diffraction modes under near-field interaction conditions [88], which are called surface lattice resonances (SLRs). When compared with pure plasmon nanoresonators, the advantages of these SLR modes are obvious: for example, Fano interference between the plasmon mode and the diffraction mode will produce a higher quality factor, and mode dispersion is produced by the two-dimensional periodic structure. The exciton emission can also be modulated effectively using a periodic plasmon structure and exciton coupling. [PERSON] et al. [89] developed a new strategy based on highly reproducible fabrication of ordered arrays of gold nanostructures coupled to CdSe/ZnS nanocrystals that were dispersed in a polymer blend; periodic triangular Au particle arrays were prepared that enhanced the fluorescence intensity of CdSe/ZnS quantum dots by 30 times, as shown in Figure 14. They dispersed the CdSe/ZnS quantum dots into PMMA colloids and used the PMMA as an isolation layer (approximately 35 nm thick) to regulate the distance between the metal and the fluorescent quantum dots and thus eliminate the effects of nonradiative energy transfer. In addition, they also compared the fluorescence enhancement effects of both triangular Au particle arrays and cylindrical particle arrays on quantum dots and found  
Figure 13: Diagram of NPoM structure. Adapted from Ref. [86]. that the fluorescence enhancement effect of the triangular Au particles with their periodic structure was better than that of the cylindrical particles: this was caused by a hot spot effect, where the local electromagnetic field was located at the tips of the particles and the field enhancement effect was greater. [PERSON] et al. [90] used the coupling of LSPs and SPPs to achieve an improved enhancement effect. They used CdSe/ZnS quantum dots with PMMA mixed films as masks to fabricate a cylindrical hole array by electron beam etching technology, and the Ag NPs were then interconnected using a continuous Ag thin film. Tuning of the SPP resonance to the quantum dot exciton emission band results in an up to 50-fold enhancement in the overall fluorescence efficiency, which is illustrated in Figure 14b. [PERSON] et al. [91] detected high fluorescence enhancement and Raman scattering on periodic nanopore arrays by tuning the diameters of the nanoholes using a simple plasma etching process, with results as shown in Figure 15. Additionally, the appearance of hybridized states in the presence of excitonic molecules (RG6) led to anticrossing behavior in extinction spectroscopy; this phenomenon, which is also known as Rabi splitting, also indicated that there was strong coupling between the plasma state and the molecular exciton. [PERSON] et al. [92] investigated strong coupling interactions between the molecules and plasmons in a mixed system composed of metal plasmon spatial arrays and polydimethylsiloxane (PDMS) films doped with porphyrin J-aggregates. Figure 16 shows the strong coupling results. Additionally, the work also demonstrated that hybrid light-matter states with greater Rabi splitting could be achieved when using plasmonic nanostructures with higher symmetry and periodicity. Figure 14: (**a**) Triangular Au particle array produced by electron beam etching and the fluorescence enhancement effect. Adapted from Ref. [89].