# 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
Researchers have demonstrated strong interactions between plasmons and excitons in various particle-exciton material systems, including use of gold or silver NPs with various shapes, such as nanoshells, nanorods, nanostars, and nanotips. Figure 11a-c depict the near-field distribution contours of silver nanospheres, nanotriangles, and nanorods, respectively [76]. The figures illustrate that the electric field enhancement of the single particle structure is weak and the enhancement factor is basically in the 1-100 range. To improve the enhancement by multiples of the near field, a coupling structure can be adopted; in Figure 11d, a silver nanosphere dimer has a maximum enhancement factor of 338. Similarly, the silver nanocube over mirror system in Figure 11e has a maximum enhancement factor of 450. These figures show that the near field distribution, the  
Figure 10: Normal incidence reflectivity vs. void thickness for a 20 nm J-aggregate film [75]. enhancement and the far field emission directions of surface plasmons can be adjusted flexibly by designing the structural characteristics of the surface plasmon nanostructure. Figure 12 illustrates some typical research work on strong interactions between LSPRs and excitons. [PERSON] et al. [77] demonstrated strong coupling between plasmons that were confined within a single silver nanoprism and excitons in molecular J-aggregates under ambient conditions through single-particle dark-field scattering measurements. They estimated that the number of excitons that contributed coherently to the coupling process was N-70-85, thus implying that single exciton strong coupling may be within reach as a result of further compression of the mode volume shown in Figure 12a. In their subsequent work, by improving the samples, the J-aggregates were wrapped uniformly around the silver nanotriangle and the redundant uncoupled molecules were removed; splitting was not only obtained on the scattering spectrum, but also was observed on the fluorescence spectrum, as seen in Figure 12c [78]. Figure 12b shows the interactions between the J-aggregate excitons and the single plasmonic dimers, and the splitting of the peak pattern can also be observed in the scattering spectrum [79]. Figure 11: Electromagnetic calculations of near-field distributions of different silver nanoparticles. (**a**) Nanosphere (diameter 60 nm) with a maximum field enhancement factor of 19; (**b**) nanoprism (edge: 60 nm; thickness: 20 nm) with a maximum enhancement factor of 103; (**c**) nanorod (length: 60 nm; width: 15 nm) with a maximum enhancement factor of 106; (**d**) silver nanosphere dimer (diameter: 60 nm; gap: 2 nm) with a maximum enhancement factor of 338; (**e**) silver nanocube over mirror system (cube side length: 65 nm; gap: 3 nm) with a maximum enhancement factor of 450. The gap modes of the coupled systems in (**d**,**e**) have stronger field enhancement when compared with the single nanoparticle plasmon modes in (**a**–**c**) [76]. Apart from the J-aggregates and dyes, strong coupling has also been achieved between photochromic molecules and surface plasmons. The photochemical effect can be used to vary the configuration of the molecule, which in turn changes the coupling strength between the molecule and the light field, thereby achieving a reversible transition between weak and strong coupling. The light-molecule interaction in a cavity containing photochromic spiropyran molecules is a reversible process and is optically addressable, which allows for fine control of the coupling conditions [80]. Active control of the coupling strength between porphyrin dyes and SPPs is supported by a thin gold layer; this control is reversible. The coupled system changes from weak coupling to the strong coupling regime (Rabi splitting of 130 meV) by controlling the effective oscillator strength in the dye layer through exposure to nitrogen dioxide [81]. [PERSON] et al.