# 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 Conductive Surfaces  
In the common strong coupling system, the most basic components are the optical resonators and the excitons. To achieve strong coupling, it is necessary to increase the coupling strength while reducing the rates of attenuation and decoherence in each component. Therefore, it becomes increasingly important to select appropriate components for the coupling system. The study of the interactions between surface plasmons and excitons dates as far back as 1902. The [PERSON] group [46] in London observed narrow-band anomalous diffraction in the reflection spectra of metal gratings for the first time. [PERSON] [47] and [PERSON] [48] then provided a mathematical description of the surface waves that were transmitted along the surface of a conductor with finite conductivity. In 1941, [PERSON] [49] connected the abnormal diffraction phenomenon that occurred in metal gratings with [PERSON]'s previous theoretical work and developed the theory of [PERSON]'s anomalous diffraction gratings before proposing the concept of the surface plasmon wave (SPW). However, it was not until 1957 that [PERSON] [50] described the conduction electrons using the hydrodynamic Bloch equations; he found that, in very thin idealized foils, energy loss may occur with a value that was less than the plasma energy, and also demonstrated that this behavior was related to the metal film interface when the foil thickness decreased below v/wp, where v and wp were the velocity and the plasma frequency, respectively. [PERSON]'s group [51] found that the general changes in the loss spectra and the specific changes in the spectrum of aluminum showed good agreement with the predictions of [PERSON]. In the same year, [PERSON] et al. [52] first proposed the concept of surface plasmons. [PERSON] [53] discovered that if a prism is brought close to a metal-vacuum interface, surface plasma waves can be excited optically using the evanescent wave present during total reflection, and the excitation of surface plasmons by attenuated total reflection, i.e., the Otto model, was then proposed. [PERSON] and [PERSON] [54] subsequently improved [PERSON]'s structure and the surface plasmon wave was successfully excited at the interface between a metal and another medium via the prism coupling method. A unified theoretical explanation for surface plasmon theory was thus established. In 1982, [PERSON] et al. [55] studied exciton-surface plasmon interactions using attenuated total reflection spectroscopy. They used Langmuir-Blodgett monolayer assemblies that contained dye molecules and had been deposited on silver films to form Kretschmann structures; these characteristic structures appeared at the transverse exciton frequency because of the interaction of the SPPs with the component of the dye transition dipole moment oriented parallel to the surface. While the angle remained unchanged, the reflection spectrum was obtained using the different wavelengths that were used to excite the SPPs, and the anticrossing dispersion relationship in the coupling system was also obtained. This showed that the coupling strength between the SPPs and the excitons could be adjusted by varying the spacing of the silver film and the molecular thin layer. During later research in 2004, [PERSON] et al. [56] performed reflectometry experiments on an organic semiconductor composed of cyanide dye J-aggregates deposited on a silver film, as illustrated in Figure 3. They observed the dispersion lines that represented an anticrossing, which is the signature of strong plasmon-exciton coupling. In addition, the Rabi splitting characteristic of their coupling reached 180 meV at room temperature. The emission of the low energy plasmon-exciton mixed state was also observed and showed a considerable shift from the corresponding uncoupled emission. In analogous experiments in 2005, [PERSON] et al.