# Principle and Applications of the Coupling of Surface Plasmons and Excitons
## 1 Introduction  
Nanoscience [1] and nanotechnology [2, 3] are recent revolutionary developments of science that are evolving at a very fast pace. Throughout the growth of modern optical technology [4, 5], photonic device size reduction [6] and improvement of the levels of device integration [7] have been important factors in promoting the development of photonics [8]. As the feature sizes of the structures of optical devices decrease toward the nanometer scale, traditional optical methods are no longer applicable. In the new category of micro-nano photonics systems [9, 10], the generation, propagation, and modulation of light and its interactions with matter have given new developments. Many unique physical and chemical properties, including quantum effects, surface effects, the macroscopic quantum tunneling effect and the microsize effect, are being discovered gradually.  
As an important branch of nanophotonics, surface plasmonics [11] has allowed important breakthroughs to be made in fields involving in super-resolution imaging [12, 13, 14], optical sensing [15, 16, 17], and sub-wavelength light transmission [18, 19, 20, 21]. Surface plasmons can achieve local sub-wavelength confinement of light fields and have a strong electromagnetic field enhancement effect within the local range of this ultra-diffraction limit. This enables nano-optical devices based on surface plasmons to control the transmission and processing of optical information at nanometer scales that are shorter than the operating wavelength, thus providing a powerful platform for the development of smaller and faster nano-optical devices that can be integrated with all-optical integration.  
In addition, many special properties that cannot be predicted or represented by traditional theories [22], such as super-strong local electromagnetic field enhancement [23], super diffraction [24], polarization regulation [25], and abnormal refractive index sensitivity [26], are also being discovered regularly. These special properties offer the possibility of breaking through the limitations of traditional optics to design integrated photonic devices with a diverse range of functions and ideal performance. With different aspects of each coupling principle, especially in nanogaps, nanoparticles, and periodic gratings, these nanostructures can lead to a new class of plasmonic nanostructures and open extraordinary potentials for diverse applications, such as detection of the biological analyte, preparation of novel two-dimensional plasmonic polaritonic devices, etc.  
Although it is largely explored in the research into atomic physics [27] and solid-state systems, the interaction between light and matter is one of the most important aspects of modern optical technology and plays an important role in microcavity quantum dynamics [28]. Generally, if the interaction between the emitter and its local optical environment is strong enough then the energy levels responsible for the emission are also changed, they become inextricably linked with the levels (modes) of the local optical environment. If this happens the energy levels of this hybrid system may be very different from those of the emitter or the optical system individually [29], this is the strong coupling conditions and relations. Under strong coupling conditions [30], a polariton exhibits half-laser and half-material properties.  
The study opens new possibilities to explore and utilize the unique properties of hybrid states over all of the visible region down to ultraviolet frequencies in nanoscale, technologically compatible, integrated platforms based on surface plasmon technology. Starting from the basic principles, we focus here on the development of surface plasmon and exciton coupling technology in recent years, and the future trends of strong coupling between light and matter are also discussed.