Exciton

When a semiconductor is excited, electrons move from the valence band to the conduction band. In the valence band, defects after the electrons leave have a positive charge and can be treated as quasiparticles. This quasiparticle is called a hole or hole. Holes are bound to electrons in the conduction band by Coulomb force and move around in the material while maintaining a certain distance between them. Therefore, this electron-hole pair can be regarded as a single particle. This particle is called an exciton. There are two types of excitons: Mott-Wannier excitons, whose electron-hole radius is　10-100 Å, it is large enough to spread throughout a crystal, and Frenkel excitons, whose radius is 1-10 Å, it is small enough to fit within a molecule. Figure 1 shows the appearance of each exciton. Mott-Wannier excitons mainly represent excitons in inorganic semiconductors, while Frenkel excitons mainly represent electronically excited states in molecular crystals.

Figure 1. Mott-Wannier Exciton and Frenkel Exciton

Exciton-polariton

Exciton-polaritons are quasiparticles that result from the coupling of the energy state of light with that of excitons, and can be confined by using a cavity (also called a resonator) structure with two mirrors facing each other. By making the distance between the mirrors an integer multiple of the wavelength of the incident light/2, the light that interferes after many round trips becomes a standing wave. This is a state in which light is quantized and discrete energy levels of light are formed. The process of standing wave formation is shown in Figure 2.

Figure 2. The process of standing wave formation

By forming an exciton in the cavity where the standing wave is generated and adjusting the cavity width so that the exciton's absorption spectrum matches the standing wave spectrum, a phenomenon occurs in which energy is given by the standing wave before the exciton releases energy. This situation can be regarded as energy sharing between the light and the exciton. This state is called a strongly coupled state, and the hybridized state produced is treated as a quasiparticle called an exciton polariton.

In this state, the energy level of the light and the energy level of the exciton combine and the energy splits into two states like molecular orbitals. The higher energy state is called the Upper Polariton (UP) and the lower energy state is called the Lower Polariton (LP), and the energy difference between the two is called Rabi splitting. This phenomenon can change the level structure of the original material. Figure 3 shows an energy level diagram of exciton-polaritons.

Figure 3. Energy level diagram of exciton-polariton

In the exciton-polariton state, we can confirm physical properties that are a mixture of the states of matter and light. Specifically, physical phenomena such as polarized light with spin information derived from matter and generation of particles with ultrafast and ultra-light properties derived from light can be observed.

In addition, it is said that in the transfer of energy between molecules, the orbital interaction between molecules can be increased and highly efficient energy transport can be achieved by putting the molecule that accepts energy into a polariton state and forming a new level that is close to the energy of the molecule that donates energy. There are growing expectations that this phenomenon can be applied to the development of solar cells that can achieve highly efficient energy conversion.

Polariton Condensation

In many-body systems in which a large number of exciton-polaritons are generated, there is a phenomenon called polariton condensation, in which individual exciton polaritons behave macroscopically as if they were a single wave. This phenomenon is very similar to Bose-Einstein Condensates (BEC), in which all exciton polaritons are degenerate and have the same level. short, and they decay before reaching thermal equilibrium. In other words, polariton condensation is a condensation observed in non-thermal equilibrium systems. This is a peculiar point compared to ordinary BECs.

In a cavity structure where polariton condensation occurs, the light leaking from the mirror is naturally coherent. Therefore, it is expected to develop a polariton laser that can oscillate with lower excitation energy than ordinary lasers that require an inverted distribution due to excitation.