Polaritointi

Polaritointi is a phenomenon in condensed matter physics where photons, the quantum particles of light, couple strongly with collective electronic excitations in a material. This coupling creates new quasiparticles called polaritons, which are neither purely photons nor purely electronic excitations but a hybrid mixture of both. The strength of this interaction is typically observed in systems where the energy of the photons is close to the energy of the electronic excitation, a condition known as resonance.

When light interacts with a material, it can excite electrons. If this excitation is collective, meaning many electrons participate in a coordinated motion, it can be viewed as a quasiparticle itself, such as a plasmon (a collective oscillation of electrons) or a exciton (a bound electron-hole pair). If the energy of the incoming photon matches the energy of this collective electronic excitation, a strong coupling can occur. This strong coupling leads to a significant exchange of energy between the photon and the electronic excitation, forming the polariton.

Polaritons exhibit properties that are distinct from both their constituent photon and electronic excitation. For example, they can possess a much larger effective mass than photons, and their optical properties can be significantly altered, such as their dispersion relation (how their energy changes with momentum). This unique behavior makes polaritons a subject of intense research for potential applications in areas like low-threshold lasers (Bose-Einstein condensation of polaritons), optical switches, and novel light-emitting devices. The study of polaritons spans various material systems, including semiconductors, insulators, and even superconductors, with research focusing on tailoring their properties through material design and structural engineering.