photonphonon

Photonphonon is a term used to describe the interaction between photons, the quanta of light, and phonons, the quanta of lattice vibrations, in a material. When these two excitations couple strongly, they form hybrid quasiparticles that mix electromagnetic and vibrational properties. The most common realization is the phonon-polariton, a result of coupling between infrared photons and optical phonons in polar crystals.

In polar dielectrics, the coupling arises from the macroscopic electric fields associated with longitudinal and transverse optical phonons. This interaction creates mixed modes whose dispersion relations exhibit an avoided crossing between the photon dispersion and the optical-phonon branches. The resulting phonon-polaritons inherit light-like propagation at higher frequencies and phonon-like character near the optical phonon resonances. Interfaces can support surface phonon-polaritons, which propagate along boundaries with strong confinement in the infrared.

Photon-phonon coupling also plays a central role in optomechanics, where the electromagnetic field in a cavity interacts with mechanical vibrations of a structure. Here, photons exchange energy with phonons via radiation pressure or photoelastic effects, enabling phenomena such as cooling of mechanical modes, quantum state control, and precision sensing. While often treated separately, both phonon-polaritons and cavity optomechanics reflect the same underlying photon-phonon coupling, viewed from different experimental regimes.

Observational techniques include infrared and terahertz spectroscopy, Raman spectroscopy, and time-domain measurements, which reveal hybrid dispersion features and lifetimes of the coupled modes. Materials commonly exhibiting strong photon-phonon interactions include polar semiconductors such as silicon carbide, gallium arsenide, zinc oxide, and aluminum nitride. The concept is foundational for understanding light–matter interactions in solids and for developing infrared photonic devices and sensors.