fononpolariteter

Fononpolariteter are quasiparticles that arise from the strong coupling between photons and optical phonons in polar dielectrics. They describe mixed light–matter excitations that can propagate as bulk modes or be confined to interfaces as surface phonon polaritons. Their characteristic frequencies lie in the mid-infrared to terahertz range, making them especially relevant for infrared nanophotonics and sensing.

The origin of fononpolariteter lies in the interaction between electromagnetic fields and optical phonons, particularly in materials with strong ionic character. In polar crystals, the longitudinal and transverse optical phonons exhibit a splitting (LO-TO) that creates a Reststrahlen band where the dielectric permittivity is negative. Within this band, surface and guided phonon polaritons can exist and exhibit dispersion relations that lie between the light line and the material’s phonon resonances. The dispersion and confinement depend on the crystal orientation, anisotropy, and damping from phonon lifetimes.

Types and notable features include bulk phonon polaritons, surface phonon polaritons at interfaces (SPhPs), and in anisotropic materials, hyperbolic phonon polaritons. Materials such as silicon carbide (SiC), silicon dioxide (SiO2), aluminum nitride (AlN), gallium nitride (GaN), and hexagonal boron nitride (hBN) support these excitations. Hyperbolic phonon polaritons in hBN and related materials enable highly confined, directional propagation and can be studied with near-field techniques.

Applications span infrared nanophotonics, sub-diffraction imaging, sensing, and nanophotonic waveguides. Phonon polaritons offer routes to compact IR components, enhanced light–matter interaction, and integration with plasmonic or dielectric platforms for advanced photonic devices.