Phonon-photon coupling at polar crystal surface (SiC, hBN)
Reststrahlen band: — THz
A surface phonon polariton (SPhP) is a coupled phonon-photon mode confined to a polar crystal surface.
The permittivity ε(ω) = ε_∞·(ω²-ω_LO²)/(ω²-ω_TO²) has a Reststrahlen band where ε < 0, confining the mode.
The SPhP dispersion k = (ω/c)√(ε/(ε+1)) shows an avoided crossing — mode hybridization between photon (k=ω/c) and phonon.
In hBN and SiC, this creates deeply subdiffractional confinement used for nanophotonics and thermal emission engineering.