Near the transverse optical (TO) phonon frequency ω_TO, light couples strongly to lattice vibrations forming phonon-polaritons. Between ω_TO and ω_LO: the Reststrahlen band where ε(ω)<0 and light cannot propagate.
Lyddane-Sachs-Teller relation: ω_LO²/ω_TO² = ε(0)/ε_∞. Dispersion relation: ck = ω√ε(ω). When Re(ε)<0, k is imaginary — total reflection (Reststrahlen = "residual rays" in German).