Avoided crossing between photon (ck) and TO phonon (ω_TO): ω±² = ½[(ck)²+ωₗₒ²) ± √(...)] — upper and lower polariton branches with forbidden Reststrahlen gap.
0.60
0.15
0.20
4.0
Upper polariton branch (UP): photon-like at high k. Lower branch (LP): phonon-like above ω_TO.