Diatomic chain - optical modes

Longitudinal optical vibrations

Long wavelength vibration (k<<π/a)

Medium wavelength vibration (k~0.5π/a)

Zone-edge vibration (k=π/a)

Transverse optical vibrations

Long wavelength vibration (k<<π/a)

Medium wavelength vibration (k~0.5π/a)

Zone-edge vibration (k=π/a)