= Optical phonon
{title2=$\omega_{\boldsymbol q}\to\omega_0>0\text{ as }q\to0$}
An optical phonon is a quantum of vibration on an <optical phonon branch>, involving relative motion of atoms within a primitive cell. Its zone-center frequency can remain nonzero. Some such modes couple strongly to light when the relative displacement changes electric polarization; the name does not imply that every optical phonon is optically active. Their finite energies produce thresholds for electron emission and thermally suppressed absorption at low temperature.
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