= 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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