Editing Phonon (section)

==Acoustic and optical phonons==<!-- the links "acoustic phonons" and "optical phonons" from the article "Photoluminescence" links here -->
Solids with more than one atom in the smallest [[unit cell]] exhibit two types of phonons: acoustic phonons and optical phonons.

'''Acoustic phonons''' are coherent movements of atoms of the lattice out of their equilibrium positions. If the displacement is in the direction of propagation, then in some areas the atoms will be closer, in others farther apart, as in a sound wave in air (hence the name acoustic). Displacement perpendicular to the propagation direction is comparable to waves on a string. If the wavelength of acoustic phonons goes to infinity, this corresponds to a simple displacement of the whole crystal, and this costs zero deformation energy. Acoustic phonons exhibit a linear relationship between frequency and phonon wave-vector for long wavelengths. The frequencies of acoustic phonons tend to zero with longer wavelength. Longitudinal and transverse acoustic phonons are often abbreviated as LA and TA phonons, respectively.

'''Optical phonons''' are out-of-phase movements of the atoms in the lattice, one atom moving to the left, and its neighbor to the right. This occurs if the lattice basis consists of two or more atoms. They are called ''optical'' because in ionic crystals, such as [[sodium chloride]], fluctuations in displacement create an electrical polarization that couples to the electromagnetic field.<ref name=girvinYang /> Hence, they can be excited by [[infrared radiation]], the electric field of the light will move every positive sodium ion in the direction of the field, and every negative chloride ion in the other direction, causing the crystal to vibrate.

Optical phonons have a non-zero frequency at the [[Brillouin zone]] center and show no dispersion near that long wavelength limit. This is because they correspond to a mode of vibration where positive and negative ions at adjacent lattice sites swing against each other, creating a time-varying [[electrical dipole moment]]. Optical phonons that interact in this way with light are called ''infrared active''. Optical phonons that are ''Raman active'' can also interact indirectly with light, through [[Raman scattering]]. Optical phonons are often abbreviated as LO and TO phonons, for the longitudinal and transverse modes respectively; the splitting between LO and TO frequencies is often described accurately by the [[Lyddane–Sachs–Teller relation]].

When measuring optical phonon energy experimentally, optical phonon frequencies are sometimes given in spectroscopic [[wavenumber]] notation, where the symbol ''ω'' represents ordinary frequency (not angular frequency), and is expressed in units of [[Centimetre|cm]]<sup>−1</sup>. The value is obtained by dividing the frequency by the [[speed of light in vacuum]]. In other words, the wave-number in cm<sup>−1</sup> units corresponds to the inverse of the [[wavelength]] of a [[photon]] in vacuum that has the same frequency as the measured phonon.<ref name="cmian">{{cite book | last = Mahan | first = Gerald | title = Condensed Matter in a Nutshell | publisher = Princeton University Press | location = Princeton | year = 2010 | isbn = 978-0-691-14016-2 }}</ref>