Why in ideal single crystals only the zone-centre optical phonons are observed using optical techniques such as Raman spectroscopy?
This is due to the conservation of both energy and momentum in the process of inelastic scattering of the incoming photons. For the energy, the situation is clear: energy conservation yields the standard interpretation of the Raman signals. For the momentum, the situation depends on the magnitude of the momentum of a photon: Because it is so small compared to the momentum of a phonon, it can safely be neglected. Therefore, only the zone-center phonons can show up in the Raman signal.
This is due to the conservation of both energy and momentum in the process of inelastic scattering of the incoming photons. For the energy, the situation is clear: energy conservation yields the standard interpretation of the Raman signals. For the momentum, the situation depends on the magnitude of the momentum of a photon: Because it is so small compared to the momentum of a phonon, it can safely be neglected. Therefore, only the zone-center phonons can show up in the Raman signal.
While the above is the standard answer (photon momentum being very small compared to typical teciprocal lattice vectors) you can have two phonon excitations with opposite momenta yielding zero total momentum for excitations typically located at high symmetry ponts at the BZ edge. This yields a prominent feature in Raman spectra of silicon for example.
Thank you all for sharing.
If two phonons of opposite momentum are involved in the scattering process, then it is second-order Raman spectra.
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