Mechanism
From a quantum perspective, Brillouin scattering is an interaction between an electromagnetic wave and a density wave (photon-phonon scattering), magnetic spin wave (photon-magnon scattering), or other low frequency quasiparticle. The scattering is inelastic: the photon may lose energy to create a quasiparticle (Stokes process) or gain energy by destroying one (anti-Stokes process). This shift in photon frequency, known as the Brillouin shift, is equal to the energy of the interacting phonon or magnon and thus Brillouin scattering can be used to measure phonon or magnon energies. The Brillouin shift is commonly measured by the use of a Brillouin spectrometer, based on a Fabry–Pérot interferometer.
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