Brillouin scattering, named after Léon Brillouin, occurs when light in a medium (such as air, water or a crystal) interacts with time dependent optical density variations and changes its energy (frequency) and path. The density variations may be due to acoustic modes, such as phonons, magnetic modes, such as magnons, or temperature gradients. As described in classical physics, when the medium is compressed its index of refraction changes, and a fraction of the traveling light wave, interacting with the periodic refraction index variations, is deflected as in a three-dimensional diffraction grating. Since the sound wave, too, is travelling, light is also subjected to a Doppler shift, so its frequency changes.
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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.
Rayleigh scattering, too, can be considered to be due to fluctuation in the density, composition and orientation of molecules, and hence of refraction index, in small volumes of matter (particularly in gases or liquids). The difference is that Rayleigh scattering considers only random and incoherent thermal fluctuations, in contrast with the correlated, periodic fluctuations (phonons) of Brillouin scattering.
Raman scattering is another phenomenon involving inelastic scattering processes of light with vibrational properties of matter. The detected frequency shift range and type of information extracted from the sample, however, are very different. Brillouin scattering denominates the scattering of photons from low-frequency phonons, while for Raman scattering photons are scattered by interaction with vibrational and rotational transitions in single molecules. Therefore the two techniques provide very different information about the sample: Raman spectroscopy is used to determine the chemical composition and molecular structure, while Brillouin scattering measures properties on a larger scale – such as the elastic behaviour. Experimentally, the frequency shifts in Brillouin scattering are detected with an interferometer, while Raman setup can be based on either interferometer or dispersive (grating) spectrometer.
For intense beams (e.g. laser light) travelling in a medium such as an optical fiber, the variations in the electric field of the beam itself may produce acoustic vibrations in the medium via electrostriction. The beam may undergo Brillouin scattering from these vibrations, usually in opposite direction to the incoming beam, a phenomenon known as stimulated Brillouin scattering (SBS). For liquids and gases, typical frequency shifts are of the order of 1–10 GHz (wavelength shifts of ~1–10 pm for visible light). Stimulated Brillouin scattering is one effect by which optical phase conjugation can take place.
Inelastic scattering of light by acoustic phonons was first predicted by Léon Brillouin in 1922. Leonid Mandelstam is believed to have recognised the possibility of such scattering as early as 1918, but he published it only in 1926.[1] In order to credit Mandelstam the effect is also called Brillouin-Mandelstam scattering (BMS). Other commonly used names are Brillouin light scattering (BLS) and Brillouin-Mandelstam light scattering (BMLS).
The process of stimulated Brillouin scattering (SBS) was first observed by Chiao et al. in 1964. The optical phase conjugation aspect of the SBS process was discovered by Zel’dovich et al. in 1972.
Brillouin scattering can also be employed to sense mechanical strain and temperature in optical fibers.[2]