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Brillouin scattering occurs when light in a medium (such as water or a crystal) interacts with density variations and changes its path. The density variations may be due to acoustic modes, such as phonons, or temperature gradients. As described in classical physics, when the medium is compressed its index of refraction changes and the light's path necessarily bends. From a quantum point of view, Brillouin scattering is considered to consist of interaction of light photons with acoustic or vibrational quanta (phonons), or with magnetic spin waves (magnons) The scattered light has a wavelength that is changed slightly by a variable quantity known as the Brillouin shift. For intense beams (e.g. laser light) travelling in a medium, 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. This phenomenon was first described by Leon Brillouin (1889-1969).
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