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 (travelling sound waves) or temperature gradients. As described in classical physics, when the medium is compressed, the index of refraction of light in the medium changes and its 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).
The scattered light has a wavelength that is changed slightly by a variable quantity known as the Brillouin shift. Stimulated Brillouin Scattering is used for Optical Phase Conjugation in optics.
This phenomenon was first described by Leon Brillouin (1889-1969).
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