Brillouin scattering

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The Brillouin scattering is a type of optical scattering , the optical waves on an interaction with acoustic lattice vibrations (acoustic phonons ) or magnetic spin waves ( magnon ) is based. Léon Brillouin theoretically predicted this type of dispersion for the first time. In 1930 this prediction was confirmed experimentally.

Scattering on phonons

When a photon interacts with a solid or a liquid , energy can be transferred to acoustic or optical phonons . The inelastic scattering of photons by acoustic phonons is called Brillouin scattering. The inelastic scattering on optical phonons is called Raman scattering .

Maximum scattering in the backward direction occurs when the reflected light components overlap in phases, which only occurs when the light and sound waves are precisely matched. Brillouin scattering therefore has an extremely frequency-selective effect from 20 to 100 MHz (frequency of the sound). Due to the Doppler shift, the reflected light has a decrease in frequency of approximately 1–15 GHz (approximately 1–10  ppm change).

The effect plays a role in optical amplifiers , which are able to amplify optical signals without first converting the optical signal into an electrical one.

Stimulated Brillouin Scattering (SBS) can be used for optical phase conjugation .

Scattering on magnons

The inelastic scattering of photons from magnons has a smaller scattering cross-section than the scattering from phonons, but can be observed with high-resolution interferometers . Light is diffracted at this phase grating, the frequency of the light being Doppler shifted by the spin wave frequency. The Doppler shift takes place towards higher (lower) frequencies when the spin wave propagates in the opposite (same) direction compared to the component of the incident light, which is parallel to the scattering surface.

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