Brillouin scattering, named for 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.

Mechanism

In gases, sound waves consist in travelling oscillations of pressure, and hence of density. In condensed matter, due to the connections between atoms, the displacement of one or more atoms from their equilibrium positions will give rise to a set of vibration waves propagating through the lattice. The amplitude of the wave is given by the displacements of the atoms from their equilibrium positions. In crystals, there is a minimum possible wavelength λ, given by twice the equilibrium separation between atoms. Not every possible lattice vibration has a well-defined wavelength and frequency. However, the normal modes do possess well-defined wavelengths and frequencies.

From a quantum point of view, Brillouin scattering is an interaction of light photons with acoustic or vibrational quanta (phonons), with magnetic spin waves (magnons), or with other low frequency quasiparticles interacting with light. The interaction consists of an inelastic scattering process in which a phonon or magnon is either created (Stokes process) or annihilated (anti-Stokes process). The energy of the scattered light is slightly changed, that is decreased for a Stokes process and increased for an anti-Stokes process. This shift, 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.

Relationship to Rayleigh scattering

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. Roughly speaking, we could say that in a silent environment we have Rayleigh, in the presence of noise we have Brillouin scattering.

Relationship to Raman 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 quasiparticles (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.

Stimulated Brillouin scattering

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.

Discovery

The phenomenon of inelastic scattering of light due to acoustic phonons was first described by Léon Brillouin (1889-1969) in 1922 and 4 years later in 1926 independently by Leonid Mandelstam. In order to credit Mandelstam it is also denoted as 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.

Fiber Optic Sensing

Brillouin scattering can also be employed to sense mechanical strain and temperature in optical fibers ^[1].

See also

• scattering
• Raman scattering

References

• Léon Brillouin, Ann. Phys. (Paris) 17, 88 (1922).
• L.I. Mandelstam, Zh. Russ. Fiz-Khim., Ova. 58, 381 (1926).
• R.Y.Chiao, C.H.Townes and B.P.Stoicheff, “Stimulated Brillouin scattering and coherent generation of intense hypersonic waves,” Phys. Rev. Lett., 12, 592 (1964)
• B.Ya. Zel’dovich, V.I.Popovichev, V.V.Ragulskii and F.S.Faisullov, “Connection between the wavefronts of the reflected and exciting light in stimulated Mandel’shtam Brillouin scattering,” Sov. Phys. JETP , 15, 109 (1972)

External links

• CIMIT Center for Intigration of Medicine and Innovative Technology
• Brillouin scattering in the Encyclopedia of Laser Physics and Technology
• Surface Brillouin Scattering, U. Hawaii
• List of labs performing Brillouin scattering measurements

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