Definitions

Brillouin_scattering

Brillouin scattering

Brillouin scattering, named for Léon Brillouin, occurs when light in a medium (such as water or a crystal) interacts with time dependent 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 the light's path necessarily bends.

Definition

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 and magnon and thus Brillouin scattering can be used to measure phonon and magnon energies. The Brillouin shift is commonly measured by the use of a Brillouin spectrometer based on a Fabry-Pérot interferometer.

Relationship to Raman scattering

The inelastic scattering process of Brillouin light scattering is in principle the same as Raman scattering. Historically Brillouin scattering denominates the scattering of acoustic phonons, while Raman scattering refers to the scattering from molecule vibrations and optic phonons. Nowadays the difference between Brillouin scattering and Raman scattering is considered to lie in the different experimental techniques and the resulting different available frequency range. The term Brillouin scattering labels an experimental detection of the frequency shift with an interferometer, while Raman scattering labels a setup employing a grating spectrometer. Brillouin scattering is technically limited to the detection of quasiparticles with frequencies below about 500 GHz, while with Raman scattering much higher frequencies in the THz range can be measured.

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 Leon 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.

Medical Applications

Researchers have developed a microscopic technique that uses Brillouin scattering to measure the elasticity of living tissue. They have begun to use this confocal Brillouin microscopy to study various eye-related conditions in mice, demonstrating that the stiffness of the lens increases with age. In the future, confocal Brillouin microscopy may be used to study important human ailments such presbyopia and cataracts.

See also

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)

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