Brillouin scattering
Encyclopedia
Brillouin scattering, named after 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 magnon
s, 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.
From a quantum perspective, Brillouin scattering is an interaction between an electromagnetic wave and a density wave (photon
-phonon
scattering), magnetic spin wave (photon-magnon
scattering), or other low frequency quasiparticle
. The scattering is inelastic
: the photon may lose energy to create a quasiparticle (Stokes
process) or gain energy by destroying one (anti-Stokes process). This shift in photon frequency, 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.
, 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.
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 photon
s from low-frequency phonon
s, 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
.
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.
in 1922. Leonid Mandelstam is believed to have recognised the possibility of such scattering as early as 1918, but he published it only in 1926.
In order to credit Mandelstam the effect is also called 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.
and temperature in optical fibers.
Léon Brillouin
Léon Nicolas Brillouin was a French physicist. He made contributions to quantum mechanics, radio wave propagation in the atmosphere, solid state physics, and information theory.-Early life:...
, occurs when light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
in a medium (such as air, water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
or a crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
) interacts with time dependent optical density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
variations and changes its energy (frequency) and path. The density variations may be due to acoustic modes, such as phonons, magnetic modes, such as magnon
Magnon
A magnon is a collective excitation of the electrons' spin structure in a crystal lattice. In contrast, a phonon is a collective excitation of the crystal lattice atoms or ions. In the equivalent wave picture of quantum mechanics, a magnon can be viewed as a quantized spin wave. As a...
s, or temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
gradients. As described in classical physics
Classical physics
What "classical physics" refers to depends on the context. When discussing special relativity, it refers to the Newtonian physics which preceded relativity, i.e. the branches of physics based on principles developed before the rise of relativity and quantum mechanics...
, when the medium is compressed its index of refraction
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
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
Diffraction grating
In optics, a diffraction grating is an optical component with a periodic structure, which splits and diffracts light into several beams travelling in different directions. The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as...
. Since the sound wave, too, is travelling, light is also subjected to a Doppler shift, so its frequency changes.
Mechanism
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
-phonon
Phonon
In physics, a phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, such as solids and some liquids...
scattering), magnetic spin wave (photon-magnon
Magnon
A magnon is a collective excitation of the electrons' spin structure in a crystal lattice. In contrast, a phonon is a collective excitation of the crystal lattice atoms or ions. In the equivalent wave picture of quantum mechanics, a magnon can be viewed as a quantized spin wave. As a...
scattering), or other low frequency quasiparticle
Quasiparticle
In physics, quasiparticles are emergent phenomena that occur when a microscopically complicated system such as a solid behaves as if it contained different weakly interacting particles in free space...
. The scattering is inelastic
Inelastic scattering
In particle physics and chemistry, inelastic scattering is a fundamental scattering process in which the kinetic energy of an incident particle is not conserved . In an inelastic scattering process, some of the energy of the incident particle is lost or gained...
: the photon may lose energy to create a quasiparticle (Stokes
Stokes shift
Stokes shift is the difference between positions of the band maxima of the absorption and emission spectra of the same electronic transition. It is named after Irish physicist George G. Stokes. When a system absorbs a photon, it gains energy and enters an excited state...
process) or gain energy by destroying one (anti-Stokes process). This shift in photon frequency, 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
Spectrometer
A spectrometer is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify materials. The variable measured is most often the light's intensity but could also, for instance, be the polarization...
, based on a Fabry–Pérot interferometer.
Relationship to Rayleigh scattering
Rayleigh scatteringRayleigh scattering
Rayleigh scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. It can occur when light travels through...
, 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.
Relationship to Raman scattering
Raman scatteringRaman scattering
Raman scattering or the Raman effect is the inelastic scattering of a photon. It was discovered by Sir Chandrasekhara Venkata Raman and Kariamanickam Srinivasa Krishnan in liquids, and by Grigory Landsberg and Leonid Mandelstam in crystals....
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 photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s from low-frequency phonon
Phonon
In physics, a phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, such as solids and some liquids...
s, 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
Raman spectroscopy
Raman spectroscopy is a spectroscopic technique used to study vibrational, rotational, and other low-frequency modes in a system.It relies on inelastic scattering, or Raman scattering, of monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range...
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
Grating
A grating is any regularly spaced collection of essentially identical, parallel, elongated elements. Gratings usually consist of a single set of elongated elements, but can consist of two sets, in which case the second set is usually perpendicular to the first...
) spectrometer
Spectrometer
A spectrometer is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify materials. The variable measured is most often the light's intensity but could also, for instance, be the polarization...
.
Stimulated Brillouin scattering
For intense beams (e.g. laserLaser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
light) travelling in a medium such as an optical fiber
Optical fiber
An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
, the variations in the electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
of the beam itself may produce acoustic vibrations in the medium via electrostriction
Electrostriction
Electrostriction is a property of all electrical non-conductors, or dielectrics, that causes them to change their shape under the application of an electric field. - Explanation :...
. 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
Picometre
A picometre is a unit of length in the metric system, equal to one trillionth, i.e. of a metre, which is the current SI base unit of length...
for visible light). Stimulated Brillouin scattering is one effect by which optical phase conjugation can take place.
Discovery
Inelastic scattering of light by acoustic phonons was first predicted by Léon BrillouinLéon Brillouin
Léon Nicolas Brillouin was a French physicist. He made contributions to quantum mechanics, radio wave propagation in the atmosphere, solid state physics, and information theory.-Early life:...
in 1922. Leonid Mandelstam is believed to have recognised the possibility of such scattering as early as 1918, but he published it only in 1926.
In order to credit Mandelstam the effect is also called 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 strainDeformation (mechanics)
Deformation in continuum mechanics is the transformation of a body from a reference configuration to a current configuration. A configuration is a set containing the positions of all particles of the body...
and temperature in optical fibers.
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