Raman scattering
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Raman scattering or the Raman effect is the inelastic scattering of a photon which creates or annihilates an optical phonon. It was first reported by C.V. Raman and K.S. Krishnan, and independently by Grigory Landsberg and Leonid Mandelstam in 1928. Raman received the Nobel Prize in 1930 for his work on the scattering of light.
The interaction of light with matter in a linear regime allows to absorb or simultaneously emit light which energy precisely matches the difference in energy levels of an interacting electron.
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History
In 1922, Chandrasekhara Venkata Raman published his work on the "Molecular Diffraction of Light," the first of a series of investigations with his collaborators which ultimately led to his discovery of the radiation effect, on the 28th of February 1928 which bears his name, and which gained him the 1930 Nobel Prize in Physics.
Raman Scattering in a Gas
When light is scattered from an atom or molecule, most photons are elastically scattered. The scattered photons have the same energy (frequency) and, therefore, wavelength, as the incident photons. However, a small fraction of light (approximately 1 in 107 photons) is scattered at optical frequencies different from, and usually lower than, the frequency of the incident photons. In a gas, Raman scattering can occur with a change in vibrational, rotational or electronic energy of a molecule. Chemists are concerned primarily with the vibrational Raman effect. The distortion of a molecule in an electric field is determined by its polarisability.
Raman Scattering in Solid State: Stokes and anti-Stokes
In the Raman effect the photon is absorbed and reemitted again via an intermediate electron state. The excess energy and impulse is transferred to the ion and is dissipated as a phonon into the material. The remaining photon of lower energy generates a Stokes line on the red side of the incident spectrum. In crystals only specific phonons are allowed (solutions of the wave equations which do not cancel themselves) by the periodic structure, so Raman scattering can only appear at certain frequencies. For amorphous materials like glasses, more phonons are allowed and thereby the discrete spectral lines become broad. There is also a process, where a phonon adds energy to the electromagnetic field and shifts the photon to the blue, thus generating an anti-Stokes line, which is less intensive than the red-shifted Stokes line in common Raman scattering.
Stimulated Raman Scattering and Raman amplification
Raman amplification can be obtained by using Stimulated Raman Scattering (SRS), which actually is a combination between a Raman process with stimulated emission. It is interesting for application in telecomunication fibers to amplify inside the standard material with low noise for the amplification process. However the process need high powers and thus imposes more stringent limits onto the material. The amplification band can be up to 100nm broad, depending on the availability of allowed photon states.
Raman spectrum generation
For high intensity cw (contiuous wave) lasers SRS can be used to produce broad bandwidth spectra. This process can also be seen as a special case of four wave mixing, where the frequencies of the two incident photons are equal and the emitted spectra are found in two bands separated from the incident light by the phonon energies. The initial Raman spectrum is build up with spontaneous emission and is amplified later on. At high pumping levels in long fibers higher order Raman spectra can be generated by using the Raman spectrum as a new starting point, thereby building a chain of new spectra with decreasing amplitude. The disadvantage of intrinsic noise due to the initial spontaneous process can be overcome by seeding a spectrum at the beginning, or even using a feedback loop like in a resonator to stabilize the process. Since this technology easily fits into the fast evolving fiber optic laser field and there is demand for transversal coherent high intensity light sources (i.e. broadband telecommunication, imaging applications), Raman amplification and spectrum generation might be widely used in the near future.
Applications
The Raman Effect is used in materials analysis. The frequency of light scattered from a molecule may be changed based on the structural characteristics of the molecular bonds. A monochromatic light source (laser) is required for illumination, and a spectrogram of the scattered light then shows the deviations caused by state changes in the molecule.
See also
- scattering
- Brillouin scattering
- Raman spectroscopy
- nonlinear optics
- fiber amplifier
- List of surface analysis methods
References
- "A new radiation", Indian J. Phys., 2 (1928) 387
External Links
- Explanation from Hyperphysics in Astronomy section of gsu.edu (http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/raman.html)
- Applications of Raman scattering (http://www.onforum.com/tutorials/acrobat/raman.pdf)
- Raman Spectroscopy - Tutorial (http://www.kosi.com/raman/resources/tutorial/)de:Ramanstreuung