Coherent anti-Stokes Raman spectroscopy
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Coherent anti-Stokes Raman spectroscopy is a form of spectroscopy used primarily in chemistry, physics and related fields. It is closely related to Raman spectroscopy and lasing processes. It is very similar to Raman spectroscopy but involves a lasing process that dramatically improves the signal. Coherent anti-Stokes Raman scattering (CARS) is a third-order nonlinear optical process involving a pump and a Stokes frequency laser beam that interacts with the sample and generates a coherent anti-Stokes frequency beam. The latter is resonantly enhanced when the difference in photon energies coincides with the frequency of a Raman resonance, which provides the intrinsic vibrational contrast mechanism. [1],[2]
In some sense, the fundamental optical phenomenon resulting in a CARS signal is not really Raman spectroscopy, because the third-order phenomenon produces a weak signal considered as a three-photon interaction of virtual states, as in multi-photon microscopy. However, the strength of the signal is greatly increased when the difference in energy of the virtual state correspond to the vibrational energy of a Raman-active state, which is why we give CARS its special name. This amplification is basically the vibrational contrast mechanism, and the (usually spatially-resolved) CARS signal observed corresponds to the presence (and concentration) of species which are Raman-active at the frequency in question, the difference between the pump beam and the Stokes beam frequencies.
CARS is used for species selective microscopy and combustion diagnostics.
[edit] See Also
[edit] References
- ^ A Review of the Theory and Application of Coherent Anti-Stokes Raman Spectroscopy (CARS) Applied Spectroscopy, Volume 31, Number 4, July/August 1977, pp. 253-271(19)
- ^ Coherent anti-Stokes Raman scattering: from proof-of-the-principle experiments to femtosecond CARS and higher order wave-mixing generalizations Journal of Raman Spectroscopy Volume 31, Issue 8-9 , Pages 653 - 667
