Coherent Anti-Stokes Raman Spectroscopy
Coherent Anti-Stokes Raman Spectroscopy (CARS)
is a nonlinear Raman spectroscopy technique that uses two very strong collinear lasers to irradiate a sample, instead of the traditional single laser. The frequency of the first laser is usually constant. The frequency of the second, however, can be tuned so that the frequency difference between the two lasers equals the frequency of a Raman-active mode of interest. This particular mode will be the only extremely strong peak in the Raman signal.
CARS is orders of magnitude stronger than normal Raman emission. A monochromator is not necessarily required to perform CARS; a wideband interference filter with a detector placed behind the filter may work instead. A more detailed description of CARS using mathematics is presented below:
Two laser beams with frequencies ω1 and ω2 (ω1 > ω2) interact coherently to produce a strong scattered light with frequency 2ω1 - ω2. If the frequency difference between the two lasers (ω1 - ω2) equals the frequency ωm of a Raman-active rotational, vibrational, or other mode, then a strong light with frequency ω1 + ωm is emitted.
In other words, to obtain a strong Raman signal, the second laser frequency should be tuned in such a way that ω2 = ω1 - ωm. Then the frequency of strong scattered light will be 2ω1 - ω2 = 2ω1 - (ω1 - ωm) = ω1 + ωm, which is higher than the excitation frequency ω1 and therefore considered an anti-Stokes frequency.
CARS derives its name from the fact that it uses two coherent laser beams in order to produce an anti-Stokes frequency signal.
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