Coherent anti-Stokes Raman spectroscopy frequency matching

An interesting variation of Raman spectroscopy is coherent anti-Stokes Raman spectroscopy (CARS) (99). If two laser beams, with angular frequencies CO and CO2 are combined in a material, and if cjj — is close to a Raman active frequency of the material, then radiation at a new frequency CJ3 = 2cJ2 — may be produced. Detection of this radiation can be used to characterize the material. Often one input frequency is fixed and the other frequency, from a tunable laser, varied until matches the Raman frequency. CARS has the capabiHty for measurements in flames, plasmas, and... 

Figure 3 Folded BOXCARS geometry applied in several transient nonlinear optical spectroscopies. In pump-probe spectroscopy, one of the three beams is blocked and the intensity of one of the incoming beams is monitored as a function of the time delay between the remaining two beams (e.g., beam 3 is blocked and beam 2 is monitored as a function of its delay with respect to beam 1, phase-matching condition would be k2 = ki — ki -I- k2>. Beams 4 and 5 are photon echo signals generated from beams 1 and 2. Beams 6 and 7 can be stimulated photon echo or transient grating signals generated from beams 1,2, and 3. In transient grating two of the beams are time coincident. In coherent anti-Stokes Raman spectroscopy, beams 1 and 3 are time coincident and carry the same frequency the difference between this frequency and that of beam 2 (so-called Stokes beam) matches a vibrational frequency of the system and beam 6 will correspond to the anti-Stokes emission.

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