Coherent anti-Stokes Raman spectroscopy

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Coherent anti-Stokes Raman scattering (CARS) is a third-order nonlinear optical process involving a pump and a Stokes laser beam that interacts with the sample and generates an anti-Stokes field.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.

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.

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