Scheme of the coherent anti-Stokes Raman scattering event: an absorption of two photons of the probe beam along with a stimulated emission of a photon in the Stokes beam results in a coherent emission of a photon at the anti-Stokes frequency. Black lines represent molecular energy levels, e.g., associated with its vibrational structure. Note that the sample is illuminated with two, pump and Stokes fields, while the anti-Stokes radiation is generated through the nonlinear process.