Coherent anti-Stokes Raman scattering (CARS) is commonly used to probe vibrational resonances mostly in biological applications. By exciting a sample with two beams at different wavelengths (respectively called pump and Stokes waves), a coherent anti-Stokes signal which corresponds to the vibrational nonlinear response of the matter can be observed. The maximum efficiency is then obtained when the frequency shift between pump and Stokes matches exactly the frequency of a vibrational mode. This resonant four-wave mixing process is also accompanied by a coherent nonresonant background (NRB) which is the signature of the pure electronic response of the matter. The total CARS intensity is then both constituted by the vibrational signal and the NRB.
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