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Coherent Anti-Stokes Raman Scattering

Pump and Stokes beams whose frequency difference matches a vibrational resonance coherently drive that vibration across many molecules, and a probe beam is then scattered into a blue-shifted anti-Stokes signal via the third-order susceptibility. The signal is coherent, directional and orders of magnitude stronger than spontaneous Raman, but it sits on a non-resonant four-wave-mixing background that distorts lineshapes.