text: Coherent anti-Stokes Raman spectroscopy, also called Coherent anti-Stokes Raman scattering spectroscopy (CARS), is a form of spectroscopy used primarily in chemistry, physics and related fields. It is sensitive to the same vibrational signatures of molecules as seen in Raman spectroscopy, typically the nuclear vibrations of chemical bonds. Unlike Raman spectroscopy, CARS employs multiple photons to address the molecular vibrations, and produces a coherent signal. As a result, CARS is orders of magnitude stronger than spontaneous Raman emission. CARS is a third-order nonlinear optical process involving three laser beams: a pump beam of frequency ω, a Stokes beam of frequency ω and a probe beam at frequency ω. These beams interact with the sample and generate a coherent optical signal at the anti-Stokes frequency (ω+ω-ω). The latter is resonantly enhanced when the frequency difference between the pump and the Stokes beams (ω-ω) coincides with the frequency of a Raman resonance, which is the basis of the technique's intrinsic vibrational contrast mechanism.
  Coherent Stokes Raman spectroscopy (CSRS pronounced as "scissors") is closely related to Raman spectroscopy and lasing processes. It is very similar to CARS except it uses an anti-Stokes frequency stimulation beam and a Stokes frequency beam is observed (the opposite of CARS).