Coherent Anti-Stokes Raman Scattering (CARS) was first reported in 1965 by Maker and Terhune1 as a method of spectroscopy for chemical analysis. The energy diagram of the process is depicted in Figure 1. CARS involve the interaction of four waves designated as pump (p), Stokes (s), probe (p') and anti-Stokes (CARS) where pump and probe are usually fixed to the same frequency (vr = vr9). If the frequency difference between the pump and Stokes waves matches the vibrational transition VR of the sample (i.e.
VR=vr-vs), CARS provides chemical selectivity due to a resonant enhancement of the third-order nonlinear signal. Being a coherent spectroscopy, the information content of CARS is the same as that of Raman spectroscopy. The main difference is that CARS is a four-wave mixing process that generates a coherent collimated directional beam with several orders of magnitude higher intensity, where the wavelength is blue-shifted with respect to the pump and Stokes beams.