Coherent anti-Stokes Raman scattering (CARS) is a nonlinear four-wave mixing process that is used to enhance the weak (spontaneous) Raman signal. In the CARS process a pump laser beam (at frequency ωpump) and a Stokes laser beam (at ωStokes) interact, producing an anti-Stokes signal at frequency ωCARS = 2ωpump – ωStokes. The Stokes beam (ωStokes) is typically provided by the 1064nm line from a Nd:Vanadate laser, which also acts as the pump source for an optical parametric oscillator (OPO), while the output from the OPO (680-1010nm) acts as the pump beam (ωpump). When the frequency difference (beat frequency) between the pump and the Stokes beams matches the frequency of a (Raman active) vibrational mode, such as the CH2 symmetric stretching mode at ~2800 cm-1, the molecular oscillators are coherently driven. This results in an enhanced anti-Stokes (shorter-wavelength) Raman signal that is the basis for the increased vibrational contrast of CARS microscopy.
Two fields that have greatly benefited from the development of CARS microscopy are cell biology and tissue imaging. Typically, cell interrogation is performed using fluorescence microscopy. With CARS it is possible to perform chemically specific, label-free imaging at the sub-micron scale. To date CARS microscopy has shown its potential in studies of lipid metabolism, organelle transport, and drug diffusion (pharmaco-kinetics) in living tissue. CARS microscopy has also found use in clinical applications and fast, video-rate imaging of tumor masses in healthy brain tissue has been demonstrated.
Learn more about CARS: