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What is Coherent anti-Stokes Raman Scattering?

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 vorticity symbolpump) and a Stokes laser beam (at vorticity symbolStokes) interact, producing an anti-Stokes signal at frequency vorticity symbolCARS = 2vorticity symbolpump - vorticity symbolStokes. The Stokes beam (vorticity symbolStokes) 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 (vorticity symbolpump). 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.
 

energy diagram for 6 coherent anti raman scattering signals and an experimental set-up for these signals


Figure 1: Coherent anti-Stokes scattering (CARS) energy diagram and schematic of the experimental setup.
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.

cholesteryl palmitate from a fixed tissue sample CARS microscopy image

Figure 2: CARS microscopy images of cholesteryl palmitate from a fixed tissue sample. The image on the left was obtained using an emission filter commonly used in CARS microscopy experiments. The image on the right was obtained using Semrock part FF01-625/90 and shows enhanced image contrast over the same sample ROI. The same laser intensities and PMT settings were used when acquiring these images.
Image courtesy of
Prof. E. Potma (UC Irvine).

Learn more about CARS:

Additional Resources:

Coherent anti-Stokes Raman scattering imaging of lipids in cancer metastasis
Biomed Central

Tissue imaging with coherent anti-Stokes Raman scattering microscopy
UC Irvine
 

Coherent-anti-stokes-Raman-Scattering CARS Optical-Parametric-Oscillator OPO