Coherent anti-Stokes Raman spectroscopy


Coherent anti-Stokes Raman Scattering (CARS) microscopy permits label-free imaging with excellent chemical selectivity and high spatial resolution. It is a very promising tool for a broad range of research areas, from biophysics studies at sub-cellular level to material science at the nanoscale.

The frequency difference of "pump" and "Stokes" photons is selected to resonantly excite the target vibration mode (ωvib). This excitation is probed by a third photon that induces a coherent emission of anti-Stokes radiation. Signal intensities several orders of magnitude stronger than spontaneous Raman can be achieved thanks to the coherent build-up of the light field and the use of pulsed lasers.

At the CNI we developed a large-field-of-view polarization-resolved CARS microscope and devised a technique, rotating-polarization CARS (RP-CARS), that allows determining the spatial anisotropy of selected chemical bonds and their average orientation. RP-CARS is the technique of choice to image myelinated axons because of the strong CARS signal from the abundant CH2 bonds.

We have shown that the degree of anisotropy of the CH2 bonds in myelin sheaths (quantified with a pixel-based numerical indicator) presents a significant correlation with their health status in a chemical model of demyelination, paving the way for clinical applications of RP-CARS in the field of neurodegenerative disorders.


Setup for CARS microscopySchematics of the four-way mixing processes







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