Coherent Anti-Stokes Raman Scattering microscopy (CARS) *
CARS microscopy is one of the label-free multiphoton imaging techniques. It allows for chemically specific imaging of biological materials without use of fluorescent labels. CARS is a three-photon nonlinear optical process where two synchronous and spatially overlapped optical beams are used to probe molecular vibrations in the sample material. The method is based on stimulated Raman scattering which makes CARS imaging much faster than confocal Raman mapping based on spontaneous Raman scattering (at least 100 times faster). Different experimental realizations of CARS microscopy exist. Typical is the visualization of a selected single vibrational frequency, for example the CH2 symmetric stretching mode around 2800 cm-1. Spectral scanning is realized by tuning the excitation wavelength step-by-step. The alternative approach is hyperspectral CARS which is based on broadband excitation and detection of vibrational frequencies in a wider spectral range simultaneously.
Modalities offered within Euro-BioImaging include single-frequency CARS imaging, combined into multi-mode simultaneous imaging of CARS, SHG and 2P-fluorescence signals.
CARS has been proven useful in some very specific application fields. The CARS technique is particularly well suited for high-resolution label-free imaging of lipids due to the high concentration of carbon-hydrogen bonds in the lipid material. Lipid imaging has been applied to a great variety of samples including lipid droplets in fixed and live cell cultures, various tissue sections, and even small animals in vivo (e.g. zebrafish). In pharmaceutical applications, CARS microscopy is gaining more and more interest. Applications include visualization of chemical component distribution in dosage forms and drug carriers, dissolution and release, solid-state transformations during dissolution, and drug delivery into cells and tissues.