Raman spectroscopy is sensitive to molecular vibrations, which in turn reflect molecular structure, composition and inter-molecular interactions. As such, Raman spectroscopy is unique in that it provides detailed intra- and inter-molecular structural information and specificity at room temperature, even within extremely complex systems such as living cells (Puppels et al., 1990) .
CARS - coherent anti-Stokes Raman scattering - is the non-linear optical analogue of spontaneous Raman scattering. In this technique a particular Raman transition is coherently driven by two laser fields - the so-called “Laser” and “Stokes” - and subsequently probed by a third laser field - the “Probe”, generating an anti-Stokes signal field (Müller et al., 2000). The coherent nature of the process permits efficient coupling of the laser fields to a particular vibrational mode, increasing the signal from this mode by many orders of magnitude.
Because there is no net energy transfer to the specimen, CARS microscopy is a mild, non-disruptive, imaging technique for biological applications; CARS microscopy has been used to image live bacteria and the mitochondria in live HeLa cells (Zumbusch et al., 1999) . Moreover, dynamic information on lipid-lipid interaction - reflecting the dynamics of the membrane structure - may be obtained from time-resolved CARS microscopy, where a time delay is introduced between the excitation by the “Laser” and “Stokes”, and the probing (see e.g. Mukamel, 1995) .
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References
Mukamel, S. 1995. Principles of nonlinear optical spectroscopy. Oxford University Press, Oxford.
Müller, M., J. Squier, C.A. De Lange, and G.J. Brakenhoff. 2000. CARS microscopy with folded BoxCARS phasematching. J. Microsc. 197:150-158.
Puppels, G.J., F.F.M. de Mul, C. Otto, J. Greve, M. Robert-Nicoud, D.J. Arndt-Jovin, and T.M. Jovin. 1990. Studying single living cells and chromosomes by confocal Raman microspectroscopy. Nature 347:301-303.
Zumbusch, A., G.R. Holtom, and X.S. Xie. 1999. Three-dimensional vibrational imaging by coherent anti-Stokes Raman scattering. Phys. Rev. Lett. 82:4142-4145.