CARS is a nonlinear variant of normal Raman vibrational spectroscopy that allows label-free chemical imaging at (sub)micron resolution. Owing to the inherently small CARS volume, depth profiling of a sample is possible with- out the need for a confocal set-up. The high sensitivity of the technique due to coherence effects overcomes the difficulty of low Raman scattering cross sections, making CARS three to eight orders of magnitude more sensitive than normal Raman scattering. Thus, vibrational fingerprints of the sample are acquired extremely fast with typical integration times between 20 and 100 ms per voxel.

The CARS set-up employed in the Bonn group consists of a ps pump/probe laser at 710 nm and a broadband fs Stokes laser tunable between 750 and 950 nm (Figure 1). With this multiplex instrument, around 500 cm-1 spec- tral width can be probed simultaneously in one shot (Figure 2), facilitating spectral analysis and sample characterization. With help of the Maximum Entropy method (MEM), quantification of the number of scatterers present in the sample is feasible.

The ongoing CARS projects include: 

• studies on lipid digestion by enzymes
• studies on the catalytic behaviour of zeolites
• studies on kinetics of Pt-catalyzed reactions in microfluidic reactors