Different contrast information obtained from CARS and nonresonant FWM images

We demonstrate that coherent anti-Stokes Raman scattering (CARS) microscopy can be used not to obtain vibrational contrast related to a particular chemical species only, but furthermore, it directly yields topological contrast. Topological contrast is due to the spatially dependent refractive index of the sample and a number of linear effects as interference, refraction and absorption contribute to the topological contrast inherent to CARS images. In order to distinguish between different contrast mechanisms we propose an image correction method based on the analysis of the nonresonant four-wave mixing (FWM) signal distribution. Copyright (C) 2009 John Wiley & Sons, Ltd.