coherent anti-stokes raman scattering (cars) microscopy is a valuable tool for label-free imaging of biological samples, since it enables providing contrast via vibrational resonances of a specific chemical bond. however, in a conventional cars image the raman resonant anti-stokes radiation is often superimposed by a nonresonant contribution arising from the electronic part of the polarization. the situation becomes worse if a sample is composed of a significant amount of water, where a strong nonresonant background over the whole image is obtained.