Nonresonant background suppression in CARS spectra of dispersive media using phase mismatching

A nonresonant background suppression technique using coherent cancellation through phase mismatching is discussed and applied for a noncollinear beam configuration. A cuvette structure consisting of a glass, a sample, and a glass layer is regarded. Phase mismatching is shown to be a useful method to suppress nonresonant contributions from cuvette glass walls as well as those originating from the sample. A numerical calculation reveals a limit for the background suppression which can be achieved with this technique. Measurements using ethanol as a sample show the possibility to compensate the nonresonant background originating from the cuvette walls and to effectively suppress the nonresonant contribution in the spectrum of the sample by a factor of 10-50, yielding Lorentzian bands biased by a constant background. Direct measurement of depolarization ratios without interfering nonresonant background is demonstrated for ethanol and shows that this technique can readily be combined with the polarization sensitive CARS technique.