Gated heterodyne coherent anti-Stokes Raman scattering for high-contrast molecule-specific imaging

The use of molecular vibrations as contrast mechanism makes coherent anti-Stokes Raman scattering (CARS) microscopy a unique approach for chemical imaging. As coherent four-wave mixing process, CARS ensures a fixed phase relationship between the involved light fields. Thus, a heterodyne detection of the CARS signal becomes possible if a tunable local oscillator pulse is available which provides phase coherence with regard to the excitation fields. If one applies appropriately time-delayed probing and heterodyning such a detection scheme - gated heterodyne CARS - is capable of significantly improving the signal-to-background ratio, i.e. the image contrast. We used three femtosecond noncollinear optical parametric amplifiers (NOPAs) seeded by a common white-light to generate the required phase-coherent tunable light pulses. A contrast improvement of more than two orders of magnitude compared to conventional CARS was achieved with deuterated benzene as sample and heavy water as a possible solvent.