High-contrast chemical imaging with gated heterodyne coherent anti-Stokes Raman scattering microscopy

A novel method is presented which substantially improves the signal- to-background ratio for coherent anti-Stokes Raman scattering (CARS) microscopy. It exploits the fixed phase relation between pump, Stokes and CARS fields together with the strong phase coherence in supercontinua generated by femtosecond lasers. Three phase-locked optical parametric amplifiers are used for the realisation of het- erodyne signal detection. Proper pulse timing yields a gating mechanism which nearly completely suppresses solvent background signals.