CARS microscopy with folded BoxCARS phasematching.

Three-dimensional microscopy based on coherent anti-Stokes Raman scattering (CARS) is a powerful new imaging technique, in which the contrast arises from molecular vibrations. Based on a simple numerical model, it is shown how the CARS interaction volume depends on the focusing parameters and the type of phasematching used. Collinear phasematching yields an ellipsoidal interaction volume, with lateral dimensions that readily cause vignetting of the CARS signal emission at the collection microscope objective. A folded BoxCARS phasematching geometry, on the other hand, results in an almost cylindrical interaction volume - at the cost of a reduced resolution, for which the possible vignetting of the CARS emission is much reduced. In addition, this type of phasematching provides spatial separation of the signal from the input laser beams, permitting simple signal detection of low frequency vibrational modes. Calculations show that when CARS is performed in a microscopic volume, the phasematching restraint on tuning over the vibrational band is strongly relaxed. A first example of CARS imaging using a folded BoxCARS imaging geometry is shown.