Coherent anti-Stokes Raman scattering microscopy within a microcavity with parallel mirrors

Coherent anti-Stokes Raman scattering (CARS) microscopy, or micro-CARS, is becoming more and more popular to identify chemical species with notable three-dimensional resolution. In view of this important application, the present work tries to extend the analysis of cavity effects that are well known in other branches of linear and nonlinear optics. In particular, the CARS process taking place within a Fabry-Perot microcavity (with mirror spacing on the order of the anti-Stokes wavelength) is here examined. Two theoretical cavity models are used and found interchangeable as to the description of the typical phenomenology of enhancement and inhibition expected for the electromagnetic confinement induced by the boundary conditions. These phenomena are responsible of strong alterations in the spatial distribution of micro-CARS, and instructive examples are shown for spherical Raman scatterers of different sizes. To sum them up, the signal enhancement occurs in correspondence of pronounced narrowing and elongation of the emission lobes surrounding the optical axis (coincident with the propagation direction of the collinear laser beams). Conversely, the inhibition is characterized by the ejection of radiation at large angles (with respect to the optical axis) for those wavelengths that do not respect the cavity tuning adjusted on the given anti-Stokes wavelength. The different angular properties of spectrally separated components of CARS signals are then proven useful for the amplification of the contrast defining the quality of CARS imaging. In the end, amplifications of one or two orders of magnitude are calculated depending on different experimental conditions. Limits and future developments of the present approach are discussed in the conclusions. Copyright (C) 2009 John Wiley & Sons, Ltd.