Nonlinear coherent four-wave mixing in optical microscopy

An analysis of the imaging properties of nonlinear coherent four-wave mixing optical microscopes is presented. The generation and propagation of coherent signals are considered under conditions of high numerical aperture with a model that circumvents the need to use the slowly varying envelope approximation. Calculations of coherent anti-Stokes Raman scattering signals show that diffraction effects play a prominent role in the spatial distribution of the coherent signal intensity. It is emphasized that, unlike for fluorescence microscopy, the detected signal is not a straightforward convolution of a point-spread function and the object but is shaped by the complex interplay of object size and coherent buildup dynamics.