Improved contrast radially polarized coherent anti-Stokes Raman scattering microscopy using annular aperture detection

We propose a unique annular aperture detection scheme in radially polarized coherent anti-Stokes Raman scattering (RP-CARS) microscopy for significantly removing nonresonant background for high contrast vibrational imaging. Our finite-difference time-domain calculations show that the maximum radiation patterns of RP-CARS signals from the scatterers vary with the scatterer's sizes, which are different from nonresonant CARS radiation from surrounding water. By applying appropriate sizes of annular stop apertures in the detection path, the nonresonant background from water can be effectively suppressed, yielding over 110-fold improvements in signal-to-background ratio for the forward-detected RP-CARS, while over 50-fold improvements for the backward RP-CARS detection.