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Annular aperture detection scheme in radially polarized coherent anti-Stokes Raman scattering (RP-CARS) microscopy for contrast improvement

We report on a unique annular aperture detection scheme in radially polarized coherent anti-Stokes Raman scattering (RP-CARS) microscopy to effectively remove the solvent background for high contrast vibrational imaging. Our finite-difference time-domain (FDTD) calculations show that the far-field RP-CARS radiation from the scatterer with size comparable to the excitation wavelength is stronger than that from the solvent at large cone angles (around 45° to 150°). The annular detection provides about one order higher contrast for both forward and backward detected RP-CARS microscopy.