Annular aperture-detected coherent anti-Stokes Raman scattering microscopy for high contrast vibrational imaging

We report on a unique annular aperture detection scheme for effective suppression of nonresonant background in coherent anti-Stokes Raman scattering (CARS) microscopy imaging. The results show that the resonant CARS signal to nonresonant background ratio (SBR) varies with both the scatterers' sizes and the annular aperture diameters used, and an approximately 20-fold improvement in SBR can be obtained when an annular aperture size of 0.9 (related to the objective diameter) is selected for imaging the scatterers with diameters close to the excitation wavelength. We demonstrate this technique by imaging 300 to 1100 nm polystyrene beads and human epithelial cells in aqueous environments.