Generation of a hollow dark spherical spot by 4π focusing of a radially polarized Laguerre-Gaussian beam

The properties of the focal spot for 4 pi focusing with radially polarized first-order Laguerre-Gaussian beams are calculated. It is shown that a focal spot that has an extremely sharp dark region at the center and an almost-perfect spherical symmetry can be achieved. When such a hollow dark spherical spot is used in 4 pi fluorescence depletion microscopy, an-axial FWHM spot size of similar to 39 run and a transverse FWHM spot size of similar to 64 nm can be achieved simultaneously in a practical system. (c) 2006 Optical Society of America.