Reducing the focal spot size under radially polarized illumination by means of a binary annular element

The influence of the width of a ring diaphragm with a radial phase jump on the focal spot size and intensity under the radially polarized illumination is analytically and numerically investigated. It is shown that due to the destructive interference of the different-phase rings, it is possible to overcome the scalar limit corresponding to the first zero of the zero-order Bessel function. Thus, the minimal focal spot size (FWHM = 0,33'lambda') is reached at the ring diaphragm width equal to 20% of the full aperture radius. In this case, the intensity of the side lobes does not exceed 30% of that of the central peak. It is also shown that due to introducing the phase jump and simultaneously widening the ring aperture it is possible to generate a focal spot whose size does not exceed a limit corresponding to the narrow ring aperture, while the intensity increases almost 6 times. In this case, the side lobes are increasing to 35% of the central peak.