We investigated the focusing properties of a phase modulated radially polarized doughnut Gaussian beam by a high numerical aperture (NA) lens based on vector diffraction theory. We observed that our proposed system generates a sub wavelength focal spot of 0.42λ having large uniform focal depth of 5 λ. This kind of non diffracting focal segments is called as dark channel, which may have application in atom trapping, nanolithography, trapping and manipulating particles.
