Generation of needle of longitudinally polarized beam by tight focusing of double-ring-shaped radially polarized beam with high NA lens axicon

A method is presented for generation of a sub wavelength (0.25 lambda) longitudinally polarized beam, which propagates without divergence over lengths of about 3 lambda in free space. This is achieved by tight focusing of double ring shaped radially polarized beam with a high NA lens axicon that utilizes spherical aberration to duplicate the performance of an axicon and to create an extended focal line. The intensity distributions were calculated based on Vector diffraction theory and it was observed that in the case of high Numerical Aperture (NA) lens axicon, the distribution of the total intensity near the focus has little effect with the degree of truncation of the incident beam by the pupil.