Abstract

We investigate the focusing properties of a double-ring-shaped azimuthally polarized beam by a high numerical aperture (NA) lens axicon based on vector diffraction theory. We observe that our proposed system generates a sub wavelength focal hole of $0.5\lambda$ having large uniform focal depth of $48\lambda$ without any annular aperture. We also observed that the distribution of the total intensity near the focus has little variation with the degree of truncation $\beta$ of the incident beam by the pupil. The authors expect such a super-long dark channel may find applications in optical, biological, and atmospheric sciences.

©2012 Optical Society of America

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