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  • Title: Continuous numerical aperture properties of a cylindrically polarized light illuminated sub-wavelength annular aperture.
    Author: Fang CC, Cheng TD, Yeh JT, Wu KC, Lee CK.
    Journal: Opt Express; 2009 Aug 03; 17(16):13646-53. PubMed ID: 19654772.
    Abstract:
    We investigated the process of focusing a radially polarized (RP) light beam through a sub-wavelength annular aperture (SAA). We found that the result was a non-diffraction doughnut-shaped light beam which propagates in free space. After analyzing the electric field component of the focus generated by the SAA structure, we identified the relationship between the focal field generated by the SAA. We then compared it to a case with a traditional objective lens. From our findings, we propose that a SAA structure can be viewed as a continuous numerical aperture optical element.
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