Subwavelength focusing by binary multi-annular plates: design theory and experiment

Subwavelength focus can be created by one single binary multi-annular plate (MAP) through diffraction interference. Based on vectorial angular spectrum theory, a universal, efficient optimization method for designing subwavelength focusing MAPs is described using genetic algorithm and fast Hankel transform algorithm. The method can use arbitrarily polarized vector beams for illumination and can be applied to design multi-amplitude MAPs. It is shown by examples that the minimum feature size is not necessarily at subwavelength, and can be extended to several wavelengths. The longitudinally polarized electric component cannot be neglected in order to accurately reconstruct the real subwavelength focus. In the experiment, the best focal plane of MAP is precisely positioned through confocal scanning mechanism, in which a virtual confocal pinhole detection method is implemented. The wide-field, high-numerical aperture microscopic imaging system predominantly detects the transversely polarized electric components. (paper)