Diffraction-limit focusing using a 60-nm-thick spiral slit.

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We demonstrate a technique for diffraction-limit focusing, on the basis of a spatial truncation of incident light using spirally structured slit motifs. The spiral pattern leads to a global phase domain where the diffractive wave vectors are distributed in phase. We fabricate such a spiral pattern on a 60-nm-thick metallic film, capable of converting an orbital-angular-momentum beam to a non-helical high-resolution diffractive focusing beam, resulting in a high numerical aperture of 0.89 in air, and of up to 1.07 in an oil-immersion scenario. The topological complementarity between the incident beam and the slit motifs generates broadband subwavelength focusing. The idea can be extended to large-scale scenarios with larger constituents. The presented technique is more accessible to low-cost fabrications as compared with metasurface-based focusing elements.