Abstract
A structure for subwavelength focusing at optical frequencies based on multiple concentric air nanorings was proposed. Using the coherent illumination light at 532 nm, a sequence of two-dimensional intensity distribution was obtained. We experimentally demonstrated the smallest focal spot observed at a distance of 2.8 μm away from the output surface of the sample with the full width at half maximum of ∼320 nm (0.6λ) and a depth of focus of ∼2.4 μm (4.51λ).
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