Experimental verification of the far-field subwavelength focusing with multiple concentric nanorings

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 mu m away from the output surface of the sample with the full width at half maximum of similar to 320 nm (0.6 lambda) and a depth of focus of similar to 2.4 mu m (4.51 lambda). (C) 2010 American Institute of Physics. [doi: 10.1063/1.3524825]