Visible light focusing demonstrated by plasmonic lenses based on nano-slits in an aluminum film.

We experimentally demonstrate plasmonic lenses working in the visible range with well controlled focal lengths using nano-slits in an aluminum film. The fabricated lenses were characterized using confocal scanning optical microscopy. Two lenses with a design focal length 3 microm and 6 microm at 633 nm were investigated in detail. The full-width half-maximum beam width at the focal point was found to be 470 nm and 490 nm, and the extension of the light spot was 1.3 mum and 2.3 mum respectively. Lens performance compared extremely well with the expected behaviour from finite-difference time-domain modeling. The focal length from experiment and simulation agreed to within 3.5%. The lens manufacture was found to be insensitive to deviations from the optimum process parameters indicating that lens components can be reliably designed and produced.