Focusing subwavelength light by using nanoholes in a transparent thin film

A circle of planar nanoholes in a dielectric thin film can focus light into a sub-half-wavelength spot. The nanostructure with higher rotational symmetry shows better focusing properties. From finite-difference time-domain calculations, we verified such a focusing spot coming from the constructive interference of diffraction beams near the nanoholes. For a 1 microm size lens with eight 200 nm diameter holes, we achieved an optical spot smaller than 250 nm measured at 650 nm wavelength. This nanostructure provides a simple way to massively fabricate a planar lens array with a scale down to the submicrometer level.