Subwavelength light focusing with a single slit lens based on the spatial multiplexing of chirped surface gratings

A lens consisting of a subwavelength slit engraved into a metal film and surrounded by periodic surface gratings and spatial multiplexing chirped surface gratings (SMCSGs) for focusing light is proposed. The focal length of the lens can be accurately designed and tuned by controlling the periods of the local gratings of the SMCSGs. Simulation results show that a subwavelength beam spot can be produced by the lens at a distance of several times the incident wavelength from the slit and the difference between the simulated and designed focal lengths can be reduced to below 3% of the designed focal length.