Ultra-subwavelength focusing of light by a monolayer of metallic nanoshells with an adsorbed defect

We study theoretically the focusing properties of a two-dimensional periodic array of silver-coated silica nanospheres. The array contains an adsorbed nanosphere of the same type which serves as point source when the array is illuminated by a plane wave. By using a rigorous multiple-scattering theory we show that under plane wave illumination, the array focuses the incident light in a tight area with size of about 1/70 of the wavelength. At the same time, the near-field is amplified, by three orders of magnitude. The frequency corresponding to the focusing and amplification effects depends on the thickness of the silver nanoshell offering a tunable response of the structure. The latter can find application,in, optical trapping, lithography and imaging below the diffraction limit.