Subwavelength focusing by a sheltered metallic waveguide array

We propose a new subwavelength focusing scheme in a limited number of two-dimensional (2D) metallic waveguide arrays. The entrances of middle waveguides are sheltered and only the waveguides on both sides of the array are open. Simulation results show that this structure can focus a plane wave (lambda=632.8 nm) to a point in the middle waveguide. The full width at half-maximum of the focusing spot is about 54 nm, whereas the FWHM without the shelter is about 150 nm. We analyze the supermode propagation of SPPs in the MWGA and find that the focusing behavior can be explained by the superposition of the supermode of SPPs. The theoretical results show a good agreement with numerical simulation results. We discuss the power loss arising from the shelter and find that the shelter in middle waveguides has little effect on the energy of the focus in the waveguide. (C) 2015 Elsevier B.V. All rights reserved.