Plasmonic lens with multi-circular-slit apertures for improvement of light utilization efficiency

A plasmonic lens (PL) is one of the promising photonic devices utilizing the surface plasmon wave. Recently, we evaluated the focusing characteristics of PLs consisting of a single-spiral-slit aperture of several µm diameter, and attained a tightly focused beam with a subwavelength spot size in the far-field region. In this study, we designed and fabricated a PL composed of concentric multi-circular-slit apertures for 405 nm wavelength to increase the light utilization efficiency compared with the conventional PLs with a single-spiral-slit aperture. We have simulated the electric field distribution of the newly developed PL, and confirmed that the light intensity increased more than twofold at the focal spot compared with the conventional PL. Also, the focal spot with subwavelength size was maintained by phase adjustment of the surface plasmon wavelength inside the multiple circular slits of different slit widths. We have measured the focusing characteristics of the PL using the near-field scanning optical microscopy (NSOM) system and confirmed a half-wavelength spot size (200 nm) at 1.3 µm above the PL surface.