Improvement of focusing characteristics of a surface plasmonic lens for UV wavelength

We study a surface plasmonic lens (SP lens) made as multicircular slit apertures of subwavelength widths milled in a thin metal film for generating a subwavelength focal spot in the far-field region. We previously developed the SP lens with a size of less than 4 mu m for a wavelength of 375 nm, consisting of three concentric circular slit apertures in aluminum (Al) thin film. We numerically calculated the electric field distribution of the SP lens and verified a tight focal spot of subwavelength size in the far-field region. However, the experimentally measured spot size at the focal position (1.3 mu m height above the SP lens surface) was more than the wavelength. The large focused spot size compared with the calculated value is attributed to the incomplete fabrication of the slit aperture delineation. In this study, we improved the design parameters and the fabrication of the SP lens for the same wavelength. We simulated the electric field distribution and confirmed that a focal spot with subwavelength size (270 nm) and enlarged focal length (2.2 mu m) was experimentally verified. (C) 2017 The Japan Society of Applied Physics