Focusing Characteristics of a Spiral Plasmonic Lens

A plasmonic lens (PL) is one of the promising photonic devices utilizing the surface plasmon wave. The surface plasmon wave can be focused using a PL structure consisting of only a single annular subwavelength slit milled into a metal film on a glass substrate. Recently, we have evaluated the beam focusing characteristics of a PL for 532 nm wavelength, which consists of a circular slit aperture of 4 μm diameter, using near-field scanning optical microscopy (NSOM), and confirmed that subwavelength focusing was attained at the center axis of the PL in both the near-field and far-field regions. However, for a PL with a circular slit, finite-difference time-domain (FDTD) simulations show that the focusing of the surface plasmon induced electric field into a single solid spot at the center cannot be achieved using circularly or linearly polarized light owing to the destructive interference between counter propagating surface plasmon waves from opposite circle slit points. In this study, we designed and fabricated a new PL with a spiral slit for 405 nm wavelength to produce a single solid focal spot due to the constructive interference at the nominal center of the PL. We measured the focusing characteristics of the PL and attained a tightly focused beam with a subwavelength spot size (${\sim}330$ nm) at 3.5 μm above the PL surface.