Subwavelength Focusing Technique Using a Plasmonic Lens

A plasmonic lens (PL) is the one of the promising photonic devices utilizing the surface plasmon wave. For example, the surface plasmon wave can be focused by the use of a PL structure consisting of only a single annular subwavelength slit milled into a metal thin film on a glass substrate. We fabricated a PL structure for 532 nm wavelength and evaluated its beam focusing characteristics using a near-field scanning microscope (NSOM). We confirmed that a plasmonic lens with only several micron size can produce a subwavelength focusing spot not only in the near-field, but also in the quasi far-field region. Moreover, we found that the PL generates a tightly focused beam through several microns with a low divergence angle, keeping a high intensity level. These unique beam focusing characteristics with adequate long working distance and depth of focus will offer many applications, such as optical memories, nanolithography, and biochemical sensing.