  Passive optical elements can play key roles in photonic applications such as
plasmonic integrated circuits. Here we experimentally demonstrate passive
gap-plasmon focusing and routing in two-dimensions. This is accomplished using
a high numerical-aperture metal-dielectric-metal lens incorporated into a
planar-waveguide device. Fabrication via metal sputtering, oxide deposition,
electron- and focused-ion- beam lithography, and argon ion-milling is reported
on in detail. Diffraction-limited focusing is optically characterized by
sampling out-coupled light with a microscope. The measured focal distance and
full-width-half-maximum spot size agree well with the calculated lens
performance. The surface plasmon polariton propagation length is measured by
sampling light from multiple out-coupler slits.
