T. Ongarello

Focusing dynamics on circular distributed tapered metallic waveguides by means of plasmonic vortex lenses

We investigate the focusing effect on circularly distributed planar tapered plasmonic waveguides by means of three-dimensional (3D) finite elements simulations. The proposed configuration allows nanofocusing on four faced planar nanotips, showing efficient condensation of surface plasmons polaritons (SPPs) at the silver/air interface toward the endpoint of the tips. By means of a plasmonic vortex lens it is possible to illuminate the tips with SPP waves carrying orbital angular momentum (OAM), namely plasmonic vortices. Our 3D simulations show that by acting on the topological charge of the plasmonic vortex the electric field charge distribution at the tips apex can be controlled accordingly to the input electric field phase distribution. The results for three particular OAM values are shown, along with a generalization for arbitrary plasmonic vortex angular momentum values.

Extraordinary optical transmission in one-dimensional gold gratings: near- and far-field analysis

One-dimensional arrays of nanoslits fabricated on silicon nitride membranes show extraordinary optical transmission. Optical characterization techniques have been used to characterize the transmission spectra and the near-field optical configuration. Experimental results have been compared with numerical simulations in order to elucidate the different modes of light propagation. Near- and far-field optical distribution is studied as a function of the polarization of light.

Mid-infrared intersubband polaritons in dispersive metal-insulator-metal resonators

We demonstrate room-temperature strong coupling between a mid-infrared (λ = 9.9 μm) intersubband transition and the fundamental cavity mode of a metal-insulator-metal resonator. Patterning of the resonator surface enables surface-coupling of the radiation and introduces an energy dispersion which can be probed with angle-resolved reflectivity. In particular, the polaritonic dispersion presents an accessible energy minimum at k = 0 where—potentially—polaritons can accumulate. We also show that it is possible to maximize the coupling of photons into the polaritonic states and—simultaneously—to engineer the position of the minimum Rabi splitting at a desired value of the in-plane wavevector. This can be precisely accomplished via a simple post-processing technique. The results are confirmed using the temporal coupled mode theory formalism and their significance in the context of the strong critical coupling concept is highlighted.

Nanoporous gold—Application to extraordinary optical transmission of light

The authors present their work in the preparation of nanoporous gold layers and their patterning with an original procedure preserving the porosity, to obtain the phenomenon of extraordinary transmission of light with a porous material. The design, fabrication, and characterization of nanoslit arrays made with bulk gold and nanoporous gold films are presented and their sensing performances are compared after coating with thiolated organic molecules. Thanks to a greatly enhanced surface-to-volume ratio, nanoporous gold reveals benefits for better reaction efficiency and detection sensitivity. Moreover, plasmonic properties in the near-IR range assure employment in plasmonic devices.

Nanofocusing on circularly distributed tapered metallic waveguides by means of plasmonic vortex lenses

We report our experimental results on the nanofocusing effect at the apex of planar nanotips placed at the center of a plasmonic vortex lens (PVL). PVLs are helical gratings that are able to generate surface plasmon polaritons (SPPs) carrying orbital angular momentum. A specific design allows us to couple the PVL with nanostructures placed at its center. The proposed configuration allows a simultaneous nanofocusing effect on four facing planar nanotips, showing efficient condensation of SPPs at the metal-air interface toward the end point of the tips. An optimized fabrication process allows us to prepare high-quality structures with a sharp tip apex. Near-field scanning optical microscopy has been used to demonstrate the nanofocusing effect.

Plasmonic nanofocusing by means of metal coated dielectric nanowedges

We report our results on arrays of transparent metal coated wedges for plasmonic nanofocusing. FIB milling and chemical etching were used for the fabrication. FEM simulations were used to design the system. The design, fabrication and characterization of wedge structures are presented. The structure shows plasmonic properties in the optical spectral range, with excitation and propagation of surface plasmon polaritons at the wedge tip. The particular designs proposed allow the condensation of plasmonic waves at the wedge tips leading to the nanofocusing effects.