Boosting Local Field Enhancement by on-Chip Nanofocusing and Impedance-Matched Plasmonic Antennas

Abstract

Strongly confined surface plasmon-polariton modes can be used for efficiently delivering the electromagnetic energy to nano-sized volumes by reducing the cross sections of propagating modes far beyond the diffraction limit, i.e., by nanofocusing. This process results in significant local-field enhancement that can advantageously be exploited in modern optical nanotechnologies, including signal processing, biochemical sensing, imaging and spectroscopy. Here, we propose, analyze, and experimentally demonstrate on-chip nanofocusing followed by impedance-matched nanowire antenna excitation in the end-fire geometry at telecom wavelengths. Numerical and experimental evidences of the efficient excitation of dipole and quadrupole (dark) antenna modes are provided, revealing underlying physical mechanisms and analogies with the operation of plane-wave Fabry-Pérot interferometers. The unique combination of efficient nanofocusing and nanoantenna resonant excitation realized in our experiments offers a major boost to the field intensity enhancement up to ∼ 12000, with the enhanced field being evenly distributed over the gap volume of 30×30×10 nm 3 , and promises thereby a variety of useful on-chip functionalities within sensing, nonlinear spectroscopy and signal processing. [This document is the unedited Author's version of a Submitted Work that was subsequently accepted for publication in Nano Letters, c American Chemical Society after peer review. To access the final edited and published work see http://dx.doi.org/10.1021/acs.nanolett.5b03593.]Keywords: Surface plasmons polaritons, nanofocusing, field enhancement, tapered waveguide, phase-resolved near-field microscopy, optical antennasThe major aspect of focusing of electromagnetic radiation is the possibility of concentrating the energy in a small volume. Because of diffraction, the focusing of freepropagating optical waves is limited in size to the half of the light wavelength in the medium the diffraction limit of light.1 One approach to overcome this limit is to use surface plasmon polaritons (SPPs) surface electromagnetic modes bound to and propagating along metaldielectric interfaces, with electromagnetic fields in a dielectric being coupled to collective free electron oscillations in a metal.2 Spatial confinement of SPP modes in the cross section perpendicular to the propagation direction depends on the material composition and geometric configuration of a waveguiding structure. Notably, some SPP modes (supported, for example, by metal nanowires 3 ) exhibit a unique scaling property in their spatial confinement: the mode is progressively better confined for smaller lateral waveguide dimensions, opening thereby the possibility for guiding extremely confined (i.e., on a deep subwavelength scale) SPP modes 4 as well as for designing SPP-based nanoantennas.5 This feature can further be used for nanofocusing, which is the process of reducing the cross sections of propagating optical modes far beyond the diffraction limit, simply by gradually decreasing lateral waveguid...

In- and out-coupling devices for subwavelength waveguides. Part 2

“…Experimentally measured CE from a waveguide of the width 1500 nm to a metallic nanowire of the width 30 nm was 56% [5] at the wavelength 1.425 µm.…”

“…For a tapered section without losses the largest CE would be reached for an infinite length of the section. In practice, however, the optimal length is determined by the balance [5] between absorption and scattering.…”

“…Экспериментально измеренный коэффициент ввода из волновода шириной 1500 нм в металличе-ский нанопровод шириной 30 нм составил 56% [5] на длине волны 1,425 мкм.…”

Three-dimensional concentration of light in deeply sub-wavelength, laterally tapered gap-plasmon nanocavities

“…3 Recently, a combination of both has also been demonstrated. 4 In particular, metal-insulator-metal (MIM) structures supporting odd surface plasmon polariton (SPP) modes that do not exhibit any cut-off thickness have been shown to be more effective than single metal-dielectric interfaces in achieving subwavelength light confinement. 5 Given the proportionality between the gap-plasmons (GP) mode group velocity and the gap thickness, 5 a number of designs have employed MIM waveguides with tapered gap thicknesses to successfully slow down and concentrate the light.…”

Design, fabrication and SNOM investigation of plasmonic devices

“…The characterisation shows an increase in the effective area (proportional to the coupling efficiency) of up to 175 times, similar to the calculated optimised parameters. We then move our attention to effective tapering of the plasmonic modes such that to achieve either strong field enhancement [3] or propagation into thin plasmonic nanowires [4]. Using onchip nanofocusing with impedance-matched nanoantenna we can obtain a field enhancement of up to ∼ 12000 evenly distributed in a volume of ∼ 30 × 30 × 10 nm 3 .…”