![]() ![]() ![]() (30) This can even be applied to reach a strong coupling between locally confined plasmonic fields and a single dye molecule. (28,29) These localized plasmon modes may, for example, be excited in a nanoparticle-on-mirror (nPoM) configuration and can be used to squeeze light to few-nanometer dimensions that result in gigantic enhancements of the electric field. In a different approach, plasmonic waveguides were built by designing chains of metal nanoparticles that transport energy via a near-field coupling of localized plasmon modes of the metal particles. Furthermore, impedance-matched slot waveguides with tapered transmission lines (24−26) and hybrid metal–silicon-on-insulator gap plasmon waveguides (27) were used to efficiently compress light to sub-100 nm dimensions. In the past, nanofocusing was studied using various geometries such as conically (3,19−21) and two- (22) or three-dimensional (23) (3D) tapered waveguides where light is readily coupled to the waveguides by metallic gratings. The strong field confinement of the SPPs that may be reached in such confined waveguide geometries makes them ideal candidates for molecular sensing (12,13) or subdiffraction all-optical information processing. (9) This confinement is crucial for increasing the interaction of SPP waves with quantum emitters attached to the waveguide, locally enhancing nonlinear optical effects (3,10,11) and thus providing the desired switching functionality. Moreover, yet more challenging to achieve experimentally, their geometry should be chosen such that it confines waveguide SPPs to deep sub-wavelength dimensions. (1−5) Ideally, such waveguides should enable an efficient coupling of far-field light to surface plasmon polaritons (SPP) and back to far-field light in a broad spectral range, as it is readily achieved by grating (6,7) or directional two-wire transmission line couplers (8) fabricated by, for example, focused ion-beam milling. Metal nanowires and two-dimensional (2D) metallic nanostructures are promising plasmonic tools for waveguiding and all-optical switching at optical frequencies since they allow for a spatial transport of ultrafast electromagnetic pulses over mesoscopic distances and at the same time for their confinement to sub-wavelength dimensions. This makes such bow-tie couplers an interesting platform for sensitively probing near-field coupling to single quantum emitters and for the ultrafast switching of light by light on the nanoscale. A finite-difference time-domain simulation supports our experimental findings. We find overall transmission efficiencies for nanofocusing, gap transmission, and plasmon outcoupling of up to 4%. A substantial increase in the coupling efficiency for antennas with gap widths below 20 nm proves that the optical near-field coupling between the two antenna arms dominates the gap transmission. We experimentally demonstrate the spectrally broadband launching and propagation of SPP waves over more than 10 μm on one arm of the antenna, their focusing into and transmission across the gap being studied by the plasmon outcoupling on the other arm. Here, we study a prototypical device geometry, a bow-tie antenna equipped with curved line gratings, for the efficient coupling of light into and across the antenna nanogap. This provides a basic functionality for designing new plasmonic devices that can greatly enhance light–matter coupling and facilitate ultrafast and efficient all-optical switching on the nanoscale. Metallic nanostructures can transport electromagnetic fields in the form of surface plasmon polariton (SPP) excitations, focus them into nanometric spots, and transfer them to nearby nanostructures by near-field coupling. ![]()
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