Beaming of Helical Light from Plasmonic Vortices via Adiabatically Tapered Nanotip

We demonstrate the generation of far-field propagating optical beams with a desired orbital angular momentum by using a smooth optical-mode transformation between a plasmonic vortex and free-space Laguerre–Gaussian modes. This is obtained by means of an adiabatically tapered gold tip surrounded by a spiral slit. The proposed physical model, backed up by the numerical study, brings about an optimized structure that is fabricated by using a highly reproducible secondary electron lithography technique. Optical measurements of the structure excellently agree with the theoretically predicted far-field distributions. This architecture provides a unique platform for a localized excitation of plasmonic vortices followed by its beaming

Probing macroscopic temperature changes with non-radiative processes in hyperbolic meta-antennas

Multilayered metal-dielectric nanostructures display both strong plasmonic behavior and hyperbolic optical dispersion. The latter is responsible for the appearance of two separated radiative and non-radiative channels in the extinction spectrum of these structures. This unique property can open a wealth of opportunities towards the development of multifunctional systems that simultaneously can behave as optimal scatterers and absorbers at different wavelengths, an important feature to achieve multiscale control light-matter interactions in different spectral regions for different types of applications, such as optical computing or detection of thermal radiation. Nevertheless, the temperature dependence of the optical properties of these multilayered systems has never been investigated. In this work we study how radiative and non-radiative processes in hyperbolic meta-antennas can probe temperature changes of the surrounding medium. We show that, while radiative processes are essentially not affected by a change in the external temperature, the non-radiative ones are strongly affected by a temperature variation. By combining experiments and temperature dependent effective medium theory, we find that this behavior is connected to enhanced damping effects due to electron-phonon scattering. Contrary to standard plasmonic systems, a red-shift of the non-radiative mode occurs for small variations of the environment temperature. Our study shows that to probe temperature changes it is essential to exploit non-radiative processes in systems supporting plasmonic excitations, which can be used as very sensitive thermometers via linear absorption spectroscopy

Fractal-Like Plasmonic Metamaterial with a Tailorable Plasma Frequency in the near-Infrared

In this work, we show that modulating the fractal dimension of nanoporous gold allows its effective dielectric response to be tailored over a wide spectral range of infrared wavelengths. In particular, the plasma edge and effective plasma frequency depend linearly on the fractal dimension, which can be controlled by varying the pore and ligament sizes. Importantly, the fractal porous metal exhibits superior plasmonic properties compared to its bulk counterpart. These properties, combined with a longer skin depth on the order of 100–200 nm, enables the penetration of optical energy deep into the nanopores where molecules can be loaded, thus, achieving more effective light–matter coupling. These findings may open new pathways to engineering the optical response of fractal-like or self-similar metamaterials without the need for sophisticated lithographic patterning

Thermoplasmonic Effect of Surface-Enhanced Infrared Absorption in Vertical Nanoantenna Arrays

Thermoplasmonics is a method for increasing temperature remotely using focused visible or infrared laser beams interacting with plasmonic nanoparticles. Here, local heating induced by mid-infrared quantum cascade laser illumination of vertical gold-coated nanoantenna arrays embedded into polymer layers is investigated by infrared nanospectroscopy and electromagnetic/thermal simulations. Nanoscale thermal hotspot images are obtained by a photothermal scanning probe microscopy technique with laser illumination wavelength tuned at the different plasmonic resonances of the arrays. Spectral analysis indicates that both Joule heating by the metal antennas and surface-enhanced infrared absorption (SEIRA) by the polymer molecules located in the apical hotspots of the antennas are responsible for thermoplasmonic resonances, that is, for strong local temperature increase. At odds with more conventional planar nanoantennas, the vertical antenna structure enables thermal decoupling of the hotspot at the antenna apex from the heat sink constituted by the solid substrate. The temperature increase was evaluated by quantitative comparison of data obtained with the photothermal expansion technique to the results of electromagnetic/thermal simulations. In the case of strong SEIRA by the CO bond of poly-methylmethacrylate at 1730 cm^–1, for focused mid-infrared laser power of about 20 mW, the evaluated order of magnitude of the nanoscale temperature increase is of 10 K. This result indicates that temperature increases of the order of hundreds of K may be attainable with full mid-infrared laser power tuned at specific molecule vibrational fingerprints

Microscopic View on a Chemical Vapor Deposition Route to Boron-Doped Graphene Nanostructures

Single layer boron-doped graphene layers have been grown on polycrystalline copper foils by chemical vapor deposition using methane and diborane as carbon and boron sources, respectively. Any attempt to deposit doped layers in one-step has been fruitless, the reason being the formation of very reactive boron species as a consequence of diborane decomposition on the Cu surface, which leads to disordered nonstoichiometric carbides. However, a two-step procedure has been optimized: as a first step, the surface is seeded with pure graphene islands, while the boron source is activated only in a second stage. In this case, the nonstochiometric boron carbides formed on the bare copper areas between preseeded graphene patches can be exploited to easily release boron, which diffuses from the peripheral areas inward of graphene islands. The effective substitutional doping (of the order of about 1%) has been demonstrated by Raman and photoemission experiments. The electronic properties of doped layers have been characterized by spatially resolved photoemission band mapping carried out on single domain graphene flakes using a photon beam with a spot size of 1 μm. The whole set of experiments allow us to clarify that boron is effective at promoting the anchoring carbon species on the surface. Taking the cue from this basic understanding, it is possible to envisage new strategies for the design of complex 2D graphene nanostructures with a spatially modulated doping