Michal Horák

The plasmonic lightning‐rod effect refers to the formation of a strong electric near field of localized surface plasmons at the sharp features of plasmonic antennas. Although this effect is intuitively utilized in the design and optimization of plasmonic antennas, the relation between the magnitude of the electric field and the local curvature of the plasmonic antenna has not yet been rigorously established. Here, we provide such a study. We design sets of plasmonic antennas that allow us to isolate the role of the local curvature from other effects influencing the field. The electric near field is inspected by electron energy‐loss spectroscopy and electrodynamic simulations. We confirm the existence of the plasmonic lightning‐rod effect, demonstrate its decoupling from concurrent effects influencing the field, and establish its quantitative description, showing that its strength can be interpreted as comparable to that of the electrostatic lightning‐rod effect. We also provide a simple empirical formula for the spatial dependence of the field, which includes the effective radius of curvature significantly smaller than the geometrical radius.

Vanadium dioxide has been identified as a promising phase-changing material for use in tunable plasmonic devices. In this study, we present a comprehensive modal analysis of single-phase and multi-phase vanadium dioxide nanoparticles. In-situ high-resolution electron energy loss spectroscopy was utilized to experimentally resolve the dipole plasmon peak, higher-order and breathing plasmonic modes, and bulk losses as a function of nanoparticle size. Furthermore, the focus is directed toward capturing the dynamic nanoscale optical response throughout the metal-insulator transition in a vanadium dioxide nanoparticle. This system possesses the ability to be gradually switched on and off in terms of the emergence of near-infrared plasmonic absorption. The switching is accompanied by a gradual spectral shift of the absorption peak, amounting to 0.18 eV for a 120 nm nanoparticle. It is envisioned that this phenomenon can be generalized to larger nanostructures with a higher aspect ratio, thereby introducing a wider tunability of the system, which is essential for functional nanodevices based on vanadium dioxide.

This review summarizes studies of localized surface plasmons in non-noble metal nanostructures. It provides an overview of their plasmonic properties, including dielectric functions, theoretical quality factors, and tunability of the dipole mode.

Plasmonic antennas exploit localized surface plasmons to shape, confine, and enhance electromagnetic fields with subwavelength resolution. The field enhancement is contributed to by various effects, such as the inherent surface localization of plasmons or the plasmonic lightning‐rod effect. Inspired by nanofocusing observed for propagating plasmons, we test the hypothesis that plasmonic antennas with a large cross‐section represent a large charge reservoir, enabling large induced charge and field enhancement. Our study reveals that a large charge reservoir is accompanied by large radiative losses, which are the dominant factor, resulting in a low field enhancement.

Bismuth nanoparticles are being investigated due to their reported photothermal and photocatalytic properties. In this study, we synthesized spherical bismuth nanoparticles (50-600 nm) and investigated their structural and optical properties at the single-particle level using analytical transmission electron microscopy. Our experimental results, supported by numerical simulations, demonstrate that bismuth nanoparticles support localized surface plasmon resonances, which can be tuned from the near-infrared to the near-ultraviolet spectral region by changing the nanoparticle size. Furthermore, plasmonic resonances demonstrate stability across the entire spectral bandwidth, enhancing the attractiveness of bismuth nanoparticles for applications over a wide spectral range. Bismuth's lower cost, biocompatibility, and oxidation resistance make bismuth nanoparticles a suitable candidate for utilization, particularly in large-scale and even industrial plasmonic applications.

Bismuth is a particularly promising alternative plasmonic metal because of its theoretically predicted wide spectral bandwidth. In this study, we experimentally demonstrate the correlation between the shape and size of individual bismuth plasmonic antennas and their optical properties. To this end, we employ a combination of scanning transmission electron microscopy and electron energy loss spectroscopy. Bar-shaped and bowtie bismuth plasmonic antennas of various sizes were fabricated by focused ion beam lithography of a polycrystalline bismuth thin film. Our experimental findings demonstrate that these antennas support localized surface plasmon resonances and their dipole modes can be tuned through their size from the near-infrared to the entire visible spectral region. Furthermore, our findings demonstrate that bismuth exhibits a plasmon dispersion relation that is nearly identical to that of gold while maintaining its plasmonic performance even at higher plasmon energies, thus rendering it a promising low-cost alternative to gold.

Gallium is a phase-changing plasmonic material offering ultraviolet-to-near-infrared tunability, facile and scalable preparation, and good stability of nanoparticles. In this work, we explore the impact of the liquid-to-solid phase transition on their plasmonic properties at the single-particle level by analytical transmission electron microscopy. We observed a phase transition from liquid to β-gallium with a freezing temperature around -135 °C and a melting temperature around -20 °C. We have shown that the dipole mode of localized surface plasmon resonances can be tuned through their size from the ultraviolet to visible spectral region, while the differences in localized surface plasmon energies between liquid gallium at 25 °C and β-gallium nanoparticles at -177 °C are minor. Our results show that the performance of gallium nanoparticles is, in the case of temperature-dependent experiments, unaffected by the liquid-to-solid phase change of gallium and paves the way for suppressing the nonradiative recombination in surface-enhanced Raman spectroscopy at cryogenic temperature.

Bismuth nanoparticles are being investigated due to their reported photothermal and photocatalytic properties. In this study, we synthesised spherical bismuth nanoparticles (50-600 nm) and investigated their structural and optical properties at the single particle level using analytical transmission electron microscopy. Our experimental results, supported by numerical simulations, demonstrate that bismuth nanoparticles support localised surface plasmon resonances, which can be tuned from the near-infrared to the ultraviolet spectral region by changing the nanoparticle size. Furthermore, plasmonic resonances demonstrate stability across the entire spectral bandwidth, enhancing the attractiveness of bismuth nanoparticles for applications over a wide spectral range. Bismuth's lower cost, biocompatibility, and oxidation resistance make it a suitable candidate for utilisation, particularly in industrial and large-scale plasmonic applications.