Resonant optical nanomaterials with a high refractive index, such as silicon, have become key elements for controlling free-space light. Here, we show that silicon nanoparticles can manipulate highly confined guided waves in the form of surface plasmon polaritons (SPPs) on a subwavelength scale. Using electron energy-loss spectroscopy in a transmission electron microscope, we demonstrate that SPPs in ultrathin metal films can be efficiently launched due to the strong coupling between the Mie resonances of the nanoparticle and the SPP modes. We find that the SPP excitation wavelength can be tuned across the entire nearinfrared by varying the particle size. For insight into the coupling mechanism, we also measure the electron-beam-induced response of the Mie resonances in isolated silicon nanostructures in a broad size range. Finally, we show that the silicon nanoparticles act as scatterers of the SPPs supported by the film. Our results may pave the way for using high-refractive-index dielectric nanoantennas as compact elements for manipulating highly confined SPPs.

Tunable high-refractive-index nanostructures are highly desired for realizing photonic devices with a compact footprint. By harnessing the large thermo-optic effect in silicon, we show reversible and wide thermal tuning of both the far- and near-fields of Mie resonances in isolated silicon nanospheres in the visible range. We perform in situ heating in a transmission electron microscope and electron energy-loss spectroscopy to show that the Mie resonances exhibit large spectral shifts upon heating. We leverage the spectral shifts to demonstrate near-field tuning between different Mie resonances. By combining electron energy-loss spectroscopy with energy-dispersive X-ray analysis, we show a reversible and stable operation of single silicon nanospheres up to a temperature of 1073 K. Our results demonstrate that thermal actuation offers dynamic near-field tuning of Mie resonances, which may open up applications in tunable nonlinear optics, Raman scattering, and light emission.

resonances have been realized at visible and infrared wavelengths thanks to the mature lithographic processing of suitable materials, [4] such as silicon (Si), [5] gallium phosphide (GaP), [6] and titanium dioxide (TiO 2 ). [7] It would be desirable to extend the operation of these materials to the ultraviolet, but their small direct band gap energies (≲3 eV) lead to significant absorption losses in the ultraviolet. Wide band gap materials, such as niobium pentoxide [8] and hafnium oxide, [9] offer transparency in the ultraviolet but at the cost of a moderate refractive index (n ≈ 2.1−2.3). Diamond has been theoretically suggested as a potential material, [10,11] but comes with significant nanofabrication challenges. [12] The scarcity of available high-index materials with wide band gap energies calls for the identification of new materials which can advance the rich optical properties of Mie resonances observed in the visible to the ultraviolet. Concurrent advances in first-principles methodology and computing power have recently made it possible to design and discover new materials via high-throughput computations. [13][14][15][16][17] The approach has been successfully applied in several domains, including photovoltaics, transparent conductors, and photocatalysis. [18][19][20] However, to the best of our knowledge, computational discovery of new high-index materials remains largely unexplored. Relevant previous work in this direction has been limited to the static response regime [21,22] reflecting the fact that the major materials databases so far has focused on ground state properties.Here we use high-throughput linear response density functional theory (DFT) to screen an initial set of 2743 elementary and binary materials with the aim to identify isotropic highindex, low loss, and broad band optical materials. For the most promising materials, the computed frequency-dependent complex refractive indices are used as input for Mie scattering calculations to evaluate their optical performance. In addition to the already known high-index materials we identify several new compounds. In particular, boron phosphide (BP) offers a refractive index above three with very low absorption losses in a spectral range spanning from the infrared to the ultraviolet. We then prepare BP nanoparticles and show, by means of darkfield optical measurements and electron energy-loss spectroscopy, that they support size-dependent Mie resonances in the visible and ultraviolet. Finally, we demonstrate a laser reshaping Controlling ultraviolet light at the nanoscale using optical Mie resonances holds great promise for a diverse set of applications, such as lithography, sterilization, and biospectroscopy. Access to the ultraviolet requires materials with a high refractive index and wide band gap energy. Here, the authors systematically search for such materials by computing the frequency-dependent optical permittivity of 338 binary semiconductors and insulators from first principles, and evaluate their scattering properties using Mie theor...

Cathodoluminescence spectroscopy performed in an electron microscope has proven a versatile tool for analyzing the near- and far-field optical response of plasmonic and dielectric nanostructures. Nevertheless, the transition radiation produced by electron impact is often disregarded in the interpretation of the spectra recorded from resonant nanoparticles. Here we show, experimentally and theoretically, that transition radiation can by itself generate distinct resonances that, depending on the time-of-flight of the electron beam inside the particle, can result from constructive or destructive interference in time. Superimposed on the eigenmodes of the investigated structures, these resonances can distort the recorded spectrum and lead to potentially erroneous assignment of modal characters to the spectral features. We develop an intuitive analogy that helps distinguish between the two contributions. As an example, we focus on the case of silicon nanospheres and show that our analysis facilitates the unambiguous interpretation of experimental measurements on Mie-resonant nanoparticles.