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.

as a low-loss nanoantenna. [1,2] Differently from plasmonic nanoantennas, they exhibit magnetic-type resonances as well as electric-type ones. Existence of magnetic and electric multipole resonances provides a large degree of freedom for tailoring light-matter interactions. [3][4][5][6][7][8][9] Up to now, the most studied dielectric material for a nanoantenna is Si because of the high refractive index (n > 3.5) in the whole visible to near IR (NIR) range and the low extinction coefficient in the red to NIR range (Figure 1). [10,11] However, the extinction coefficient of Si increases at shorter wavelength, and below ≈600 nm, the quality factor (Q-factor) of the resonance degrades and the albedo, that is the ratio of scattering to extinction efficiencies, decreases. This limits the performance of a nanoantenna in the short wavelength range. An alternative material of a nanoantenna operating in a short wavelength range is GaP. It has a moderately high refractive index (n > 3) and a small extinction coefficient at >470 nm (Figure 1a) because of the much larger indirect band gap 2.26 eV (≈550 nm) [12][13][14][15][16][17][18] than that of Si (1.12 eV). For example, the extinction coefficient of GaP at 514 nm is 0.004, which is an order of magnitude smaller than that of Si (0.06). However, despite the expected high performance, research on GaP nanoantennas is very scarce. [13,16,19] For example, even the highest symmetry nanoantenna, i.e., a spherical GaP nanoparticle, has not been developed and the antenna performance has not been studied.In this work, we develop colloidal solution of spherical GaP NP nanoantennas operating below 600 nm. We produce spherical GaP NPs by the combination of mechanical milling and a pulsed laser melting in solution process. In general, the Q-factor of Mie resonances of a spherical NP nanoantenna is higher than that of irregular-shape NP nanoantennas. We first study the effect of the shape on the Mie resonances by single particle scattering spectroscopy and electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM). We then discuss scattering spectra of single GaP NPs with different sizes and demonstrate the existence of distinctive Mie resonances below 600 nm when the diameter is above ≈200 nm. Finally, we show Purcell enhancement of fluorescence of dye molecules by a GaP nanoantenna.

The fundamental requirement for strong coupling is bringing the cavity-emitter interaction to such an extent that the coherent energy exchange between light and molecular transitions becomes greater than the individual decay rates. This can be achieved by exposing molecules to the confined electromagnetic field of a resonator.

A dielectric core–metal shell nanosphere has attracted scientific and technological interests due to the unique optical resonances arising from the hybridization of surface plasmon modes and cavity modes. The previous studies focus on a low‐index dielectric core without its own optical resonances. Here, optical resonances of a core–shell nanosphere with a high refractive index (n ≈ 4) core with the lowest order Mie resonances in the visible range are investigated theoretically and experimentally. Scattering and absorption spectra of a core–shell nanosphere for different values of the core refractive index are first analyzed, and there is a transition of the hybridization scheme around n ≈ 2. Above the value, a characteristic hybridized mode with strong absorption and weak scattering emerges in the near‐infrared range. A core–shell nanosphere composed of a silicon core and a gold shell is prepared, and the resonance modes are studied by single particle scattering spectroscopy and electron energy loss spectroscopy (EELS) in a transmission electron microscope. The core–shell nanospheres exhibit the hybridized modes depending on the core diameter. The hybridized mode as well as the higher order one that is not observable in the scattering spectroscopy is observed in the EELS.

No abstract