A number of plasmonic devices and applications, such as chemical and biological sensors, plasmon-enhanced solar cells, optical nanoantennas, metamaterials and metasurfaces, require the deposition of plasmonic metal nanocrystals on various substrates. Because the localized plasmon resonance modes, energies and strengths are strongly dependent on the dielectric function of the surrounding environment, the substrate is expected to largely affect the plasmonic properties of supported metal nanocrystals. Therefore, understanding the effects of the substrate on the plasmonic properties of metal nanocrystals and the roles of the involved factors will be crucial for designing various plasmonic devices that are made of metal nanocrystals deposited on different substrates. Herein we report on our study and results of the effects of substrates with distinct dielectric functions on the plasmonic properties of three types of Au nanocrystals. A combination of experiments and numerical simulations shows that the presence of a substrate causes plasmonic shifts as well as the appearance of new plasmon modes. The plasmonic shifts and the emergence of new plasmon modes are found to be dependent on the particle shape of Au nanocrystals and in turn on the fractional particle surface area that is in contact with the supporting substrate. For Au nanospheres and nanorods, plasmonic shifts, less than 100 nm, are observed on the scattering spectra by changing the supporting substrate from indium tin oxide to silicon. In comparison, a giant spectral shift of more than 300 nm is obtained for Au nanoplates. Moreover, silicon substrates induce the emergence of an out-of-plane quadrupolar mode of Au nanoplates, which interacts with an out-of-plane octupolar mode to give rise to a distinct Fano resonance. The Fano resonance is found to become stronger as the thickness of Au nanoplates is decreased. These results are of great importance for understanding the plasmonic properties of noble metal nanocrystals supported on various substrates, and for designing novel plasmonic nanostructures with desired optical properties and functions.

research fields, such as medicine, biology, and materials science. [1] These different types of optical components usually utilize the reflection, refraction, and diffraction of light to reshape the wavefronts and control the propagation direction of light in real space. [2] Although great achievements have been made in controlling light in various scopes, it is still challenging to manipulate the light direction at the subwavelength scale with the conventional optical devices. [3] The major constraint of controlling light at nanoscale is the diffraction limit of light, where the smallest resolvable feature is at the wavelength scale. The limited resolution hinders the observation of many intriguing phenomena and features at the subwavelength scale, such as biomolecules, cell components, nanoparticles and nanoscale devices. [4-7] New techniques are therefore highly demanded for subwavelength light manipulation. Nanoparticles have been introduced as a bridge to connect the gap between the macroscopic and the microscopic scale owing to their extraordinary optical properties in the manipulation of light. [8,9] In order to design nanoantennas with nanoparticles, many ideas have been borrowed from conventional macroscopic schemes. One of the most famous examples is Yagi-Uda antennas. This antenna structure was first invented by Hidetsugu Yagi and Shintaro Uda in 1926. [10] The traditional Yagi-Uda antennas are usually composed of multiple parallel elements in a line, usually halfwave dipoles made of metal rods, which function as a reflector, a driving element (feed), and several directors. Each director reradiates the electromagnetic waves from the driving element at a different phase, and the reradiated waves from the multiple directors superpose and interfere to enhance the radiation in a single direction. The Yagi-Uda antennas have been widely used in analog television, shortwave communication links, radar antennas, and broadcasting stations. Inspired by this idea, Yagi-Uda nanoantennas consisting of a reflector, a feed, and several directors have also been invented. As the nanoscale counterpart, Yagi-Uda nanoantennas can redirect light emission in a desired direction with a narrow angle and enhanced signal intensity in a nanoscale region. [11,12] The detailed analysis of different Yagi-Uda nanoantennas will be introduced in Sections 3-5. The inspiring work has opened a new era for directional light control at the subwavelength scale. After the development for a decade, the techniques for directional light scattering and emitting have been enriched. Many different Light manipulation has been widely employed in lighting, display, and energy storage, becoming an inseparable part of human lives. However, the conventional optical devices suffer from the diffraction limit of electromagnetic waves. To overcome the limitation, plasmonic and dielectric nanoantennas are introduced for the control of light direction at nanoscale. The directionality of the nanoantennas stems from their electromagnetic resonance properties or ...

Gold nanorings are attractive as plasmonic metal nanocrystals because they have a hollow inner cavity. Their enhanced electric field inside the ring cavity is accessible, which is highly desirable for assembling with other optical components and studying their plasmon‐coupling behaviors. However, the lack of robust methods for synthesizing size‐controllable and uniform Au nanorings severely impedes the study of their attractive plasmonic properties and plasmon‐driven applications. Herein, an improved wet‐chemistry method is reported for the synthesis of monodisperse colloidal Au nanorings. Using circular Au nanodisks with different thicknesses and diameters as templates, Au nanorings are synthesized with thicknesses varied from ≈30 to ≈50 nm and cavity sizes varied from ≈90 to ≈40 nm. The produced Au nanorings are assembled with colloidal Au nanospheres to yield Au nanoring–nanosphere heterodimers in sphere‐in‐ring and sphere‐on‐ring configurations on substrates. The sphere‐in‐ring heterodimers exhibit the interesting feature of plasmonic Fano resonance upon the excitation of the dark quadrupolar plasmon mode of the Au nanorings. The open cavity in a nanoring holds a great promise for studying plasmon‐coupled systems, which will facilitate the construction of advanced metamaterials and high‐performance Fano‐based devices.

Toroidal electrodynamics has been established as a new branch of electromagnetics research since Y. B. Zel'Dovich proposed the concept of toroidal moments in 1957. [1-4] The introduction of toroidal moments complements the theoretical framework of standard multipole expansion, providing a complete

Quantum tunneling plays an important role in coupled plasmonic nanocavities with ultrasmall gap distances. It can lead to intriguing applications such as plasmon mode excitation, hot carrier generation, and construction of ultracompact electro-optic devices. Molecular junctions bridging plasmonic nanocavities can provide a tunneling channel at moderate gap distances and therefore allow for the facile fabrication of quantum plasmonic devices. Herein we report on the large-scale bottom-up fabrication of molecular junction-bridged plasmonic nanocavities formed from Au nanoplate−Au nanosphere heterodimers. When the molecular junction turns from insulating to conductive, a distinct spectral change is observed, together with the emergence of a high-order charge transfer plasmon mode. The evolution of the electron tunneling-induced plasmon mode also greatly affects the Fano resonance feature in the scattering spectrum of the individual heterodimers. The molecular conductance at optical frequencies is estimated. The molecular junction-assisted electron tunneling is further verified by the reduced surface-enhanced Raman intensities of the molecules in the plasmonic nanocavity. We believe that our results provide an interesting system that can boost the investigation on the use of molecular junctions to modulate quantum plasmon resonances and construct molecular plasmonic devices.