Colloidal Gold Nanorings and Their Plasmon Coupling with Gold Nanospheres

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.     

Theta-shaped plasmonic nanostructures: bringing "dark" multipole plasmon resonances into action via conductive coupling

Quadrupole plasmon and (octupolar) Fano resonances are induced in lithographically fabricated theta-shaped ring-rod gold nanostructures. The optical response is characterized by measuring the light scattered by individual nanostructures. When the nanorod is brought within 3 nm of the ring wall, a weak quadrupolar resonance is observed due to capacitive coupling, and when a necklike conductive bridge links the nanorod to the nanoring the optical response changes dramatically bringing the quadrupolar resonance into prominence and creating an octupolar Fano resonance. The Fano resonance is observed due to the destructive interference of the octupolar resonance with the overlapping and broadened dipolar resonance. The quadrupolar and Fano resonances are further enhanced by capacitive coupling (near-field interaction) that is favored by the theta-shaped arrangement. The interpretation of the data is supported by FDTD simulation.     

Dark‐Exciton‐Mediated Fano Resonance from a Single Gold Nanostructure on Monolayer WS2 at Room Temperature

Strong spatial confinement and highly reduced dielectric screening provide monolayer transition metal dichalcogenides with strong many‐body effects, thereby possessing optically forbidden excitonic states (i.e., dark excitons) at room temperature. Herein, the interaction of surface plasmons with dark excitons in hybrid systems consisting of stacked gold nanotriangles and monolayer WS₂ is explored. A narrow Fano resonance is observed when the hybrid system is surrounded by water, and the narrowing of the spectral Fano linewidth is attributed to the plasmon‐enhanced decay of dark K‐K excitons. These results reveal that dark excitons in monolayer WS₂ can strongly modify Fano resonances in hybrid plasmon–exciton systems and can be harnessed for novel optical sensors and active nanophotonic devices.     

Directional Fano Resonance in an Individual GaAs Nanospheroid

Fano resonance has been observed in a wide variety of nanophotonic structures such as photonic crystals, plasmonic structures, and metamaterials. It arises from the interference of discrete resonance states with broadband continuum states. As an emerging nanophotonic material, high‐index all‐dielectric nanomaterials provide a new platform to achieve Fano resonance by virtue of the simultaneous excited electric and magnetic resonances. However, to date, Fano resonance in the visible region has not been observed in individual high‐index all‐dielectric nanoparticles. Here, for the first time, the experimental observation of the directional Fano resonance is reported in an individual GaAs nanospheroid. The special geometry enables GaAs nanospheroids to generate spectrally overlapped electric and magnetic dipole resonances, which enhances their spectral coupling, giving rise to asymmetric‐shaped backward scattering spectrum. This directional Fano resonance can be tuned by the aspect ratio of nanospheroids as well as excitation polarization. In addition, efficient directional light scattering is realized at the total scattering peak of the GaAs nanospheroid. The forward‐to‐backward scattering ratio can be largely enhanced due to Fano dip in the backward scattering spectrum. These findings suggest that high‐index all‐dielectric nanospheroid is a promising candidate for directional sources and optical switches.     

Designing and deconstructing the Fano lineshape in plasmonic nanoclusters

By varying the relative dimensions of the central and peripheral disks of a plasmonic nanocluster, the depth of its Fano resonance can be systematically modified; spectral windows where the scattering cross section of the nanocluster is negligible can be obtained. In contrast, electron-beam excitation of the plasmon modes at specific locations within the nanocluster yields cathodoluminescence spectra with no Fano resonance. By examining the selection rules for plasmon excitation in the context of a coupled oscillator picture, we provide an intuitive explanation of this behavior based on the plasmon modes observed for optical and electron-beam excitation in this family of nanostructures.     

Magnetic Plasmon Networks Programmed by Molecular Self‐Assembly

Nanoscale manipulation of magnetic fields has been a long‐term pursuit in plasmonics and metamaterials, as it can enable a range of appealing optical properties, such as high‐sensitivity circular dichroism, directional scattering, and low‐refractive‐index materials. Inspired by the natural magnetism of aromatic molecules, the cyclic ring cluster of plasmonic nanoparticles (NPs) has been suggested as a promising architecture with induced unnatural magnetism, especially at visible frequencies. However, it remains challenging to assemble plasmonic NPs into complex networks exhibiting strong visible magnetism. Here, a DNA‐origami‐based strategy is introduced to realize molecular self‐assembly of NPs forming complex magnetic architectures, exhibiting emergent properties including anti‐ferromagnetism, purely magnetic‐based Fano resonances, and magnetic surface plasmon polaritons. The basic building block, a gold NP (AuNP) ring consisting of six AuNP seeds, is arranged on a DNA origami frame with nanometer precision. The subsequent hierarchical assembly of the AuNP rings leads to the formation of higher‐order networks of clusters and polymeric chains. Strong emergent plasmonic properties are induced by in situ growth of silver upon the AuNP seeds. This work may facilitate the development of a tunable and scalable DNA‐based strategy for the assembly of optical magnetic circuitry, as well as plasmonic metamaterials with high fidelity.     

Double Fano-type resonances in heptamer-hole array transmission spectra with high refractive index sensing

Nanohole arrays or individual nanohole oligomers in metallic films have attracted intense attention due to their unique optical properties such as extraordinary optical transmission or Fano resonance. However, the nanohole oligomer array still remains largely unexplored. In this work, we numerically investigate the heptamer-hole arrays in an optically thin silver film, which can support double Fano-type resonances in the transmission spectra. The two Fano-type transmissions arise from the interference between the non-resonant direct transmission through holes and the resonant indirect scatterings based on the excitations of surface plasmons polaritons (SPPs, set up by the array periodicity) and a sub-radiant localized surface plasmon resonance (LSPR, arising from the anti-bonding hybridization between the central and the surrounding holes). Because of their different physical mechanisms, the two Fano resonances can be tuned independently. In addition, the LSPR-related Fano resonance shows an ultra-high sensitivity to surrounding dielectric medium with a figure of merit of 25 due to its sub-radiant feature, far larger than the SPP-related Fano resonance, offering tremendous potentials for plasmonic biosensors.     

Symmetry breaking in plasmonic nanocavities: subradiant LSPR sensing and a tunable Fano resonance

A metallic nanostructure consisting of a disk inside a thin ring supports superradiant and very narrow subradiant modes. Symmetry breaking in this structure enables a coupling between plasmon modes of differing multipolar order, resulting in a tunable Fano resonance. The LSPR sensitivities of the subradiant and Fano resonances are predicted to be among the largest yet for individual nanostructures.     

Cascades of Fano resonances in light scattering by dielectric particles

Fano lineshapes are fundamental spectroscopic signatures that quantitatively characterize the structural and dynamic properties of physical objects, from nuclei to three-dimensional solids and liquids. The study of cascades of Fano resonances is a fresh approach to the classical problem that greatly expands our knowledge and the scope of practical applications. Here it is demonstrated that in solid state physics cascades of Fano resonance can be considered as a general property of light scattering by dielectric particles with the Mie and Fabry-Perot manifold of narrow photonic eigenmodes. A general picture of Fano resonance cascades in the spectra of all basic elementary scatterers (spheres, cylinders, rings, split rings, rectangular cuboids) is presented. For rings, split rings, and cuboids, the scattering spectrum is divided into separate spectral regions, which begin with a broad transverse resonance of the Lorentz-type or Fano-type lineshape, genetically related to the modes of the disk that generates the ring, and continue with a gallery of longitudinal modes with exponentially increasing Q factors. The alternation of a cascade of transverse modes in a strict sequence of Lorentz-Fano-Lorentz-Fano-… is shown theoretically, as well as experimentally in the case of a ring. This picture opens the door to new fundamental phenomena and extended functionality of dielectric elementary scatterers due to the highly asymmetric controllable shape of the Fano line, periodically repeating in the scattering spectrum.     

Tunable Fano Resonance and Plasmon–Exciton Coupling in Single Au Nanotriangles on Monolayer WS2 at Room Temperature

Tunable Fano resonances and plasmon–exciton coupling are demonstrated at room temperature in hybrid systems consisting of single plasmonic nanoparticles deposited on top of the transition metal dichalcogenide monolayers. By using single Au nanotriangles (AuNTs) on monolayer WS₂ as model systems, Fano resonances are observed from the interference between a discrete exciton band of monolayer WS₂ and a broadband plasmonic mode of single AuNTs. The Fano lineshape depends on the exciton binding energy and the localized surface plasmon resonance strength, which can be tuned by the dielectric constant of surrounding solvents and AuNT size, respectively. Moreover, a transition from weak to strong plasmon–exciton coupling with Rabi splitting energies of 100–340 meV is observed by rationally changing the surrounding solvents. With their tunable plasmon–exciton interactions, the proposed WS₂–AuNT hybrids can open new pathways to develop active nanophotonic devices.     

Fano resonances of a metal nanorod array with a symmetry breaking wedge and gain medium filling

Fano resonances of a metal nanorod array with a symmetry breaking wedge and gain medium filling have been explored using the finite-difference time-domain method. Results show that a periodic symmetry breaking nanorod array supports Fano resonance due to the interaction between a hybridized dipolar plasmon mode of the nanorod and a narrower quadrupolar mode of the slice. By a tiny increase of the imaginary part of the dielectric constant of the gain medium, the Ohmic loss can be counteracted, and Fano resonance dip gets deeper significantly. Additionally, the modulation depth can be improved by changing the real part of the gain medium dielectric constant. The results found are useful for further develop of the devices on the basis of the Fano resonance and its modulation.     

High‐Quality‐Factor Mid‐Infrared Toroidal Excitation in Folded 3D Metamaterials

With unusual electromagnetic radiation properties and great application potentials, optical toroidal moments have received increasing interest in recent years. 3D metamaterials composed of split ring resonators with specific orientations in micro‐/nanoscale are a perfect choice for toroidal moment realization in optical frequency considering the excellent magnetic confinement and quality factor, which, unfortunately, are currently beyond the reach of existing micro‐/nanofabrication techniques. Here, a 3D toroidal metamaterial operating in mid‐infrared region constructed by metal patterns and dielectric frameworks is designed, by which high‐quality‐factor toroidal resonance is observed experimentally. The toroidal dipole excitation is confirmed numerically and further demonstrated by phase analysis. Furthermore, the far‐field radiation intensity of the excited toroidal dipoles can be adjusted to be predominant among other multipoles by just tuning the incident angle. The related processing method expands the capability of focused ion beam folding technologies greatly, especially in 3D metamaterial fabrication, showing great flexibility and nanoscale controllability on structure size, position, and orientation.     

Optofluidic Platform for Real‐Time Monitoring of Live Cell Secretory Activities Using Fano Resonance in Gold Nanoslits

An optofluidic platform for real‐time monitoring of live cell secretory activities is constructed via Fano resonance in a gold nanoslit array. Large‐area and highly sensitive gold nanoslits with a period of 500 nm are fabricated on polycarbonate films using the thermal‐annealed template‐stripping method. The coupling between gap plasmon resonance in the slits and surface plasmon polariton Bloch waves forms a sharp Fano resonance with intensity sensitivity greater than 11 000% per refractive index unit. The nanoslit array is integrated with a cell‐trapping microfluidic device to monitor dynamic secretion of matrix metalloproteinase 9 (MMP‐9) from human acute monocytic leukemia cells in situ. Upon continuous lipopolysaccharide (LPS) stimulation, MMP‐9 secretion is detected within 2 h due to ultrahigh surface sensitivity and close proximity of the sensor to the target cells. In addition to the advantage of detecting early cell responses, the sensor also allows interrogation of cell secretion dynamics. Furthermore, the average secretion per cell measured using our system well matches previous reports while it requires orders of magnitude less cells. The optofluidic platform may find applications in fundamental studies of cell functions and diagnostics based on secretion signals.     

Observation of Fano Resonances in All‐Dielectric Nanoparticle Oligomers

It is well‐known that oligomers made of metallic nanoparticles are able to support sharp Fano resonances originating from the interference of two plasmonic resonant modes with different spectral width. While such plasmonic oligomers suffer from high dissipative losses, a new route for achieving Fano resonances in nanoparticle oligomers has opened up after the recent experimental observations of electric and magnetic resonances in low‐loss dielectric nanoparticles. Here, light scattering by all‐dielectric oligomers composed of silicon nanoparticles is studied experimentally for the first time. Pronounced Fano resonances are observed for a variety of lithographically‐fabricated heptamer nanostructures consisting of a central particle of varying size, encircled by six nanoparticles of constant size. Based on a full collective mode analysis, the origin of the observed Fano resonances is revealed as a result of interference of the optically‐induced magnetic dipole mode of the central particle with the collective mode of the nanoparticle structure. This allows for effective tuning of the Fano resonance to a desired spectral position by a controlled size variation of the central particle. Such optically‐induced magnetic Fano resonances in all‐dielectric oligomers offer new opportunities for sensing and nonlinear applications.     

Plasmon transmutation: inducing new modes in nanoclusters by adding dielectric nanoparticles

Planar clusters of coupled plasmonic nanoparticles support nanoscale electromagnetic "hot spots" and coherent effects, such as Fano resonances, with unique near and far field signatures, currently of prime interest for sensing applications. Here we show that plasmonic cluster properties can be substantially modified by the addition of individual, discrete dielectric nanoparticles at specific locations on the cluster, introducing new plasmon modes, or transmuting existing plasmon modes to new ones, in the resulting metallodielectric nanocomplex. Depositing a single carbon nanoparticle in the junction between a pair of adjacent nanodisks induces a metal-dielectric-metal quadrupolar plasmon mode. In a ten-membered cluster, placement of several carbon nanoparticles in junctions between multiple adjacent nanoparticles introduces a collective magnetic plasmon mode into the Fano dip, giving rise to an additional subradiant mode in the metallodielectric nanocluster response. These examples illustrate that adding dielectric nanoparticles to metallic nanoclusters expands the number and types of plasmon modes supported by these new mixed-media nanoscale assemblies.     

Fano resonance in (gold core)-(dielectric shell) nanostructures without symmetry breaking

A Fano resonance is observed in highly symmetric nanostructures comprising Au nanosphere cores and dielectric shells. It arises from the interference between the narrow plasmon resonance of the Au nanosphere core and the broad scattering background of the dielectric shell. The Fano resonance behavior is dependent on the gap distance between the core and shell and the shell material.     

Giant Optical Activity in an All‐Dielectric Spiral Nanoflower

Optical activity is an effect of prominent importance in stereochemistry, analytical chemistry, metamaterials, spin photonics, and astrobiology, but is naturally minuscule. Metallic nanostructures are commonly exploited as basic elements for artificially producing large optical activity by virtue of surface plasmon resonance (SPR) on the nanostructures. However, their intrinsic high ohmic loss amplified by the SPR results in low energy efficiency and large photothermal heat generation, severely limiting their performance and practical utility. Giant optical activity by inducing magnetic resonance in an all‐dielectric spiral nanoflower (spiral‐flower‐shaped nanostructure) is demonstrated here. Specifically, a large circular‐intensity difference of ≈35% is theoretically predicted and experimentally demonstrated by optimizing the magnetic quadrupole contribution of the nanoflower to scattered light. The nanoflower overcomes the bottleneck of the traditional metallic platforms and enables the development of diverse chiroptical devices and applications.     

Controlling steady-state second harmonic signal via linear and nonlinear Fano resonances

ABSTRACT

Nonlinear signal even from a single molecule becomes visible at hot spots of plasmonic nanoparticles. In these structures, Fano resonances can control the nonlinear response in two ways. (i) A linear Fano resonance can enhance the hot spot field, resulting enhanced nonlinear signal. (ii) A nonlinear Fano resonance can enhance the nonlinear signal without enhancing the hot spot. In this study, we compare the enhancement of second harmonic signal at the steady-state obtained via these two methods. Since we are interested in the steady-state signal, we adapt a linear enhancement which works at the steady-state. This is different than the dark-hot resonances that appear in the transparency window due to enhanced plasmon lifetime.     

Substrate-induced Fano resonances of a plasmonic nanocube: a route to increased-sensitivity localized surface plasmon resonance sensors revealed

Symmetry-breaking introduced by an adjacent semi-infinite dielectric can introduce coupling and hybridization of the plasmon modes of a metallic nanostructure. This effect is particularly large for entities with a large contact area adjacent to the dielectric. For a nanocube, a nearby dielectric mediates an interaction between bright dipolar and dark quadrupolar modes, resulting in bonding and antibonding hybridized modes. The Fano resonance that dominates the scattering spectrum arises from the interference of these modes. This analysis provides a strategy for optimizing the sensitivity of nanostructures, whether chemically synthesized or grown by deposition methods, as high-performance localized surface plasmon resonance sensors.