Fano resonance in dual-disk ring plasmonic nanostructures

Ultrafast and low-power multichannel all-optical switcher based on multilayer graphene

A metal-insulator-metal waveguide structure composed of a hexagonal resonator cavity and a ring with a slit is proposed. By using the finite difference time domain method, the transmission properties of the structure were studied. It was found that three distinct plasmon-induced transparency peaks appear in the visible and near-infrared bands, and the transmissivity of the three peaks is more than 80%. By tuning the structure size, the positions of the peaks can be adjusted. Then we introduced graphene, covering the surface of the cavity. By adjusting the refraction index of the graphene using light, the position of the three transmission peaks can be changed correspondingly. Based on the effect, we designed an all-optical switcher with ultrafast optical response time (about 2 ps) and low light absorption (about 2.3%). The proposed waveguide structure provides a way for the development of visible and near-infrared filters and all-optical switchers.

Periodic arrays of plasmonic crossed-bowtie nanostructures interspaced with plasmonic nanocrosses for highly sensitive LSPR based chemical and biological sensing

In this paper, we present novel localized surface plasmon resonance (LSPR) sensors based on periodic arrays of gold crossed-bowtie nanostructures interspaced with gold nanocross pillars. Finite difference time domain (FDTD) numerical simulations were carried out to model bulk sensors as well as localized sensors based on the plasmonic nanostructures being proposed. The geometrical parameters of the plasmonic nanostructures are varied to obtain the best possible sensing performance in terms of sensitivity and figure of merit. A very high bulk sensitivity of 1753 nm per unit change in refractive index (nm RIU-1), with a figure of merit for bulk sensing (FOMbulk) of 3.65 RIU-1, is obtained for these plasmonic nanostructures. This value of bulk sensitivity is higher in comparison to previously proposed LSPR sensors based on plasmonic nanopillars and nanocrosses. Moreover, the optimized LSPR sensors being proposed in this paper provide maximum sensitivity of localized refractive index sensing of 70 nm/nm with a FOMlocalized of 0.33 nm-1. This sensitivity of localized refractive index sensing is the highest reported thus far in comparison with previously reported LSPR sensors. It is also demonstrated that the operating resonance wavelengths of these LSPR sensors can be controllably tuned for specific applications by changing the dimensions of the plasmonic nanostructures.

Tunable Multipolar Fano Resonances and Electric Field Enhancements in Au Ring-Disk Plasmonic Nanostructures

We theoretically research the characteristics of tunable multipolar Fano resonances in novel-designed Au ring-disk plasmonic nanostructures. We systematically study some structural parameters that influence the multipolar Fano resonances of the nanostructures. Adjustment of the radius (R₁ and R₂) of the Au ring, the radius (R₃) of the Au disk and the thickness (H) of the Au ring-disk can effectively adjust the multipolar Fano resonances. The complex field distributions excited by a Au ring-disk can produce dark resonance modes. At the frequency of the multipolar Fano resonances, strong localized field distributions can be obtained. The Fano resonances exhibit strong light-extinction properties in Au ring-disk nanostructures, which can be applied to an optical tunable filter and optical switch.

Fano-resonance-based mode-matching hybrid metasurface for enhanced second-harmonic generation

Plasmonic nanostructures have been considered as potential candidates for enhancing the nonlinear upconversion rate at nanoscale levels due to their strong near-field enhancement. Here, we propose a Fano-resonance-based mode-matching hybrid metasurface that combines the advantages of Fano resonances and mode-matching for boosting second-harmonic conversion. A confined and strong near-field intensity is generated by gold nanoantennas within the volume of polycrystalline zinc sulfide nanoparticles, thus resulting in a larger effective second-harmonic coefficient. The combination of the abovementioned features allows for the realization of a second-harmonic generation (SHG) conversion efficiency of 5.55×10^-8, and the SHG signal is twice that obtained with dipole hybrid metasurfaces. Our designed metasurface may pave the way for optimizing nonlinear light-matter interactions at the nanoscale.

Investigations of high order plasmonic resonance features of the nano hyper ring

A novel silver hyper ring and its complex nanostructures are designed and its plasmonic properties are investigated numerically. It is found that these hyper ring structures have relative stable optical features. The absorption cross section of the structure changes slightly when the direction and polarization of incident light is adjusting. For the complex structure, the position of each resonance peak does not present obvious change when the relative position of the inner hyper ring and outside larger ring changes. The result of the investigation has great significance for the production of practical nanostructures and the improvement of possible applications.

Plasmonic mode interferences and Fano resonances in Metal-Insulator-Metal nanostructured interface

Metal-insulator-metal systems exhibit a rich underlying physics leading to a high degree of tunability of their spectral properties. We performed a systematic study on a metal-insulator-nanostructured metal system with a thin 6 nm dielectric spacer and showed how the nanoparticle sizes and excitation conditions lead to the tunability and coupling/decoupling of localized and delocalized plasmonic modes. We also experimentally evidenced a tunable Fano resonance in a broad spectral window 600 to 800 nm resulting from the interference of gap modes with white light broad band transmitted waves at the interface playing the role of the continuum. By varying the incident illumination angle shifts in the resonances give the possibility to couple or decouple the localized and delocalized modes and to induce a strong change of the asymmetric Fano profile. All these results were confirmed with a crossed comparison between experimental and theoretical measurements, confirming the nature of different modes. The high degree of control and tunability of this plasmonically rich system paves the way for designing and engineering of similar systems with numerous applications. In particular, sensing measurements were performed and a figure of merit of 3.8 was recorded ranking this sensor among the highest sensitive in this wavelength range.

Fano resonance by dipole-hexapole coupling in a χ-shaped plasmonic nanostructure

We demonstrate that pronounced Fano resonance can be obtained by dipole-hexapole coupling in a χ-shaped plasmonic nanostructure. The confined local field in the vicinity of dipole-resonant nanorods excites and coherently couples to the hexapolar mode in the rod perpendicular to the polarization direction of the incident wave, leading to strong Fano resonance. By using the discrete dipole approximation, we numerically investigate the behavior of Fano resonance by dipole-hexapole coupling in the plasmonic nanostructure. We obtain the parameters relevant to Fano resonance by fitting the coupled oscillator model equation to the calculated extinction data and investigate their dependencies on the structural parameters. The results demonstrate the possibility of obtaining high-contrast Fano resonance with narrow features by using this plasmonic nanostructure, which has potential applications for highly sensitive biological sensing, low-loss metamaterials, and others.

Self-assembled large-area annular cavity arrays with tunable cylindrical surface plasmons for sensing

Surface plasmons that propagate along cylindrical metal/dielectric interfaces in annular apertures in metal films, called cylindrical surface plasmons (CSPs), exhibit attractive optical characteristics. However, it is challenging to fabricate these nanocoaxial structures. Here, we demonstrate a practical low-cost route to manufacture highly ordered, large-area annular cavity arrays (ACAs) that can support CSPs with great tunability. By employing a sol-gel coassembly method, reactive ion etching and metal sputtering techniques, regular, highly ordered ACAs in square-centimeter-scale with a gap width tunable in the range of several to hundreds of nanometers have been produced with good reproducibility. Ag ACAs with a gap width of 12 nm and a gap height of 635 nm are demonstrated. By finite-difference time-domain simulation, we confirm that the pronounced dips in the reflectance spectra of ACAs are attributable to CSP resonances excited in the annular gaps. By adjusting etching time and Ag film thickness, the CSP dips can be tuned to sweep the entire optical range of 360 to 1800 nm without changing sphere size, which makes them a promising candidate for forming integrated plasmonic sensing arrays. The high tunability of the CSP resonant frequencies together with strong electric field enhancement in the cavities make the ACAs promising candidates for surface plasmon sensors and SERS substrates, as, for example, they have been used in liquid refractive index (RI) sensing, demonstrating a sensitivity of 1505 nm/RIU and a figure of merit of 9. One of the CSP dips of ACAs with a certain geometry size is angle- (0-70 degrees) and polarization-independent and can be used as a narrow-band absorber. Furthermore, the nano annular cavity arrays can be used to construct solar cells, nanolasers and nanoparticle plasmonic tweezers.

Strong dipole-quadrupole coupling and Fano resonance in H-like metallic nanostructures

Under certain conditions of the incident light polarization direction a Fano resonance arises in small gold nanorods arranged in a H-like configuration. This stems from the coupling between a bright dipole plasmon mode excited in the horizontal rod and dark quadrupole plasmon modes in both vertical rods. We investigate these surface plasmon modes, and analyze the dependence of the Fano resonance on the geometry parameters such as rod size and interparticle separation, and refractive index of embedding medium. To describe the degree of this energy transfer, we introduce a new parameter: the Fano resonance efficiency. We calculate absorption cross-sections for visible and NIR spectrum in each element of the structure, and near-field distributions at different wavelengths. We show that Fano resonance in small H-like structures exhibits high sensitivity with respect to the refractive index of the host medium, outperforming the values for larger plasmonic structures based on nanorods already investigated.

Narrow and deep fano resonances in a rod and concentric square ring-disk nanostructures

Localized surface plasmon resonances (LSPRs) in metallic nanostructures have been studied intensely in the last decade. Fano interference is an important way to decrease the resonance linewidth and enhance the spectral detection resolution, but realizing a Fano lineshape with both a narrow linewidth and high spectral contrast-ratio is still challenging. Here we propose a metallic nanostructure consisting of a concentric square ring-disk (CSRD) nanostructure and an outside nanorod. Fano linewidth and spectral contrast ratio can be actively manipulated by adjusting the gap between the nanorod and CSRD, and by adjusting the gap between the ring and disk in CSRD. When the gap size in CSRD is reduced to 5 nm, the quadrupolar Fano linewidth is of 0.025 eV, with a contrast ratio of 80%, and the figure of merit reaches 15.

Generating and manipulating higher order Fano resonances in dual-disk ring plasmonic nanostructures

In this article, we investigate higher order (quadrupolar, octupolar, hexadecapolar, and triakontadipolar) Fano resonances generated in disk ring (DR) silver plasmonic nanostructures. We find that the higher order Fano resonances are generated when the size of the disk is reduced and falls into a certain range. With dual-disk ring (DDR) nanostructures, a rich set of tunable Fano line shapes is provided. More specifically, we report our observations on the optical behavior of the DDRs including asymmetric cases either in two disks with different sizes or their asymmetric locations inside the ring. In the case of symmetric dual-disk ring (SDDR) nanostructures, we demonstrate that the quadrupolar and the hexadecapolar Fano resonances are suppressed, which can reduce the cross-talk in spectroscopic measurements, while the octupolar and the triakontadipolar Fano resonances are enhanced. The potential of using the studied plasmonic nanostructures as biochemical sensors is evaluated with the figure of merit (FOM) and the contrast ratio (CR). The values of the FOM and the CR achieved using the triakontadipolar Fano resonance in the SDDR are 17 and 57%, respectively. These results indicate that the SDDRs could be developed into a high-performance biochemical sensor in the visible wavelength range.