Plasmon resonances and strong electric field enhancements in side-by-side tangent nanospheroid homodimers

Single-double-band switchable optical circular polarizers based on surface plasmon resonance

A single-double-band switchable circular polarization filter based on surface plasmon resonance exhibits significant potential for applications in fields such as communication and sensing due to its adjustable, low-cost, and easy integration features. In this study, we propose a bi-layer rod nanostructure and use FEM simulation to study the transmission spectra of the structure. The results demonstrate that the structure exhibits both single- and double-band circular polarization filtering effects, which can be switched by varying geometric parameters such as the distance between the two layers and the width of nanorods. Furthermore, the filtering effects of both single- and double-band are highly dependent on the length of the nanorods, with average extinction rates reaching 486 and 2020/129, respectively; the operating bandwidths (defined as extinction ratio >10) can reach 170 nm and 35 nm/70 nm, respectively. The underlying physical mechanisms are clarified by analyzing the electric dipole, magnetic dipole resonance modes, and induced chiral fields on nanostructures.

Optical Ultracompact Directional Antennas Based on a Dimer Nanorod Structure

Controlling directionality of optical emitters is of utmost importance for their application in communication and biosensing devices. Metallic nanoantennas have been proven to affect both excitation and emission properties of nearby emitters, including the directionality of their emission. In this regard, optical directional nanoantennas based on a Yagi-Uda design have been demonstrated in the visible range. Despite this impressive proof of concept, their overall size (~λ²/4) and considerable number of elements represent obstacles for the exploitation of these antennas in nanophotonic applications and for their incorporation onto photonic chips. In order to address these challenges, we investigate an alternative design. In particular, we numerically study the performance of a recently demonstrated "ultracompact" optical antenna based on two parallel gold nanorods arranged as a side-to-side dimer. Our results confirm that the excitation of the antiphase mode of the antenna by a nanoemitter placed in its near-field can lead to directional emission. Furthermore, in order to verify the feasibility of this design and maximize the functionality, we study the effect on the directionality of several parameters, such as the shape of the nanorods, possible defects in the dimer assembly, and different positions and orientations of the nanoemitter. We conclude that this design is robust to structural variations, making it suitable for experimental upscaling.

Au-on-Ag nanostructure forin-situSERS monitoring of catalytic reactions

Dual-functionality Au-on-Ag nanostructures (AOA) were fabricated on a silicon substrate by first immobilizing citrate-reduced Ag nanoparticles (Ag NPs, ∼43 nm in diameter), followed by depositing ∼7 nm Au nanofilms (Au NFs) via thermal evaporation. Au NFs were introduced for their catalytic activity in concave-convex nano-configuration. Ag NPs underneath were used for their significant enhancement factor (EF) in surface-enhanced Raman scattering (SERS)-based measurements of analytes of interest. Rhodamine 6G (R6G) was utilized as the Raman-probe to evaluate the SERS sensitivity of AOA. The SERS EF of AOA is ∼37 times than that of Au NPs. Using reduction of 4-nitrothiophenol (4-NTP) by sodium borohydride (NaBH₄) as a model reaction, we demonstrated the robust catalytic activity of AOA as well as its capacity to continuously monitor via SERS the disappearance of reactant 4-NTP, emergence and disappearance of intermediate 4,4'-DMAB, and the appearance of product 4-ATP throughout the reduction process in real-time andin situ.

Broadband circular polarizer based on twisted plasmonic nano-disks

The broadband circular polarizer based on the surface plasmon of a metal nanostructure has important practical application due to its adjustability, low cost, and easy integration. In this article, a twisted multilayer double semi-disc structure is designed to allow for a broadband circular polarizer, whose transmission spectra are simulated using the finite element method under left-handed circularly polarized and right-handed circularly polarized extinction. The results show a large extinction ratio up to 900 for properly designed geometry and a broad bandwidth (defined as extinction ratio >10) of 50-160 nm. Additionally, the broadband position, bandwidth, and extinction rate are all sensitive to some geometric parameters, such as rotation angle, disk radius, and more, and the mechanism of this phenomenon is clarified by exploring the plasmon electromagnetic resonance on nanostructures.

Tunable broadband circular polarizer based on ultrahigh-order surface plasmonic resonance

Circular polarizers have potential applications in optical communication and liquid crystal display. In this paper, a multilayer twisted nanoring-rod nanostructure is designed. The finite element method is used to explore the surface plasmon of the structure under the excitation of left-handed circularly polarized light and right-handed circularly polarized light. The results show that the structure can be used as a polarizer in the tunable operating bandwidth of 400-1290 nm, and tunability is achieved by changing the filling medium near the structure instead of the geometric parameters of the nanostructures. We qualitatively reveal the physical mechanism of this phenomenon from the perspective of plasmon resonance coupling by plotting the charge distribution at several specific wavelengths.

Plasmonic Spherical Heterodimers: Reversal of Optical Binding Force Based on the Forced Breaking of Symmetry

The stimulating connection between the reversal of near-field plasmonic binding force and the role of symmetry-breaking has not been investigated comprehensively in the literature. In this work, the symmetry of spherical plasmonic heterodimer-setup is broken forcefully by shining the light from a specific side of the set-up instead of impinging it from the top. We demonstrate that for the forced symmetry-broken spherical heterodimer-configurations: reversal of lateral and longitudinal near-field binding force follow completely distinct mechanisms. Interestingly, the reversal of longitudinal binding force can be easily controlled either by changing the direction of light propagation or by varying their relative orientation. This simple process of controlling binding force may open a novel generic way of optical manipulation even with the heterodimers of other shapes. Though it is commonly believed that the reversal of near-field plasmonic binding force should naturally occur for the presence of bonding and anti-bonding modes or at least for the Fano resonance (and plasmonic forces mostly arise from the surface force), our study based on Lorentz-force dynamics suggests notably opposite proposals for the aforementioned cases. Observations in this article can be very useful for improved sensors, particle clustering and aggregation.

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.

Generation and manipulation of ultrahigh order plasmon resonances in visible and near-infrared region

Optical properties of periodic nanorings with built-in V-shaped nanowedges (NRBV) are investigated theoretically. Tunable ultrahigh order Fano resonances are achieved and they are found to be sensitive to geometric parameters and surrounding dielectric environment of the planar nanostructure. High order Fano resonances can be suppressed or enhanced by adjusting the opening angle of the nanowedge, the size of the nanoring and the aspect ratio of the nanowedge. Moreover, manipulating the offset of the built-in nanowedge, or filling dielectrics asymmetrically can revive suppressed Fano resonances when the V-shaped nanowedge develops into a straight nanorod. Meanwhile, stronger plasmon resonances emerge alternately in the two parts of this planar nanostructure. This periodic plasmonic nanostructure produces ultrahigh order plasmon resonances and stronger electric field enhancement, which have great potential applications in multi-wavelength surface enhanced spectroscopy and biochemical sensing.

Plasmonic-enhanced perovskite solar cells using alloy popcorn nanoparticles

This article demonstrates a significant broadband enhancement of light absorption and improvement of photon-generated-charge transfer in CH₃NH₃PbI₃ perovskite solar cells by incorporating plasmonic Au–Ag alloy popcorn-shaped nanoparticles (NPs).

Angle-Resolved Plasmonic Properties of Single Gold Nanorod Dimers

Through wet-chemical assembly methods, gold nanorods were placed close to each other and formed a dimer with a gap distance ~1 nm, and hence degenerated plasmonic dipole modes of individual nanorods coupled together to produce hybridized bonding and antibonding resonance modes. Previous studies using a condenser for illumination result in averaged signals over all excitation angles. By exciting an individual dimer obliquely at different angles, we demonstrate that these two new resonance modes are highly tunable and sensitive to the angle between the excitation polarization and the dimer orientation, which follows cos² φ dependence. Moreover, for dimer structures with various structure angles, the resonance wavelengths as well as the refractive index sensitivities were found independent of the structure angle. Calculated angle-resolved plasmonic properties are in good agreement with the measurements. The assembled nanostructures investigated here are important for fundamental researches as well as potential applications when they are used as building blocks in plasmon-based optical and optoelectronic devices.