Multipolar Fano Resonances Research Articles

Fano resonances in the corrugated disk resonator and their applications

We have experimentally excited terahertz multipolar Fano resonances in two asymmetrical metal particles: a defective corrugated metallic disk(CMD) structure and a hybrid structure consisted of a C-shaped resonator and a CMD. Furthermore, the Fano resonance modes can also be excited by the interaction between plasmonic waveguide and CMD. Our findings have shed light into the terahertz multipolar Fano resonances in asymmetrical CMD and opened the way to the design of terahertz plasmonic devices.

Multipolar Fano resonances in concentric semi-disk ring cavities

The plasmon properties of the concentric cavity consisting of a semi-disk inside a thin ring are theoretically investigated by the finite difference time domain and plasmon hybridization methods. According to the simulated absorption and scattering spectra, electric field distributions and charge diagrams, multiplar Fano resonances are successfully generated, mainly due to the coupling between the dark multipolar modes and the bright dipolar mode originating from the hybridization between the primitive modes of the semi-disk and the ring. Moreover, the intensities and spectral positions of the Fano resonances can be manipulated by modifying the geometric parameters of the cavity. The cavity for potential use as a multi-wavelength biochemical sensor is evaluated with the refractive index sensitivity and figure of merit.

Switch ON and OFF Dipolar Plasmon Dark Mode in Four-Fold Triangle Nanoclusters

In this letter, we experimentally and theoretically investigate the optical properties of four-fold symmetric triangle nanoclusters (TNCs) in nanoholes. Multipolar Fano resonances are observed in the presented TNCs, while dipolar Fano resonance is switched OFF. A plasmon hybridization model is proposed to investigate the underlying reasons of the absence of dipolar Fano resonance through decomposing the optical properties of TNCs into plasmon couplings between the horizontal and vertical bow-ties. Near-field charge density distributions show that the dipolar bonding mode (dipolar dark mode) between the two bow-ties is forbidden due to the four-fold symmetry of TNCs. Moreover, we demonstrate that the dipolar plasmon dark mode can be switched ON by breaking the structural symmetry. The presented nanostructures are helpful to investigate the optical properties of rotational symmetry 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.

Nanostrip-Induced High Tunability Multipolar Fano Resonances in a Au Ring-Strip Nanosystem.

Surface plasmon resonances of a Au ring-strip nanosystem with tunable multipolar Fano resonances have been investigated based on the finite-difference time-domain (FDTD) method. Abundant plasmon properties of a Au ring-strip nanosystem can be obtained on the basis of the unique electronic properties of different geometry parameters. In our research models, these multipolar Fano resonances are induced and can be tuned independently by changing the geometry parameters of the Au ring-strip nanosystem. Complex electric field distributions excited by the Au ring-strip nanosystem provide possibility to form dark plasmonic modes. Multipolar Fano resonances display strong light extinction in the Au ring-strip nanosystem, which can offer a new approach for an optical tunable filter, optical switching, and advanced biosensing.

Lateral sorting of chiral nanoparticles using Fano-enhanced chiral force in visible region.

Chiral gradient force allows a passive separation of an enantiomer since its direction is dependent on the handedness of its chiral entities. However, chiral polarisability is much weaker than electric polarisability. As a consequence, the non-chiral gradient force dominates over chiral force, which makes enantioselective sorting challenging. We present here, both numerically and analytically, that the chiral gradient force acting on chiral nanoparticles can overcome the non-chiral force when specimens are placed in a Fano-enhanced chiral gradient near-field using a plasmonic nanoaperture. Under circularly polarized light illumination, the interaction between the resonant modes of symmetric outer and asymmetric inner Au split-rings results in a splitting of the modal energies, which excites multipolar interference Fano resonances (FRs). This enables a local aperture between the two split-rings to possess very large optical chirality gradients while maintaining low gradients of electromagnetic energy density around the FRs from the visible region. By way of the lateral resultant force composed of both chiral and non-chiral gradient forces, we can accomplish a helicity-dependent transverse deflection of the chiral nanoparticles positioned above the aperture, which may offer a good platform for all-optical enantiopure compounds.

Enhancement of multipolar Fano resonances by nanocrescent elliptical disk structures

Multipolar Fano resonance is important for sensing, nonlinear optics, and enhanced spectroscopy at short wavelength. A symmetry breaking nanocrescent elliptical disk (NCED) structure is employed to demonstrate enhanced multipolar Fano resonances. The local-electric-field enhancement (LFE) factor of hexapole and octupole modes in NCED reaches approximately 1000. This is due to the strong coupling of nanocrescent and the nanogap modes formed by a nanocrescent and elliptical disk. The multipolar resonance wavelengths can be tuned by the short axis of the ellipse and is sensitive to the surrounding refractive index in an ultraviolet and visible region. This work presents the promising applications of the multipolar Fano resonance in surface Raman spectroscopy, bio-chemistry sensing, and nonlinear optical response.

Tunability of Multipolar Plasmon Resonances and Fano Resonances in Bimetallic Nanoshells

We study the multipolar surface plasmon modes and its link to Fano resonances in bimetallic nanoparticles. General expressions for the multipolar surface plasmon frequencies and damping rates are analytically derived by long-wave approximation. The results are in agreement with that from plasmon hybridization model for nanoshell system. Numerical results show that the surface plasmon resonances and damping rates for Au/Ag and Ag/Au nanoshells exhibit quite different behavior with the increase of shell thickness. In addition, both the near-field diagram and the far-field quantity are theoretically investigated. Fano resonances of scattering efficiency by Ag/Au core-shell nanoparticles occur due to the dipole-dipole coupling. The asymmetry of Fano profiles can be tuned by adjusting the size ratio of the core and the shell. Our results may found some potential applications in optical devices and biomedicine.

Excitation and tuning of Fano-like cavity plasmon resonances in dielectric-metal core-shell resonators.

Fano resonances have been realized in plasmonic systems and have found intriguing applications, in which, however, precisely controlled symmetry breaking or particular arrangement of multiple constituents is usually involved. Although simple core-shell type architectures composed of a spherical dielectric core and a concentric metallic shell layer have been proposed as good candidates that support inherent Fano resonances, these theoretical predictions have rarely seen any detailed experimental investigation. Here, we report on the experimental investigation of the magnetic and electric-based multipolar plasmonic Fano resonances in the dielectric-metal core-shell resonators that are formed by wrapping a nearly perfect metal shell layer around a dielectric sphere. We demonstrate that these Fano resonances originate from the interference between the Mie cavity and sphere plasmon resonances. Moreover, we present that the variation on either the dielectric core size or core refractive index allows for easily tuning the observed Fano resonances over a wide spectral range. Our findings are supported by excellent agreement with analytical calculations, and offer unprecedented opportunities for realizing ultrasensitive bio-sensors, lasing and nonlinear optical devices.

Low threshold tunable spaser based on multipolar Fano resonances in disk–ring plasmonic nanostructures

In this paper, we study theoretically spasers based on multipolar Fano resonances in disk-ring nanostructures covered with a silica layer doped with Yb3+:Er3+ used as the gain material. The electric field amplitudes at the quadrupolar mode (lasing wavelength) and the octupolar mode (pumping wavelength) are simultaneously enhanced by tens of times. Moreover, the spaser operates in a dark mode, which can reduce the radiation loss and enhance the confinement effectively. These factors work together to decrease greatly the critical gain coefficient and threshold. By adjusting the elliptic partial degrees of the nano-ring, the spasers can be tuned in the range of 1550 to 1650 nm, while the pumping light remains at the 980 nm absorption band of Yb3+ ions. Moreover, the spasers at three Fano resonance wavelengths of the disk-ring nanostructures appear in sequence with increasing the gain coefficient. The effects of the extinction coefficient on the electric field amplitudes of the pump light are also studied.

Fano resonance assisting plasmonic circular dichroism from nanorice heterodimers for extrinsic chirality.

In this work, the circular dichroisms (CD) of nanorice heterodimers consisting of two parallel arranged nanorices with the same size but different materials are investigated theoretically. Symmetry-breaking is introduced by using different materials and oblique incidence to achieve strong CD at the vicinity of Fano resonance peaks. We demonstrate that all Au-Ag heterodimers exhibit multipolar Fano resonances and strong CD effect. A simple quantitative analysis shows that the structure with larger Fano asymmetry factor has stronger CD. The intensity and peak positions of the CD effect can be flexibly tuned in a large range by changing particle size, shape, the inter-particle distance and surroundings. Furthermore, CD spectra exhibit high sensitivity to ambient medium in visible and near infrared regions. Our results here are beneficial for the design and application of high sensitive CD sensors and other related fields.

Multipolar Fano Resonances and Fano-Assisted Optical Activity in Silver Nanorice Heterodimers

We propose a simple structure of a silver nanorice heterodimer to achieve both Fano resonances and strong optical activity. Due to the ellipsoid shape of nanorice, the dimer shows a unique plasmon hybridization picture, being different from those in previously studied two-nanoparticle coupling, which benefits the generation of Fano resonances here. Under oblique incidence, this nanorice dimer exhibits not only multipolar Fano resonances but also very strong optical activity in the Fano resonant ranges. A chiral molecule model is proposed to explain the generation of chirality. It is also shown that Fano resonance has an assisting effect on optical activity (especially for subradiant modes), which can be ascribed to the energy transfer from the super-radiant modes to the subradiant modes during Fano interference.

High Tunability Multipolar Fano Resonances in Dual-Ring/Disk Cavities

Metallic nanostructures that support multipolar Fano resonances have drawn much attention in recent years. Such structures are applicable especially to enhanced nonlinear optics, where two resonance wavelengths need to be modulated simultaneously. However, how to tune multipolar Fano resonances independently remains a challenge. In the paper, the plasmonic nanostructure consisting of two ring/disk cavities (RDCs) is investigated using the finite element method. The dark multipolar modes of each RDC are excited, and sharp multipolar Fano resonances are induced. The multipolar modes supported by different RDCs can be tuned independently by changing the sizes. The line-widths of such Fano resonances nearly keep below 0.05 eV, and the contrast ratio (CR) of the two quadrupolar Fano dips mostly maintain above 50 %. In addition, the exciting bonding modes of different RDCs make the selective storage of resonance energy available. Such plasmonic nanostructures may find applications in enhanced nonlinear optics or nano-optical elements.

Multiple and Multipolar Fano Resonances in Plasmonic Nanoring Pentamers

The optical properties of plasmonic nanoring pentamers are experimentally and theoretically investigated in the near‐infrared spectral region, where the nanoring metamolecules allow complex optical design by tuning a single geometrical parameter (the inner radius of the nanoring). Even in fully four‐fold symmetric nanoring pentamers, unconventional plasmonic coupling paths generate multiple Fano resonances which would not appear in symmetric nanodisc oligomers. In addition, inhomogeneous elementary units in the pentamer induce additional resonances due to high‐order dark modes, while complete breaking of the four‐fold symmetry results in sharp multipolar Fano resonances upon the hybridization of dark and bright modes. This approach demonstrates a simple way to engineer plasmonic couplings by subtle changes of uniform metamaterial design.