Multiple Fano Resonances Research Articles

Optical magnetism and plasmonic Fano resonances in metal-insulator-metal oligomers.

The possibility of achieving optical magnetism at visible frequencies using plasmonic nanostructures has recently been a subject of great interest. The concept is based on designing structures that support plasmon modes with electron oscillation patterns that imitate current loops, that is, magnetic dipoles. However, the magnetic resonances are typically spectrally narrow, thereby limiting their applicability in, for example, metamaterial designs. We show that a significantly broader magnetic response can be realized in plasmonic pentamers constructed from metal-insulator-metal (MIM) sandwich particles. Each MIM unit acts as a magnetic meta-atom and the optical magnetism is rendered quasi-broadband through hybridization of the in-plane modes. We demonstrate that scattering spectra of individual MIM pentamers exhibit multiple Fano resonances and a broad subradiant spectral window that signals the magnetic interaction and a hierarchy of coupling effects in these intricate three-dimensional nanoparticle oligomers.

Multiple Fano interferences in a plasmonic metamolecule consisting of asymmetric metallic nanodimers

We theoretically explore signatures of plasmonic Fano interferences in a subwavelength plasmonic metamolecule consisting of closely packed asymmetric gold nanodimers, which lead to the possibility of generating multiple Fano resonances in the scattering spectrum. This spectral feature is attributed to the interference between bright and dark plasmonic modes sustained by the constituent nanodimers. The excited Fano dips are highly sensitive in both wavelength and amplitude to geometry and background dielectric medium. The tunability of induced Fano resonances associated with enhanced electric fields from the visible to infrared region provides promising applications, particularly in refractive index sensing, light-trapping, and photon up-converting.

Multiple Fano Resonances in Plasmonic Metamaterials Composed of Al/Al2O3 Nanomatryushka Structures

ABSTRACTMetamaterial structures composed of ordered arrays of metallic nanoparticles (NPs) and nanocavities are able to support strong plasmon and Fano resonances in the optical frequencies, where the appeared Fano dips can be utilized in bio/chemical sensing and spectroscopic purposes with a significant sensitivity. Herein, we utilize two concentric compositional Aluminum (Al) nanoshells (Al/Al2O3) to design nanomatryushka (NM) structures in periodic arrays, where each one of Al NPs is covered by a certain thickness of the oxide layer. Depositing studied Al NM arrays on metasurfaces, we determined the optical response of the metamaterial. It is shown that the proposed structure is able to support multiple strong Fano resonances in the visible spectrum. Evaluating the plasmon response of the metamaterial configuration for the presence of various semiconductor metasurfaces (Silicon and GaP), the quality of Fano dips is analyzed for different regimes. In this method, we measured the accuracy and sensitivity of the metamaterial structure by plotting the linear figure of merit (FoM) and quantifying this parameter.

Multiple Fano Resonances Based on Different Waveguide Modes in a Symmetry Breaking Plasmonic System

Multiple Fano resonances are numerically investigated based on different waveguide modes in a nanoscale plasmonic waveguide resonator system, which consists of two grooves coupled with a metal-insulator-metal (MIM) waveguide. Simulation results show that by introducing a small structural breaking in the plasmonic resonator, both symmetric and antisymmetric waveguide modes can be excited. Due to the interaction of the symmetric and antisymmetric waveguide modes, the transmission spectra possess a sharp asymmetrical profile. Because of different origins, these Fano resonances exhibit different dependence on the parameters of the structure and can be easily tuned. These characteristics offer flexibility to design the device. This nanosensor yields a sensitivity of ~820 nm/RIU and a figure-of-merit of ~ 3.2× 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> . The utilization of the antisymmetric mode in the MIM waveguide provides a new possibility for designing high-performance plasmonic devices.

Multiple coil-type Fano resonances in all-dielectric antisymmetric quadrumers

In this study, we examined the optical properties of an antisymmetric quadrumer composed of silicon (Si) nanodisks that are located in a close proximity to each other as a molecular cluster. Considering the magnetic response of the presented cluster and modifying the geometrical sizes of employed nanodisks, we calculated the extinction cross-sectional profile in a numerical methods by using two-dimensional finite-difference time-domain model. We verified that proposed configuration can be tailored to support multiple strong and deep coil-type Fano resonances without significant radiation losses. Comparing the performance of the structure with analogous metallic nanoclusters, the superior behavior of the investigated quadrumer is evaluated and confirmed. We verified that localized magnetic fields can be exploited to intensify the energy of produced hot-spots.

Fano line shapes in the branch–baffle system

A compact branch–baffle system, consisting of a branch resonator and a baffle in the metal–dielectric–metal waveguide (MDM), is proposed. A novel and efficient circuit model is developed to present the mechanisms of the plasmonic system. The model predicts two sharp asymmetric Fano line shapes, which are caused by the interactions between the branch resonator and the constructed branch–baffle resonator, exhibiting good agreement with the finite-element simulation results. The peaks of the two Fano line shapes are investigated by the relative phase method and demonstrated referring to the field distributions. The physical mechanisms of the remaining three resonances in the transmission spectrum are also carefully studied. From the analysis, the five resonances can all be precisely calculated and designed in theory. Considering the compact structure, multiple Fano resonances may be adjusted in different ways. Also, the transmittance of the Fano resonances is highly efficient (0.8). The proposed structure can have important applications in high-resolution and high-sensitivity nano-plasmonic devices.

Interaction between single nano-emitter and plasmonic disk–ring nanostructure with multiple Fano resonances

We study the emission behavior of an electric dipolar nano-emitter coupled with a disk–ring nanostructure (DRN) that sustains multiple plasmonic Fano resonances. The emitter–DRN electromagnetic coupling efficiency strongly depends on the relative position of the nano-emitter and the DRN, which determines whether the multiple Fano interactions are visibly activated. More specifically, for longitudinal polarization, the multiple Fano resonances are pronounced when the nano-emitter is at the outer apex of the disk or at the gap center of the DRN, observable in the far-field and/or near-field characteristics. However, no obvious Fano feature shows up when the nano-emitter is near the outer apex of the ring. For the case in which the nano-emitter oscillates vertically with respect to the DRN axis, Fano resonance is dramatic only when the nano-emitter is inside the gap of the DRN. We show that the cascading amplification of the dipole moment by the nanodisk is crucial for the excitation of the multiple Fano resonances. Our results are useful in engineering plasmon-modified optical spectroscopy and photon emission control, particularly in resonant plasmonic heterostructures.

Double Fano resonances in nanoring cavity dimers: The effect of plasmon hybridization between dark subradiant modes

Dark mode which is subradiant plays a key role in the generation of Fano effect. This study proposes that plasmon interaction between dark modes is a favorable method to generate multiple Fano resonances, where plasmon hybridization leads to the formation of a subradiant bonding and a subradiant antibonding combination. It demonstrates that a concentric ring/ring cavity dimer introduces interactions that render bonding quadrupolar ring mode dipole active, resulting in a pronounced Fano resonance. The corresponding antibonding quadrupolar ring mode is excited in a symmetry breaking nonconcentric cavity dimer, and double Fano resonances appear in the spectra.

Multiple Fano resonances in spoof localized surface plasmons.

We present the occurrence of bright modes and dark modes in spoof localized surface plasmons (LSPs) generated by ultrathin corrugated metallic disks. As two such disks with asymmetric geometries are placed in close proximity, we find that dark modes (in multipoles) of one disk emerge by coupling with the bright modes (in dipoles) of the other disk. Then we further observe multiple Fano resonances due to destructive interferences of dark modes with the overlapping and broadened bright modes. These Fano line-shapes clearly exhibit the strong polarization dependence. We design and fabricate the ultrathin corrugated bi-disk structure in the microwave frequency, and the measurement results show reasonable agreement with theoretical predictions and numerical simulations. Such multiple Fano resonances could be exploited for the plasmonic devices at lower frequencies.

Multiple Fano resonances in bimetallic layered nanostructures

Plasmonic Fano resonances arise in bimetallic layered nanostructures (metal–dielectric–metal and dielectric–metal–dielectric–metal) are studied theoretically as the function of their geometrical parameters. Multiple Fano resonances are generated in these nanostructures where several subradiant dark modes appear due to the geometrical symmetry breaking induced by offsetting different layers. The plasmonic responses of the proposed nanoparticles with equal volumes are compared and multiple Fano resonances with large modulation depths are obtained in metal–dielectric–metal structure which is highly suitable for multi-wavelength sensing and plasmon line shaping. However, the dielectric–metal–dielectric–metal nanostructure is found to provide a slightly better tunability of higher-order plasmonic modes compared to metal–dielectric–metal nanostructure due to which it can be useful for biomedical applications.

Generation of Multiple Fano Resonances in Plasmonic Split Nanoring Dimer

We present a computational study of the plasmonic response of a split nanoring dimer resonator which supports multiple plasmonic Fano-like resonances that arises by the coupling and interference of the dimer plasmon modes. For the generation of Fano resonances with large modulation depths, numerous configurations of the dimer resonator are analyzed which are observed to be highly dependent on the polarization of incident light. Moreover, the influence of dimension of the split nanoring structure on the spectral positions and intensities of the higher order Fano resonances are also investigated, and it is found that the asymmetric Fano line shapes can be flexibly tuned in the spectrum by varying various geometrical parameters. Such Fano resonators are also discovered to offer high values of figure of merit and contrast ratio due to which they are suitable for high-performance biological sensors.

MULTIPLE FANO RESONANCES STRUCTURE FOR TERAHERTZ APPLICATIONS

A new planar engineered material structure, which has multiple Fano resonances at the terahertz range of frequency, is presented. Starting with a double Fano resonance structure, it is shown that by considering several unit cells as a larger unit cell and creating new asymmetries in the super- cell, we can have five Fano resonances in one structure. Analysis of current distributions at resonance frequencies clarifies the origin of different resonances. We show that all of these resonances come from different arrangement of magnetic dipoles.

Tunable Plasmonic and Hyperbolic Metamaterials Based on Enhanced Nonlinear Response

We present here tunable and reconfigurable designs of linear and nonlinear plasmonic and hyperbolic metamaterials. Rich scattering features of multilayered composite nanoparticles are demonstrated, which include complex and exotic scattering signatures combining multiple dipolar Fano resonances and electromagnetic induced transparency (EIT) features. These dipole-dipole multi-Fano scattering responses can be further tuned through altering the plasmonic properties of the concentric layers or the permittivity of the core, for instance, by the presence of nonlinearities. Strong third-order nonlinear effects, such as optical bistability, may also be induced in the scattering response of nonlinear nanoparticles due to the highly enhanced and confined fields inside their core. Nonlinear hyperbolic metamaterial designs are also explored, which can realize tunable positive-to-negative refraction at the same frequency, as a function of the input intensity. Negative Goos-Hänchen shift is demonstrated based only on the hyperbolic dispersion properties of these layered metamaterials without the usual need of negative index metamaterials. The Goos-Hänchen shift may be tuned from positive-to-negative values, when the structure is illuminated with different frequencies. A plethora of applications are envisioned based on the proposed tunable metamaterials, such as ultrafast reconfigurable imaging devices, tunable sensors, novel nanotag designs, and efficient all-optical switches and memories.

Excitation of Multiple Fano-Like Resonances Induced by Higher Order Plasmon modes in Three-Layered Bimetallic Nanoshell Dimer

The presence of plasmonic Fano-like resonances in the optical response of isolated and dimer metal-dielectric-metal nanostructures are investigated theoretically. The nanostructures are engineered in such a way to support multiple Fano-like resonances that are induced by the interference of bright and dark plasmon modes. It is found that the dimer resonators exhibit different types of Fano resonances for both the transverse and longitudinal polarizations unlike conventional nanodimers. Several configurations of the dimer Fano resonator are analyzed with special emphasis on the Fano spectral line shape. Breaking the symmetry of the dimer nanostructure in various directions control the asymmetric line shape and provides different kinds of unique Fano resonances. In certain cases, the Fano resonators exhibit multiple Fano resonances that are particularly significant for plasmon line shaping and can serve as platforms for multi-wavelength sensing applications.

Multiple Magnetic Mode-Based Fano Resonance in Split-Ring Resonator/Disk Nanocavities

Plasmonic Fano resonance, enabled by the weak interaction between a bright super-radiant and a subradiant resonance mode, not only is fundamentally interesting, but also exhibits potential applications ranging from extraordinary optical transmission to biosensing. Here, we demonstrate strong Fano resonances in split-ring resonators/disk (SRR/D) nanocavities. The high-order magnetic modes are observed in SRRs by polarization-resolved transmission spectroscopy. When a disk is centered within the SRRs, multiple high-order magnetic modes are coupled to a broad electric dipole mode of SRR/D, leading to significant Fano resonance spectral features in near-IR regime. The strength and line shape of the Fano resonances are tuned through varying the SRR split-angle and interparticle distance between SRR and disk. Finite-difference-time-domain (FDTD) simulations are conducted to understand the coupling mechanism, and the results show good agreement with experimental data. Furthermore, the coupled structure gives a sensitivity of ∼282 nm/RIU with a figure of merit ∼4.

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.

Equivalent Permittivity and Permeability and Multiple Fano Resonances for Nonlocal Metallic Nanowires

Equivalent permittivity and permeability for metallic nanowires with spatial dispersion are derived analytically. The extinction cross section based on the local full-wave theory with our equivalent permittivity and permeability is found to be in agreement with the one based on the nonlocal full-wave theory. When the electromagnetic wave is incident on the nanocylinder with nonlocal permittivity, multiple Fano resonances exist because of the interference of different longitudinal modes with the dipole moments, and the nonlocal nanocylinder can be designed to yield an abrupt transition between full transparency and strong scattering. Interestingly, both the frequencies at which the transparency and strong scattering occur can be described well by the equivalent permittivity. Our study may be helpful in the design of optical nanoswitches and nanomemories.

Tuning multiple Fano and plasmon resonances in rectangle grid quasi-3D plasmonic-photonic nanostructures

Plasmonic and photonic nanostructures can manipulate light-matter interaction, leading to a wide range of tunable properties. Here, we show that multiple Fano and plasmon resonances can be generated in quasi-3D plasmonic nanostructure arrays on rectangle grid photonic nanostructure substrates. The Fano resonances are the quasiguided modes coupled with the plasmon resonance while the distinct plasmon resonances are the localized surface plasmon resonances of the top gold thin film with sub-wavelength nanohole array and the bottom gold nanodisk array. The Fano and plasmon resonances can be tuned separately and selectively by changing the dimension of photonic and plasmonic nanostructures.

Tuning multiple Fano resonances in plasmonic pentamer clusters

Multiple Fano resonances in plasmonic pentamer clusters composed of nanorings are observed and investigated. Molecular point group theory is used to understand the formation of multiple Fano resonances. By modifying the radius of the center ring or the angle between the center and the surrounding rings, the modulation depths and the spectral positions of the multiple Fano resonances can be tuned within a wide range. For pentamers composed of split nanorings, another Fano resonance can be excited because of the excitation of the quadrupole mode of the surrounding split nanorings.

Multiple Fano resonances in single-layer nonconcentric core-shell nanostructures

Multiple plasmonic Fano resonances are generally considered to require complex nanostructures, such as multilayer structure, to provide several dark modes that can couple with the bright mode. In this paper, we show the existence of multiple Fano resonances in single layer core-shell nanostructures where the multiple dark modes appear due to the geometrical symmetry breaking induced by axial offset of the core. Both dielectric-core-metal-shell (DCMS) and metal-core-dielectric-shell (MCDS) configurations have been studied. Compared to the MCDS structure, the DCMS configuration provides higher modulation depth. Analytical studies based on transformation optics and numerical simulations have been performed to investigate the role of geometrical and material parameters on the optical properties of the proposed nanostructures. Refractive index sensing with higher-order Fano resonances has also been described, providing opportunity for multiwavelength sensing with high figure of merit.