Ultra-High Sensitivity Nanosensor Based on Multiple Fano Resonance in the MIM Coupled Plasmonic Resonator

This paper proposes a compact plasmonic structure that is composed of a metal-insulator-metal (MIM) waveguide coupled with a groove and stub resonators, and then investigates it by utilizing the finite element method (FEM). Simulation results show that the interaction between the local discrete state caused by the stub resonator and the continuous spectrum caused by the groove resonator gives rise to one of the two Fano resonances, while the generation of the other resonance relies only on the groove. Meanwhile, the asymmetrical linear shape and the resonant wavelength can be easily tuned by changing the parameters of the structure. By adding stubs on the groove, we excited multiple Fano resonances. The proposed structure can serve as an excellent plasmonic sensor with a sensitivity of 2000 nm/RIU and a figure of merit of about 3.04?×?10³, which can find extensive applications for nanosensors.     

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.     

Ultra-high Sensitivity Plasmonic Nanosensor Based on Multiple Fano Resonance in the MDM Side-Coupled Cavities

We propose a compact plasmonic structure comprising a metal-dielectric-metal (MDM) waveguide coupled with a side cavity and groove resonators. The proposed system is investigated by the finite element method. Simulation results show that the side-coupled cavity supports a local discrete state and the groove provides a continuous spectrum, the interaction between them, gives rise to the Fano resonance. The asymmetrical line shape and the resonant wavelength can be easily tuned by changing the geometrical parameters of the structure. Moreover, we can extend this plasmonic structure by the double side-coupled cavities to gain the multiple Fano resonances. The proposed structure can serve as an excellent plasmonic sensor with a sensitivity of ～1900 nm/RIU and a figure of merit of about ～3.8?×?10⁴, which can find wide applications for nanosensors.     

Tunable Multi-Fano Resonances in MDM-Based Side-Coupled Resonator System and its Application in Nanosensor

In this paper, two Fano resonances are achieved in the proposed plasmonic system. Theoretical analysis and simulation results show that two discrete states coupled with a continua lead to these Fano resonances. The discrete states are provided by the side-coupled square cavity, and a baffle plate placed in metal-dielectric-metal waveguide is used to produce a continuous transmission spectrum. The resonant wavelengths and the linewidth of these Fano resonances can be easily tuned by adjusting the parameters of system. This system exhibits high sensitivities as high as 850 and 1120 nm/RIU corresponding to two Fano resonances, and the figure of merit can reach to 1.7 × 10⁵ by optimizing the system. By introducing another square cavity, four Fano resonances are obtained which originate from four discrete states coupled with continua, and they can be tuned independently. The flexible multi-Fano resonances system has significant application bio-nanosensor, nonlinear, and slow light devices.     

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.     

Magneto-Electric Double Fano Resonances in Hybrid Split Ring/Disk Hetero-Cavity

In this work, we conceive and demonstrate the magneto-electric double Fano resonances of a hetero-cavity composed of Si disk and Au split ring, where Si disk can provide additional magnetic responses besides electric responses. The interference between electric and magnetic responses in proposed hetero-cavity gives rise to magneto-electric double Fano resonances with magnetic and electric near-field enhancements. Dipole radiative enhancement is used to analyze magnetic and electric responses of hetero-cavity and the spectral features of hetero-cavity can be used to quantitatively characterize by coupled oscillator model. And the spectral tunability of magneto-electric double Fano resonances is investigated, highlighting a potential for applications in low-loss sensing and nanophotonic devices.     

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.     

Electromagnetically Induced Transparency and Sharp Asymmetric Fano Line Shapes in All-Dielectric Nanodimer

Low-loss electromagnetically induced transparency (EIT) and asymmetric Fano line shapes are investigated in a simple planar silicon dimer resonator. The EIT and Fano effects emerge due to near-field coupling of the modes supported by both the nanoparticles in a dimer structure. Different configurations of the dimer nanostructure are analyzed, which provide distinct EIT and Fano resonances. Furthermore, the tunability of EIT and Fano resonant modes are incorporated by changing the structural parameters. It is also found that the dimer resonator exhibits high Q factor and large electromagnetic field enhancement at Fano resonance and EIT window due to extremely low absorption loss. Such values and narrow resonances are supposed to be useful highly sensitive sensors and slow-light applications.     

High-Q Fano Resonances in Asymmetric and Symmetric All-Dielectric Metasurfaces

We present high-quality (high-Q) Fano resonances in all-dielectric metasurfaces consisting of a periodic array of air holes on silicon (Si) film, deposited on the top of quartz substrate. With the control of the radius difference Δr and center distance Δd between the air holes, two asymmetric all-dielectric metasurfaces are proposed to achieve extremely high-Q Fano resonances. Numerical method with finite difference time domain and equivalent circuit model is employed to analyse the excitation mechanism of the sharp Fano resonances. It is shown that the high-Q Fano resonances come from the interference of two Fabry-Perot resonances, resulting in an extremely narrow window. Moreover, we also demonstrate that the high-Q Fano resonances can also be realized as electromagnetic wave is obliquely incident on the symmetric all-dielectric metasurface. Finally, we show the high-Q Fano resonances caused by asymmetric configurations can coexist with the Fano resonances in the symmetric configuration induced by oblique incidence. As a result, a tri-band Fano resonance is obtained. It is expected that our results will provide important mechanisms for tuning and switching a wide variety of optical devices such as angular sensors, filters, switches, and modulators.     

Fano Resonances Induced by Strong Interactions Between Dipole and Multipole Plasmons in T-Shaped Nanorod Dimer

A simple T-shaped plasmonic nanostructure composed of two perpendicular coupled nanorods is proposed to produce strong Fano resonances. By the near-field coupling between the “bright” dipole and “dark” quadrupole plasmons of the nanorods, a deep Fano dip is formed in the extinction spectrum, which can be well fitted by the Fano interference model. The effects of the geometry parameters including nanorod length, coupling gap size, and coupling location to the Fano resonances are analyzed in detail, and a very high refractive index sensitivity is achieved by the Fano resonance. Also by adjusting the incident polarization direction, double Fano resonances can be formed by the interferences of the dipole, quadrupole, and hexapole plasmons. The proposed nanorod dimer structure is agile, and a trimer which supports double Fano resonances can be easily formed by introducing a third perpendicular coupled nanorod. The proposed T-shaped nanorod dimer structure may have applications in the fields of biological sensing and plasmon-induced transparency.     

Fano Resonance Based on End-Coupled Cascaded-Ring MIM Waveguides Structure

Plasmonics - Though adding a groove to a plasmonic end-coupled perfect ring (PR) resonator, two additional resonance modes, which can be controlled by the length of the groove, will arise in this...     

Tuning the Fano Resonance Between Localized and Propagating Surface Plasmon Resonances for Refractive Index Sensing Applications

Localized and propagating surface plasmon resonances are known to show very pronounced interactions if they are simultaneously excited in the same nanostructure. Here, we study the Fano interference that occurs between localized surface plasmon resonance (LSPR) and propagating surface plasmon polariton (SPP) modes by means of phase-sensitive spectroscopic ellipsometry. The sample structures consist of periodic gratings of gold nanodisks on top of a continuous gold layer and a thin dielectric spacer, in which the structural dimensions were tuned in such a way that the dipolar LSPR mode and the propagating SPP modes are excited in the same spectral region. We observe pronounced anti-crossing and strongly asymmetric line shapes when both modes move to each other’s vicinity, accompanied of largely increased phase differences between the respective plasmon resonances. Moreover, we show that the anti-crossing can be exploited to increase the refractive index sensitivity of the localized modes dramatically, which result in largely increased values for the figure-of-merit which reaches values between 24 and 58 for the respective plasmon modes.     

Plasmonic Fano Resonances in Single-Layer Gold Conical Nanoshells

Plasmonic Fano resonances arise in symmetric single-layer conical nanoshells, which can be switched on and off by changing the polarization of the incident electric field. By breaking the symmetry, higher-order dark hybridized modes emerge in the spectrum, which couple to the superradiant bright mode and induce higher-order plasmonic Fano resonances. From a comparison with spherical nanostructures, it comes out that single-layer conical nanoshells are found to be highly capable in the generation of higher-order Fano resonances with larger modulation depths in the optical spectra. Such nanostructures are also found to offer high values of figure of merit and contrast ratio due to which they are highly suitable for biological sensors.     

High Sensitivity Nanoplasmonic Sensor Based on Plasmon-Induced Transparency in a Graphene Nanoribbon Waveguide Coupled with Detuned Graphene Square-Nanoring Resonators

A novel nanoscale structure for high sensitivity sensing which consists of a graphene nanoribbon waveguide coupled with detuned graphene square-nanoring resonators (GSNR) based on edge mode is investigated in detail. By altering the Fermi energy level of the graphene, the plasmon-induced transparency (PIT) window from the destructive interference between a radiative square-nanoring resonator and a dark square-nanoring resonator can be easily tailored. The coupled mode theory (CMT) is used to show that the theoretical results agree well with the finite difference time domain (FDTD) simulations. This nanosensor yields a ultrahigh sensitivity of ∼2600 nm/refractive index unit (RIU) and a figure of merit (FOM) of ∼54 in the mid-infrared (MIR) spectrum. The revealed results indicate that the Fermi energy level of the graphene and the coupling distance play important roles in optimizing the sensing properties. Our proposed structure exerts a peculiar fascination on the realization of ultra-compact graphene plasmonic nanosensor in the future.

     

Higher Order Tunable Fano Resonances in Multilayer Nanocones

We present a computational study of the plasmonic response of a gold–silica–gold multilayered nanostructure based on truncated nanocones. Symmetry breaking is introduced by rotating the nanostructure and by offsetting the layers. Nanocones with coaxial multilayers show dipole–dipole Fano resonances with resonance frequencies depending on the polarization of the incident light, which can be changed by rotating the nanostructure. By breaking the axial symmetry, plasmonic modes of distinct angular momenta are strongly mixed, which provide a set of unique and higher order tunable Fano resonances. The plasmonic response of the multilayered nanocones is compared to that of multishell nanostructures with the same volume and the former are discovered to render visible high-order dark modes and to provide sharp tunable Fano resonances. In particular, higher order tunable Fano resonances arising in non-coaxial multilayer nanocones can vary the plasmon lines at various spectral regions simultaneously, which makes these nanostructures greatly suitable for plasmon line shaping both in the extinction and near field spectra.     

A Nanoscale Fano Resonator by Graphene-Gold Dipolar Interference

A nanoscale Fano resonator composed of a hybrid graphene disk-gold ring combination is reported in this letter. The inner narrow dipolar resonance of a discrete state induced by graphene interferes with the outside broad dipolar resonance of a continuum state induced by gold, thus forming an asymmetric Fano transparency within the absorption window. The metastructure exhibits a wide tunable band along with an excellent refractive index sensing capability of 2344 nm/RIU. The geometry adjustment modulates the spectral response giving chances to the equivalent of electromagnetically induce transparency. Moreover, the group index exceeds 760 within the transparency window enabling a potential use in slow light or light storage applications. The analytic analysis is in accordance with the numerical simulation results.     

Tailoring the Multiple Fano Resonances in Nanobelt Plasmonic Cluster

Plasmonics - We theoretically explore the appearances and characteristics of Fano resonances in novel-designed nanobelt cluster, which shows strong modulation depths, and the Fano dips can be...     

Coupled-Resonator-Induced Fano Resonances for Plasmonic Sensing with Ultra-High Figure of Merits

Fano resonances are numerically predicted in an ultracompact plasmonic structure, comprising a metal-isolator-metal (MIM) waveguide side-coupled with two identical stub resonators. This phenomenon can be well explained by the analytic model and the relative phase analysis based on the scattering matrix theory. In sensing applications, the sensitivity of the proposed structure is about 1.1?×?10³ nm/RIU and its figure of merit is as high as 2?×?10⁵ at λ?=?980 nm, which is due to the sharp asymmetric Fano line-shape with an ultra-low transmittance at this wavelength. This plasmonic structure with such high figure of merits and footprints of only about 0.2 μm² may find important applications in the on-chip nano-sensors.     

Tunable Fano Resonances in Mid-Infrared Waveguide-Coupled Otto Configuration

A planar silicon carbide/dielectric multilayer structure is investigated in Otto geometry, where surface phonon polaritons and planar waveguide mode can be coupled to realize Fano resonances under transverse magnetic polarization. The resonance coupling is analytically demonstrated using the coupled harmonic oscillator model and numerically presented through rigorous coupled-wave analysis calculations, which shows that the coupling strength between different resonances and the resonant wavelength matching condition plays an important role in the bandwidth and position of the Fano resonance (FR); the magnetic field distribution was also shown to explain the origin of FRs qualitatively.

     

Magnetic-Based Fano Resonance by a Trimer with Y-shaped Gap

We introduce a Y-shaped gap into a silver disk to break the structure symmetry which can be looked as a loop-linked structure. Magnetic resonances are excited by incident light when incident electric field is parallel to the trimer plane. Fano resonance is generated by the coupling between bright electric mode and dark magnetic mode. These resonances can be adjusted by tuning the gap size, the radius of trimer, and the position of Y-shaped gap. The extinction cross section of the structure is calculated with the finite element method (FEM). The maximum figure of merit (FOM) is 37.8. Both the magnetic and electric field are greatly enhanced at the Fano dip and the magnetic resonance peak.