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.     

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.     

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.     

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.     

Higher Order Fano Resonances and Electric Field Enhancements in Disk-Ring Plasmonic Nanostructures with Double Symmetry Breaking

Optical properties of disk-ring plasmonic nanostructures with double symmetry breaking are investigated theoretically. Tunable higher order Fano resonance is achieved, and it is sensitive to the degree of asymmetry of the nanoring, the offset and the dimension of the nanodisk. It is demonstrated that such higher order Fano resonances originate from the destructive interference between the bright mode of the displaced nanodisk and the dark mode of the asymmetric nanoring. By tunning the asymmetry degree of the nanoring, the offset, and the dimension of the nanodisk, certain higher order Fano resonances can be suppressed or enhanced. Double asymmetry breaking also allows the realization of the stronger electric field enhancement, resulting from the stronger interaction between the displaced nanodisk and the asymmetric nanoring.     

Independently Formed Multiple Fano Resonances for Ultra-High Sensitivity Plasmonic Nanosensor

We proposed a plasmonic nanosensor with an ultra-high sensitivity based on groove and ring resonator. Simulation results show that these sharp Fano profiles originate from the interference between the groove and ring resonator. The profile can be easily tuned by changing the parameters of the structure. Moreover, we introduce a new way to achieve multiple Fano resonances through independent processes by adding a side-coupled stub cavity, and the Fano resonances can be tuned independently. These characteristics offer flexibility in the design of the devices. This nanosensor yields an ultra-high sensitivity of ～2000 nm/RIU, which is rarely seen in the previous report. Our structures may have potential applications for nanosensors, slow light, and nonlinear devices in highly integrated circuits.     

Highly Sensitive Plasmonic Sensor Based on Fano Resonance from Silver Nanoparticle Heterodimer Array on a Thin Silver Film

We investigate the optical spectrum of a multilayer metallic slab using multiple-scattering formalism. A thin silver film is attached to a periodic array of heterodimers consisting of two vertically spaced silver nanoparticles of different radii. Depending on the radius of nanoparticles, heterodimer array presents a simple nanoscale geometry which gives rise to remarkable plasmonic properties of multipolar resonances. Due to the coherent interference of the localized nanoparticle plasmons (discrete mode) and surface plasmon polaritons of metallic film (continuous mode), the reflection spectrum represents a sharp asymmetric Fano resonance dip, which is strongly sensitive to the refractive index of the surrounding embedded dielectric host. The physical features contribute to a highly efficient plasmonic sensor for refractive index sensing with sensitivity of ～1.5?×?10^?3 RIU/nm.     

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 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.     

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.     

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.     

Plasmonically Tunable Blue-Shifted Emission from Coumarin 153 in Ag Nanostructure Random Media: A Demonstration of Fast Dynamic Surface-Enhanced Fluorescence

Enhancement of intensity and wavelength tunability of emission are desirable features for light-emitting device applications. We report on the large and tunable blue shift (60 nm) in emission from an environment-sensitive fluorophore (Coumarin153) embedded in Ag plasmonic random media. Coumarin 153 having emission at 555 nm, show a systematic blue shift (to 542, 503 and 495 nm) upon infiltration into random media fabricated by Ag nanowires of different aspect ratio (hence, surface plasmon resonances at 426, 445 and 464 nm). The blue shift is due to the fast dynamic surface-enhanced fluorescence mechanism and can be tuned by controlling the surface plasmon resonance and hotspot density in random media. Enhanced emission at desired wavelength is achieved by using nanostructures having higher extinction coefficient but same-surface plasmon resonance. Ag nanostructures of different aspect ratio used for fabricating the random media are synthesized by chemical route.     

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.     

Tunability of Multipolar Plasmon Resonances and Fano Resonances in Bimetallic Nanoshells

Plasmonics - 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...     

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.     

Förster Resonance Energy Transfer Between Molecules in the Vicinity of Graphene-Coated Nanoparticles

The recent demonstration of the plasmonic-enhanced Förster resonance energy transfer (FRET) between two molecules in the vicinity of planar graphene monolayers is further investigated using graphene-coated nanoparticles (GNP). Due to the flexibility of these nanostructures in terms of their geometric (size) and dielectric (e.g., core material) properties, greater tunability of the FRET enhancement can be achieved employing the localized surface plasmons. It is found that while the typical characteristic graphene plasmonic enhancements are manifested from using these GNPs, even higher enhancements can be possible via doping and manipulating the core materials. In addition, the broadband characteristics are further expanded by the closely spaced multipolar plasmon resonances of the GNPs.     

Fano Resonance-Assisted Plasmonic Trapping of Nanoparticles

Plasmonic optical trapping is widely applied in the field of bioscience, microfluidics, and quantum optics. It can play a vital role to extend optical manipulation tools from micrometer to nanometer scale level. Currently, it is a challenge to obtain the highly stable optical trapping with low power and less damage. In this paper, we propose Fano resonance-assisted self-induced back-action (FASIBA) method, through which a single 40-nm gold particle can be trapped in hole-slit nano-aperture milled on metallic film. It is used to achieve ultra-accurate positioning of nanoparticle, metallic nanostructures at wide infrared wavelength range, quite effectively and evidently. The stable plasmonic trapping is achieved by tuning the transmission wavelengths and modifications of nanoslit, indicating that the depth of potential well can be increased from minus 8KT to 12KT, with the input power of 10⁹ W/m². This can be attributed to great modifications in Fano resonance transmissions according to self-induced back-action (SIBA) theory. The results are basically helpful to facilitate the trapping with lower power and less damage to the objects, which enables new scenario for the treatment of undesirable spread of a single nanoscale creature, such as virus.     

Tunable Ultrahigh Order Surface Plasmonic Resonance in Multi-Ring Plasmonic Nanocavities

Optical properties of multi-ring with spatial symmetry breaking are investigated theoretically. Tunable ultrahigh order surface plasmonic resonance is achieved, which is found to be sensitive to geometric parameters. Certain high-order surface plasmonic resonances can be either suppressed or enhanced when geometrical parameters are adjusted. Moreover, more than one quadrupolar-dipolar, octupolar-dipolar, and hexadecapolar-dipolar mode of the surface plasmonic resonance can be achieved. The asymmetry also allows the generation of strong electric field enhancement with these nanostructures that can be applied in the field of surface-enhanced spectroscopy and biosensing.     

Plasmonic Fano Resonance in Homotactic Aluminum Nanorod Trimer: the Key Role of Coupling Gap

Recently, the Fano effect of aluminum nanostructures has attracted a lot of attentions in several detector and sensor applications, but the role of coupling gap in it remains unintuitive. In this paper, a homotactic aluminum rod trimer (HART) is designed to form the plasmonic Fano resonances and visualize the important role of coupling gap size. The plasmon hybridization model and far field images were used to qualitatively describe the formation mechanism of Fano resonance. The simulation results intuitively show that the Fano dip of HART with a smaller coupling gap size has a higher red-shift speed when increasing the refractive index of surrounding environment or the length of HART with a fixed axial ratio (L_S/L_L = 0.6). Our study provides the insights to the key role of coupling gap in the performance of Fano structures.

     

Search of Extremely Sensitive Near-Infrared Plasmonic Interfaces: A Theoretical Study

The sensitivity of the wavelength position of localized surface plasmon resonance (LSPR) in metal nanostructures to local changes in the refractive index has been widely used for label-free detection strategies. Tuning the optical properties of the nanostructures from the visible to the infrared region is expected to have a drastic effect on the refractive index sensitivity. Here, we theoretically investigate the optical response of a newly designed plasmonic interface to changes in the bulk refractive index by the finite difference time domain method. It consists of a structured interface, where the planar interface is superposed with dielectric pillars 30 nm in height and 125 nm in length with a separation distance of 15 nm. The pillars are covered with U-shaped gold nanostructures of 50 nm in height, 125 nm in length, and 5 nm of gold base thickness. The whole structure is finally covered with a 5-nm thick dielectric layer of n ₂?=?2.63. This plasmonic structure shows bulk refractive index sensitivities up to 1750 nm/RIU (RIU : refractive index unit) in the near infrared (λ?=?2621 nm). The enhanced sensitivity is a consequence of the extremely enhanced electrical field between the gold nanopillars of the plasmonic interface.