Subradiant Modes Research Articles

Hybridized plasmon resonant modes in molecular metallodielectric quad-triangles nanoantenna

In this study, we examined the plasmon response for both metallic and metallodielectric nanoantennas composed of four gold (Au) triangles in a quadrumer orientation. Tailoring an artificial metallic quad-triangles nanoantenna, it is shown that the structure is able to support pronounced plasmon and Fano resonances in the visible spectrum. Using plasmon transmutation effect, we showed that the plasmonic response of the proposed cluster can be enhanced with the placement of carbon nanoparticles in the offset gaps between the proximal triangles. It is verified that this structural modification gives rise to formation of new collective magnetic antibonding (dark) plasmon modes. Excitation of these subradiant dark modes leads to formation of narrower and deeper Fano resonances in the spectral response of the metallodielectric nanoantenna. To investigate the practical applications of the metallodielectric structure, we immersed the nano-assembly in various liquids with different refractive indices to define its sensitivity to the environmental perturbation as a plasmonic biological sensor.

Surface Plasmon Polariton Unidirectional Nano-Launcher Based on the Strong Coupling Effects in an Asymmetric Optical Slot Nanoantenna Pair

We investigate the strong coupling effects in asymmetric optical slot nanoantenna pairs (AOSNPs) consisting of two parallelly positioned optical slot nanoantennas with different lengths and propose a surface plasmon polariton (SPP) unidirectional nano-launcher based on the strongly coupled AOSNPs (SCAOSNPs). Two subradiant modes can be excited in the SCAOSNPs, which originate from the hybridizations of the dipolar plasmon resonant modes in the two slot antennas. The subradiant modes give rise to large phase difference and nearly equal near-field electric field amplitudes between the two slot antennas, which can be used for unidirectional SPP launching and enable an ultra-compact nano-launcher. An extinction ratio up to 246 is achieved in the calculation via an SCAOSNP with a footprint of 170 nm by 260 nm on the surface of the gold film. The ultra-small footprint and tunable working wavelength enable the SCAOSNP important applications in future optical circuits and on-chip plasmonic devices.

Numerical Realization of Fano-Type Resonances in Cascaded Plasmonic Heterodimers for Refractive Index Sensing

We propose a simple heterodimer system consisting of a long and a short Au nanobar to achieve Fano-type resonances based on the destructive interference between the superradiant and subradiant modes without involvement of higher order multipolar modes of individual components. The plasmonic modes are identified by the distributions of the electric field and charge density. By introducing another short nanobar on the other side of the long nanobar, the dipole moment of the system can be further canceled and the spectral detuning between the superradiant and subradiant mode is decreased. As a result, the Fano-type resonance becomes deeper and its shape is more symmetric. This Fano-type resonance can be utilized in the refractive index sensing because of its narrow linewidth and steep dip. The figure of merit (FoM) can reach as high as 8.74, which is comparable with the results realized in more complicated systems involved with higher order modes.

Double Fano resonances in plasmon coupling nanorods

Fano resonances are investigated in nanorods with symmetric lengths and side-by-side assembly. Single Fano resonance can be obtained by a nanorod dimer, and double Fano resonances are shown in nanorod trimers with side-by-side assembly. With transverse plasmon excitation, Fano resonances are caused by the destructive interference between a bright superradiant mode and dark subradiant modes. The bright mode originates from the electric plasmon resonance, and the dark modes originate from the magnetic resonances induced by near-field inter-rod coupling. Double Fano resonances result from double dark modes at different wavelengths, which are induced and tuned by the asymmetric gaps between the adjacent nanorods. Fano resonances show a high figure of merit and large light extinction in the periodic array of assembled nanorods, which can potentially be used in multiwavelength sensing in the visible and near-infrared regions.

Fano resonances in complex plasmonic super-nanoclusters: The effect of environmental modifications on the LSPR sensitivity

In this study, gold nanodisk clusters in heptamer orientations as clusters were used to design a super-heptamer consisting of one central and six peripheral heptamers. We examined the position and movement of the plasmon and Fano resonances by sketching the spectral response of the superstructure for various nanodisk dimensions. The quality of the interference between the superradiant and subradiant plasmon resonance modes of the nanodisk clusters was found to depend strongly on the structural configuration and the refractive index of the environmental medium. We replaced the central heptamer with a nanodisk and probed the position of the Fano resonance by geometrically altering the nanodisk structure. Finally, the effect of the dielectric environment on the plasmon response of both of the studied structures was examined numerically and theoretically. The localized surface plasmon resonance sensitivity of the finite plasmonic structures to the presence of liquid substances was investigated and shown by plotting the linear figure of merit. The finite-difference time-domain method was used as a numerical tool to investigate the plasmon response of the structure.

Fano Resonances Generated in a Single Dielectric Homogeneous Nanoparticle with High Structural Symmetry

Fano resonances in plasmonic nanostructures suppress radiative losses effectively, but nonradiative Ohmic losses limit the performance of many important applications. In addition, it is hard to generate strong Fano resonances in a single plasmonic homogeneous nanoparticle with high structural symmetry. Dielectric nanostructures offer a potential solution to the above issues. There are various subradiant hybrid modes in a single dielectric nanoparticle, making it possible to generate Fano resonances. This study shows that due to the excitation of the subradiant hybrid EH12δ mode a strong Fano resonance is generated in a single silicon nanodisk. Higher-order subradiant hybrid modes (EH13δ and EH14δ) are excited by manipulating the disk radius, and multiple Fano resonances arise in spectra. These optical responses are not dependent on a retardation effect, and strong Fano resonances are generated even for a very thin disk. One can get similar results in a single dielectric triangle, square, or rectangle nanoplate. The simple geometry and high structural symmetry make these dielectric nanoparticles promising for practical implementations in biosensing and optoelectronics.

Plasmon resonance hybridization in self-assembled copper nanoparticle clusters: efficient and precise localization of surface plasmon resonance (LSPR) sensing based on Fano resonances.

In this work, we have investigated the hybridization of plasmon resonance modes in completely copper (Cu)-based subwavelength nanoparticle clusters from simple symmetric dimers to complex asymmetric self-assembled structures. The quality of apparent bonding and antibonding plasmon resonance modes for all of the clusters has been studied, and we examined the spectral response of each one of the proposed configurations numerically using the finite-difference time domain (FDTD) method. The effect of the geometric sizes of nanoparticles used and substrate refractive index on the cross-sectional profiles of each of the studied structures has been calculated and drawn. We proved that Fano-like resonance can be formed in Cu-based heptamer clusters as in analogous noble metallic particles (e.g., Au and Ag) by determining the coupling strength and interference between sub-radiant and super-radiant resonance modes. Employing certain Cu nanodiscs in designing an octamer structure, we measured the quality of the Fano dip formation along the scattering diagram. Accurate tuning of the geometric sizes for the Cu-based octamer yields an opportunity to observe isotropic, deep, and narrow Fano minima along the scattering profile that are in comparable condition with the response of other plasmonic metallic substances. Immersing investigated final Cu-based octamer in various liquids with different refractive indices, we determined the sensing accuracy of the cluster based on the performance of the Fano dip. Plotting a linear diagram of plasmon energy differences over the refractive index variations as a figure of merit (FoM), which we have quantified as 13.25. With this method, the precision of the completely Cu-based octamer is verified numerically using the FDTD tool. This study paves the way toward the use of Cu as an efficient, low-cost, and complementary metal-oxide semiconductor (CMOS)-compatible plasmonic material with optical properties that are similar to analogous plasmonic substances.

Twin Dipole Fano Resonances in Symmetric Three-Layered Plasmonic Nanocylinder

A unique metal-dielectric structure composed of symmetric multilayered nanocylinder is demonstrated to support unconventional superradiant and subradiant plasmon modes. Here, the twin distinctive dipole-dipole Fano resonances are observed in the structure within the visible spectrum by carefully selecting the geometrical parameters. The spectral features of these Fano resonances are controlled by changing the incident field polarization and parameters of the nanoparticle. Eventually, an RLC circuit model is proposed to reproduce the optical response of the nanostructure.

Coupled Mode Modeling To Interpret Hybrid Modes and Fano Resonances in Plasmonic Systems

By generalizing the concept of extinction cross-section to complex valued extinction cross-section we analyze the coupling between plasmon modes in metallic dimers or quadrumers. Identifying the phase information in the field scattered by subsets of the whole plasmonic system allows to infer the formation of subradiant or super-radiant hybrid modes. We also propose a phenomenological modeling based on the use of coupled mode equations to deduce from rigorous calculations a quantitative estimate of mutual coupling coefficients when only two modes interfere. These coefficients determine the spectral position of hybrid modes. This approach is applied to two interacting silver spheres; the parameters of the energetic diagram are calculated as a function of the gap between spheres. In the case of two identical spheres illuminated with a linearly polarized light parallel or perpendicular to the dimer, only one hybrid mode is excited. The phenomenological modeling is then applied to a four-particle system, where the interaction between the initial dipolar modes gives rise to Fano resonances. In a weak coupling regime of the system, the asymmetric line profile in the extinction spectra of the system emerges from the superposition of a broad super-radiant mode and a sharp subradiant mode. A strong coupling regime is characterized by a broadened subradiant mode and a larger Fano resonance. The sharpness of the Fano resonance in the weak coupling regime makes this structure well suited for sensing applications.

Dynamically configurable hybridization of plasmon modes in nanoring dimer arrays.

We present a novel strategy capable of dynamically configuring the plasmon-induced transparency (PIT) effect with a polarization-dependent controllability based on a nanoring dimer array. The controllable coupling strength between the superradiant and subradiant modes is due to the polarization-dependent field distributions. It is shown that this dynamically controlled PIT is realized with a modulation depth as high as 95%, and a linear dependence of the coupling strength on polarization angle is deduced using a coupled-oscillator model. We believe that our results will inspire further exciting achievements that utilize various polarization states of the electromagnetic wave and pave a way towards applications using PIT with dynamic controllability such as slow light, optical nonlinearities and chemical/bio-sensing.

Periodicity-induced symmetry breaking in a Fano lattice: hybridization and tight-binding regimes.

We investigate experimentally and theoretically the role of periodicity on the optical response of dolmen plasmonic arrays that exhibit a Fano line shape. Contrary to previous works on single nanostructures, this study deals with the in-plane near-field coupling between adjacent unit cells. By making an analogy to the electronic properties of atoms in the tight-binding model, specific behaviors of photonic states are investigated numerically as a function of the structural asymmetry for different coupling directions. These predictions are verified experimentally with dark-field measurements on nanostructure arrays which exhibit high tunability and fine control of their spectral features as a function of the lattice constants. These effects, originated from symmetry-breaking and selective excitation of the subradiant mode, provide additional degree of freedom for tuning the spectral response and can be used for the sensitive detection of local perturbations. This study provides a general understanding of the near-field interactions in Fano resonant lattices that can be used for the design of plasmonic nanostructures and planar metamaterials.

Refractive index sensing with Fano resonant plasmonic nanostructures: a symmetry based nonlinear approach.

Sensing using surface plasmon resonances is one of the most promising practical applications of plasmonic nanostructures and Fano resonances allow achieving a lower detection limit thanks to their narrow spectral features. However, a narrow spectral width of the subradiant mode in a plasmonic system, as observed in the weak coupling regime, is in general associated with a low modulation of the complete spectral response. In this article, we show that this limitation can be overcome by a nonlinear approach based on second harmonic generation and its dependence on symmetry at the nanoscale. The Fano resonant systems considered in this work are gold nanodolmens. Their linear and nonlinear responses are evaluated using a surface integral equation method. The numerical results demonstrate that a variation of the refractive index of the surrounding medium modifies the coupling between the dark and bright modes, resulting in a modification of the electromagnetic wave scattered at the second harmonic wavelength, especially the symmetry of the nonlinear emission. Reciprocally, we show that evaluating the asymmetry of the nonlinear emission provides a direct measurement of the gold nanodolmens dielectric environment. Interestingly, the influence of the refractive index of the surrounding medium on the nonlinear asymmetry parameter is approximately 10 times stronger than on the spectral position of the surface plasmon resonance: hence, smaller refractive index changes can be detected with this new approach. Practical details for an experimental realization of this sensing scheme are discussed and the resolution is estimated to be as low as Δn = 1.5 × 10(-3), respectively 1.5 × 10(-5), for an acquisition time of 60 s for an isolated gold nanodolmen, respectively an array of 10 × 10 nanodolmens.

Application of Fano resonance effects in optical antennas formed by regular clusters of nanospheres

This paper describes an analytical model developed to study the Fano resonance effect in clusters of spherical plasmonic nanoparticles under local excitation. The model depicted the case of a parallel single dipole emitter that was near-field coupled to a pentamer or heptamer cluster of nanospheres. Spatial polarization and field distributions of the optical states and resonance spectra for these cluster configurations were calculated. It was discovered that polarization interference between the nanoparticles triggered the formation of a second peak in the directivity spectra at 690 nm, and this in turn provided a mechanism for the occurrence of subradiant mode effects. The directivity calculation was analyzed in order to qualify the redirection of emission. Performances of various nanoantennae were investigated and fully characterized in terms of spatial geometric differences and the Fano resonance effect on plasmonic nanoparticles in the optical domain. Light radiation patterns were found to be significantly affected by nanosphere sizes and positioning of nanospheres with respect to the dipole. The analytical treatment of these modeled nanoantennae yielded results that are applicable to physical design and utilization considerations for pentamer and heptamer clusters in nanoantennae mechanisms.

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.

The modulation effect of transverse, antibonding, and higher-order longitudinal modes on the two-photon photoluminescence of gold plasmonic nanoantennas.

Plasmonic nanoantennas exhibit various resonant modes with distinct properties. Upon resonant excitation, plasmonic gold nanoantennas can generate strong two-photon photoluminescence (TPPL). The TPPL from gold is broadband and depolarized, and may serve as an ideal local source for the investigation of antenna eigenmodes. In this work, TPPL spectra of three arrays of single-crystalline gold nanoantennas are comprehensively investigated. We carefully compare the TPPL spectra with dark-field scattering spectra and numerically simulated spectra. We show the modulation effect of the transverse resonant mode and the nonfundamental longitudinal mode on the TPPL spectrum. We also demonstrate suppression of TPPL due to the subradiant antibonding modes and study the influence of antenna resonant modes on the overall TPPL yield. Our work provides direct experimental evidence on nanoantenna-mediated near-to-far-field energy coupling and gains insight into the emission spectrum of the TPPL from gold nanoantennas.

Circuit Model of Fano Resonance on Tetramers, Pentamers, and Broken Symmetry Pentamers

This paper presents the representation circuit model for Fano resonance of plasmonic nanoparticles in the optical domain. An intuitive explanation is provided for the physical nature of Fano resonance based on the three-level quantum system, and the Fano resonance effects of three basic nanoparticle arrangements, namely tetramer, pentamer, and symmetry broke pentamer are discussed. A coupling capacitor is calculated as an equivalent component in the proposed circuit model in order to describe the coupling effect between subradiant and superradiant mode in the Fano resonance. The circuit impedances of tetramer, pentamer, and broken symmetry pentamer are simulated, with resultant circuit models in agreement with the calculated results based on S-parameters.

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.

Superconductivity-Induced Transparency in Terahertz Metamaterials

A plasmonic analogue of electromagnetically induced transparency is activated and tuned in the terahertz (THz) range in asymmetric metamaterials fabricated from high critical temperature (Tc) superconductor thin films. The asymmetric design provides a near-field coupling between a superradiant and a subradiant plasmonic mode, which has been widely tuned through superconductivity and monitored by Fourier transform infrared spectroscopy. The sharp transparency window that appears in the extinction spectrum exhibits a relative modulation up to 50% activated by temperature change. The interplay between ohmic and radiative damping, which can be independently tuned and controlled, allows for engineering the electromagnetically induced transparency of the metamaterial far beyond the current state-of-the-art, which relies on standard metals or low-Tc superconductors.

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.

Third Harmonic Mechanism in Complex Plasmonic Fano Structures.

We perform third harmonic spectroscopy of dolmen-type nanostructures, which exhibit plasmonic Fano resonances in the near-infrared. Strong third harmonic emission is predominantly radiated close to the low energy peak of the Fano resonance. Furthermore, we find that the third harmonic polarization of the subradiant mode interferes destructively and diminishes the nonlinear signal in the far-field. By comparing the experimental third harmonic spectra with finite element simulations and an anharmonic oscillator model, we find strong indications that the source of the third harmonic is the optical nonlinearity of the bare gold enhanced by the resonant plasmonic polarization.