Higher Order Fano Resonances Research Articles

Dual-Stimulus Control for Ultra-Wideband and Multidimensional Modulation in Terahertz Metasurfaces Comprising Graphene and Metal Halide Perovskites.

Perovskites and graphene are receiving a meteoric rise in popularity in the field of active photonics because they exhibit excellent optoelectronic properties for dynamic manipulation of light-matter interactions. However, challenges still exist, such as the instability of perovskites under ambient conditions and the low Fermi level of graphene in experiments. These shortcomings limit the scope of applications when they are used alone in advanced optical devices. However, the combination of graphene and perovskites is still a promising route for efficient control of light-matter interactions. Here, we report a dual-optoelectronic metadevice fabricated by integrating terahertz metasurfaces with a sandwich complex composed of graphene, polyimide, and perovskites for ultra-wideband and multidimensional manipulation of higher-order Fano resonances. Owing to the photogenerated carriers and electrostatic doping effect, the dual optoelectronic metadevice showed different manipulation behavior at thermal imbalance (electrostatic doping state of the system). The modulation depth of the transmission amplitude reached 200%, the total resonant frequency shift was 800 GHz, and the tunable range of the resonant frequency was 68.8%. In addition, modulation of the maximum phase reached 346°. This work will inspire a new generation of metasurface-based optical devices that combine two active materials.

Graphene–polyimide-integrated metasurface for ultrasensitive modulation of higher-order terahertz fano resonances at the Dirac point

The initial Fermi level of graphene is quite close to the Dirac point. It is ultrasensitive to external stimuli in the modulation of Fano resonance. Thus, the Fermi level directly skips the Dirac point during modulation with strong external excitation. Therefore, the modulation behavior of the Fano resonance is not easy to capture at the Dirac point. Here, we applied a laser pump with micropower densities and a mains with weak voltages to the terahertz metasurface and observed the modulation behavior of higher-order Fano resonance at the Dirac point. Accompanied by Fermi level shifts between the Dirac point, conduction band, and valence band, higher-order Fano resonances exhibited the process of disappearance, appearance, and re-disappearance. Concomitantly, the transmission amplitude underwent an increasing stage and a decreasing stage. More importantly, a 0.05 V change in the bias voltage or a 7.5 mW/cm2 change in the power density enabled a modulation depth of 90%. These findings could be potentially used for designing ultrasensitive terahertz metaphotonic devices.

Effect of Symmetry Breaking on Plasmonic Coupling in Nanoring Dimers

Breaking the morphological and compositional symmetries of metallic nanoparticle (NP) dimers provides a novel approach to modulate plasmon coupling between the NPs. In this study, we theoretically investigate the effect of symmetry breaking on the plasmonic coupling in morphologically asymmetric Au nanoring-nanodisk (NR-ND) dimer, compositionally heterogeneous Au-Ag nanoring dimer, and compositionally as well as morphologically asymmetric Au-Ag NR-ND heterodimers. It has been found that when the inner radii of symmetrical Au NR dimer is decreased, the scattering spectral intensity of coupled bonding mode and higher-order coupled bonding mode drastically decreases and increases, respectively. Besides, the effect of aspect ratio on the plasmon resonance modes is investigated. Furthermore, with fixed geometric parameters of one NR and by morphing the shape of the other NR into ND through inner radius, we show the generation and modification of a Fano resonance. As a result of introducing morphological and compositional asymmetry separately, single Fano-type spectral feature is observed in the scattering spectra. We demonstrate that the double Fano resonances can be easily achieved in compositionally and morphologically asymmetric heterodimer with double symmetry breaking. These dual modes are caused by the interference of dipolar bright mode (localized LSPR modes) of Ag NP with the quadrupole and hexapole modes (interband transitions) of the Au NP. Finally, at optimum asymmetric heterodimers, we discussed the effect of refractive index on the Fano-type resonance. These new and simple designs provide a novel insight to modulate optical response and the generation of higher-order Fano resonances, which have many potential applications such as in photonics and refractive index sensing.

Higher order fano resonances in plasmonic nanostructures with two disks outside one ring

A kind of plasmonic nanostructure with two disks placed outside a ring is introduced. When incident light is perpendicular to the surface of the structure, the extinction spectra in the observed wavelength range show that octupolar modes are reduced as the radii of two-disks are different. In the case of two disks with the same size symmetrically around the ring (the SDDR structure), octupolar modes are suppressed absolutely. For SDDR structure, the larger the size ratios of the ring to the disks, the easier higher order dark modes in the ring are excited; as higher order resonances appear, lower order resonances are reduced. The decapolar mode of the SDDR structure with the ring radius of 250nm and two-disk radius of 60nm is in the near-infrared wavelength region, its contrast ratio (CR) and figure of merit (FOM) can reach 87% and 22.5, respectively, which demonstrates application potentials for high-performance biochemical sensing.

Enhanced plasmonic Fano-like resonances in multilayered nanoellipsoid

Plasmonic Fano-like resonances, which arise from the destructive interference of bright and dark plasmon modes, are theoretically investigated in gold–silica–gold multilayered nanoellipsoid. It is found that the higher-order Fano resonances with sharp modulation depth can be achieved in the symmetric case by simply varying the structural parameters of the nanoparticle. Moreover, for the generation of unique multiple Fano-like resonances, two types of symmetry-breaking conception are also introduced: first by offsetting the inner metal core from its central position and then by relaxing the rotational symmetry of the nanoparticle. The multiple plasmonic Fano-like resonances make the proposed nanostructure highly suitable for plasmon line shaping, electromagnetic-induced transparency, and slow light applications.

Multiple higher-order Fano resonances in plasmonic hollow cylindrical nanodimer

The optical properties of a nanodimer composed of hollow nanocylinders that are located in a close proximity to each other are investigated. The plasmon modes of the dimer resonator spectrally overlap and induce plasmonic Fano resonances due to destructive interference. For the generation of multiple Fano resonances with large modulation depths and sharp linewidths, several configurations of the dimer nanostructure are analyzed. Different kinds of unique Fano resonances are obtained by changing the polarization of incident light. Moreover, the spectral positions and modulation depths of the higher-order multiple Fano resonances can be flexibly tuned and controlled in the extinction spectrum by varying the geometrical parameters. The proposed resonator has the advantage to exhibit multiple Fano resonances with large modulation depths and offers high values of figure of merit and contrast ratio due to which it can be greatly appropriate for plasmon line shaping, slow light and broadband biosensing 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.

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.

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.

Generating and Manipulating Higher Order Fano Resonances in Dual-Disk Ring Plasmonic Nanostructures

In this article, we investigate higher order (quadrupolar, octupolar, hexadecapolar, and triakontadipolar) Fano resonances generated in disk ring (DR) silver plasmonic nanostructures. We find that the higher order Fano resonances are generated when the size of the disk is reduced and falls into a certain range. With dual-disk ring (DDR) nanostructures, a rich set of tunable Fano line shapes is provided. More specifically, we report our observations on the optical behavior of the DDRs including asymmetric cases either in two disks with different sizes or their asymmetric locations inside the ring. In the case of symmetric dual-disk ring (SDDR) nanostructures, we demonstrate that the quadrupolar and the hexadecapolar Fano resonances are suppressed, which can reduce the cross-talk in spectroscopic measurements, while the octupolar and the triakontadipolar Fano resonances are enhanced. The potential of using the studied plasmonic nanostructures as biochemical sensors is evaluated with the figure of merit (FOM) and the contrast ratio (CR). The values of the FOM and the CR achieved using the triakontadipolar Fano resonance in the SDDR are 17 and 57%, respectively. These results indicate that the SDDRs could be developed into a high-performance biochemical sensor in the visible wavelength range.

Excitation and Tuning of Higher-Order Fano Resonances in Plasmonic Oligomer Clusters

Plasmonic oligomer clusters are assemblies of closely packed metallic nanoparticles. They provide a rich set of spectral features such as Fano lineshapes and a simultaneous tunability of the supported resonances in the optical wavelength regime. In this study, we investigate numerically and experimentally clusters of plasmonic nanoparticles that exhibit multiple Fano resonances due to the interference of one broad superradiant mode and multiple narrow subradiant modes. In particular we investigate oligomers with multiple ring modes and elongated chains of nanoparticles surrounded by one ring of nanoparticles. We show that the number of nanoparticles and their respective arrangement in the cluster strongly influence the spectral position and modulation depth of the spectral signature of the supported modes. Our study opens up the pathway to "plasmonic super molecules" that show unprecedented tunability, which renders them highly suitable for applications such as multiwavelength surface-enhanced Raman scattering.