Sharp Fano Resonance Research Articles

Quenching effect of oscillating potential on anisotropic resonant transmission through a phosphorene electrostatic barrier

The anisotropy in resonant tunneling transport through an electrostatic barrier in monolayer black phosphorus either in presence or in absence of an oscillating potential is studied. Non-perturbative Floquet theory is applied to solve the time dependent problem and the results obtained are discussed thoroughly. The resonance spectra in field free transmission are Lorentzian in nature although the width of the resonance for the barrier along the zigzag (Г–Y) direction is too thinner than that for the armchair (Г–X) one. Resonant transmission is suppressed for both the cases by the application of oscillating potential that produces small oscillations in the transmission around the resonant energy particularly at low frequency range. Sharp asymmetric Fano resonances are noted in the transmission spectrum along the armchair direction while a distinct line shape resonance is noted for the zigzag direction at higher frequency of the oscillating potential. Even after the angular average, the conductance along the Г–X direction retains the characteristic Fano features that could be observed experimentally. The present results are supposed to suggest that the phosphorene electrostatic barrier could be used successfully as switching devices and nano detectors.

High-Q Fano Resonance in Subwavelength Stub-Wall-Coupled MDM Waveguide Structure and Its Terahertz Sensing Application

Waveguide structures effectively controlling and guiding terahertz (THz) waves can achieve interesting resonance effects when combined with resonators. At present, achieving high quality factor (Q-factor) resonance in THz resonator-coupled waveguide structure is still a critical consideration to expand its practical application. Here, a high Q-factor Fano resonance based on a metal-dielectric-metal (MDM) waveguide consisting of a stub resonator and a metal wall with an aperture in the center is investigated theoretically and numerically in the THz region. The results show that the sharp and asymmetric Fano resonance peak is induced by the destructive interference between the stub resonator and metal wall which act as a Fabry-Perot cavity. Q-factor is obviously improved about 60 times (3.72 to ~225) by introducing the metal wall into the stub-coupled MDM waveguide. Moreover, Fano resonance can be effectively tuned by varying different structure parameters. Owing to the high sensitivity of Fano resonance peak to dielectric surroundings, a large-range refractive index (RI) sensor based on the proposed structure with a high sensitivity of 96480 nm/RIU is obtained. The figure of merit (FOM) of 195 is greatly improved compared to other THz Fano-based RI sensors. These results provide possibilities for subwavelength MDM waveguide structure to apply for THz bio/chemical sensing, bandpass filters, and on-chip highly integrated plasmonic device.

Imaging Through a Fano-Resonant Dielectric Metasurface Governed by Quasi--bound States in the Continuum

Fano resonance has raised great attention in nanophotonics attributing to its unique properties.In this work, we study the imaging function of Fano-resonant silicon metasurfece governed by quasi-bound states in the continuum (BICs). First, by breaking the in-plane symmetry of nanodisks, a symmetry-protected quasi-BIC is excited with the emergence of a sharp Fano resonance. The near field distributions, multipole contributions and radiation patterns of the metasurface are performed to uncover the mechanism and characteristics of this resonance. In addition, we investigate the imaging function of this Fano-resonant metasurface assisted by phase-change material Ge_2Sb_2Te_5(GST). Through selective modification of different units from a-GST to c-GST, the produced transmitted image well reconstructs the target letter. Our finding may provide a route to achieve efficient metasurface-based imaging and fast spatial modulations.

Active nonlocal metasurfaces

AbstractActively tunable and reconfigurable wavefront shaping by optical metasurfaces poses a significant technical challenge often requiring unconventional materials engineering and nanofabrication. Most wavefront-shaping metasurfaces can be considered “local” in that their operation depends on the responses of individual meta-units. In contrast, “nonlocal” metasurfaces function based on the modes supported by many adjacent meta-units, resulting in sharp spectral features but typically no spatial control of the outgoing wavefront. Recently, nonlocal metasurfaces based on quasi-bound states in the continuum have been shown to produce designer wavefronts only across the narrow bandwidth of the supported Fano resonance. Here, we leverage the enhanced light-matter interactions associated with sharp Fano resonances to explore the active modulation of optical spectra and wavefronts by refractive-index tuning and mechanical stretching. We experimentally demonstrate proof-of-principle thermo-optically tuned nonlocal metasurfaces made of silicon and numerically demonstrate nonlocal metasurfaces that thermo-optically switch between distinct wavefront shapes. This meta-optics platform for thermally reconfigurable wavefront shaping requires neither unusual materials and fabrication nor active control of individual meta-units.

Plasmonic nanosensor based on sharp Fano resonances induced by aperture-coupled slot system

We theoretically investigate a plasmonic nanostructure, which comprises a groove and aperture-coupled slot resonators integrated on a metal–dielectric–metal (MDM) waveguide. The interference between the mode of the slot resonator and groove give rise to the Fano resonances which can be easily tuned by changing the parameters of the slot. Furthermore, this structure can be extended to generate double Fano resonances by introducing another slot. The calculated wavelength shift between the peak and valley of the Fano line shape is reduced to 10 nm, and the sensitivity and FOM for the structure serving as a nanosensor can reach ∼850 nm/RIU and ∼14000. Thus, our results reveal that the structures may have great potential for highly integrated circuits, nanosensors and slow-light.

Sharp Fano resonance in 2D photonic crystal and the applications

Fano resonance is realized theoretically in a photonic crystal. The structure is composed of a cavity side coupled to a partially transmitting waveguide. By optimizing the structure parameters, asymmetric sharp Fano resonance transmit-tance spectrum is achieved with quality factor of 2 213, extinction ratio of 57 dB and peak loss of 0.2 dB. The sharp spectrum can be used in sensor applications. Such as pressure sensor, the pressure sensitivity is about 9.15 nm/GPa. and for refractive index sensing application, the sensitivity is about 800 nm/RIU, and the maximum of figure of merit can reach 1 000. Besides, this sharp Fano resonance based on photonic crystal has potential applications in optical switches, filters etc. And it can be integrated into optical communications and optical integration circuits.

Multiple Fano resonances in all-dielectric elliptical disk-ring metasurface for high-quality refractive index sensing

In this study, an all-dielectric metasurface is proposed, whose unit cell is composed of a silicon elliptical disk and a silicon elliptical ring. The transmission of the structure were analyzed by using three-dimensional finite-difference time-domain method. In the wavelength range of 800 to 1200 nm, four sharp Fano resonances appeared in the transmission spectrum. The maximum quality factor reached 7421. Furthermore, we analyzed the electromagnetic field distributions corresponding to the four peaks, and evaluated the contributions of multipoles in forming the resonant responses. Besides, we investigated the influence of different structural parameters, including the length of the semi-major axis, the height of the elliptical disk and ring, and the period length, on the four resonance peaks. Finally, based on the high quality factor, we studied the refractive index sensing characteristics of the structure. By optimizing the structural parameters, the sensitivity and the figure of merit could reach up to 392 nm/RIU and 3001, respectively. Therefore, we believe that the proposed structure exhibits broad application prospects in refractive index sensors.

High-sensitivity complex refractive index sensor by designing a slot-waveguide side-coupled Fano resonant cavity

In this paper, we proposed an integrated slotted photonic crystal (PhC) waveguide (SPCW) Fano resonant cavity (SPCW-FRC) for complex refractive index (RI) sensing, in which the device consists of three slot-waveguides and a H1 cavity. The light is coupled input and output the PhC structure by the slot waveguide. By adjusting the parameters of the PhC structure, a sharp asymmetric Fano resonance line shape can be obtained around 1550 nm. An optimal structure is achieved and the optimal parameters are 80 nm of the red air hole diameter, 680 nm of W1 (one line missing) waveguide, 120 nm of the gap (G) of the slots, and 100 nm of the slot width (ws), respectively. Through the three dimensional finite-difference time-domain (3D-FDTD) simulation, the sensitivities of the real part and imaginary part for the complex RI sensing are 494 nm/RIU and ∼8850/RIU, respectively. The simulation results show that the proposed structure is a promising candidate for the ternary mixture concentration detection in medical diagnosis and environmental monitoring.

Tunable Bound States in the Continuum in All-Dielectric Terahertz Metasurfaces.

In this paper, a tunable terahertz dielectric metasurfaces consisting of split gap bars in the unit cell is proposed and theoretically demonstrated, where the sharp high-quality Fano resonance can be achieved through excitation of quasi-bound states in the continuum (quasi-BIC) by breaking in-plane symmetry of the unit cell structure. With the structural asymmetry parameter decreasing and vanishing, the calculated eigenmodes spectra demonstrate the resonance changes from Fano to symmetry-protected BIC mode, and the radiative quality factors obey the inverse square law. Moreover, combining with graphene monolayer and strontium titanate materials, the quasi-BIC Fano resonance can be tuned independently, where the resonance amplitude can be tuned by adjusting the Fermi level of graphene and the resonance frequency can be tuned by controlling the temperature of strontium titanate materials. The proposed structure has numerous potential applications on tunable devices including modulators, switches, and sensors.

Study on the sensing characteristics of Fano resonance based on a coupled streamlined resonance cavity

Based on the theory of surface plasmon polaritons (SPPs), a metal–insulator–metal (MIM) waveguide structure consisting of a streamlined resonant cavity and a baffle is proposed. When the incident light waves enter into the waveguide structure, through the interaction between the three discrete states and the continuous state by the streamlined cavity and the baffle, three sharp asymmetric Fano resonance spectra can be formed. The transmission spectrum characteristics of the structure are simulated and analyzed by the finite element method (FEM), and the effects of structure parameters and refractive index on the transmission spectra characteristics are studied. By optimizing the structure parameters, it is found that the performance of the system can be adjusted and optimized flexibly by changing the structure parameters, the figure of merit (FOM) can reach 1.99 × 106, and the sensitivity is 2960 nm/RIU. This flexible Fano resonant-structure has several applications in micro-nano biosensor, nonlinear optics, and slow light devices.

Plasmonic nanosensor based on multiple independently tunable Fano resonances.

A novel refractive index nanosensor with compound structures is proposed in this paper. It consists of three different kinds of resonators and two stubs which are side-coupled to a metal–dielectric–metal (MDM) waveguide. By utilizing numerical investigation with the finite element method (FEM), the simulation results show that the transmission spectrum of the nanosensor has as many as five sharp Fano resonance peaks. Due to their different resonance mechanisms, each resonance peak can be independently tuned by adjusting the corresponding parameters of the structure. In addition, the sensitivity of the nanosensor is found to be up to 1900 nm/RIU. For practical application, a legitimate combination of various different components, such as T-shaped, ring, and split-ring cavities, has been proposed which dramatically reduces the nanosensor dimensions without sacrificing performance. These design concepts pave the way for the construction of compact on-chip plasmonic structures, which can be widely applied to nanosensors, optical splitters, filters, optical switches, nonlinear photonic and slow-light devices.

Symmetry-protected bound states in the continuum supported by all-dielectric metasurfaces

Symmetry-protected bound states in the continuum (BICs) are nonradiative states with infinite lifetime and perfect confinement of energy even though lying in the radiation continuum due to the symmetry incompatibility. Herein, we study the symmetry-protected BIC supported by metasurfaces composed of silicon nanodisks. Through adding or removing parts of the nanodisks from the edge, a sharp Fano resonance emerges and demonstrates the excitation of quasi-BIC. Their $Q$ factors exhibit the same dependence on the asymmetry degree with these two opposite operations. Furthermore, from both qualitative and quantitative perspectives, analysis on far-field contributions from multipole moments along different directions combining with near-field distributions explains the evolution from BIC to quasi-BIC. The dominant contributor to the quasi-BIC is illustrated to be the electric quadrupole in the $x\ensuremath{-}y$ plane. Finally, the topological charge carried by the BIC is calculated to be $\ensuremath{-}1$, demonstrating the topological characteristics of our design. Such metasurfaces are robust in nanofabrication. Our results may provide a route for resonators with better performance applied in sensing, switching, nonlinear optics, and so on.

Hybrid metal-graphene plasmonic sensor for multi-spectral sensing in both near- and mid-infrared ranges.

This paper proposes a hybrid metal-graphene plasmonic sensor which can simultaneously perform multi-spectral sensing in near- and mid-IR ranges. The proposed sensor consists of an array of asymmetric gold nano-antennas integrated with an unpatterned graphene sheet. The gold antennas support sharp Fano-resonances for near-IR sensing while the excitation of graphene plasmonic resonances extend the sensing spectra to the mid-IR range. Such a broadband spectral range goes far beyond previously demonstrated multi-spectral plasmonic sensors. The sensitivity and figure of merit (FOM) as well as their dependence on the thickness of the sensing layer and Fermi energy of graphene are studied systematically. This new type of sensor combines the advantages of conventional metallic plasmonic sensors and graphene plasmonic sensors and may open a new door for high-performance, multi-functional plasmonic sensing.

Sharp Fano resonance induced by all-dielectric asymmetric metasurface

Plasmonic metasurfaces have attracted tremendous attentions due to the broad applications in chemical and biomedical sensing, surface enhanced spectroscopy, and near-field scanning optical microscopy. However, it is still a great challenge to enhance the figure of merit and sensitivity of sensors in near-infrared wavelengths due to the inherent resistive loss of conventionally used metallic materials. In this paper, we designed an all-dielectric metasurface composed of asymmetric double nanorods to achieve ultra-narrow band Fano resonance. Benefitting from the interference of bright mode and dark mode, a sharp and wavelength-adjustable reflectance peak can be obtained, offering an excellent platform for biological sensing and detection.

Photosensitive chalcogenide metasurfaces supporting bound states in the continuum.

We study, both theoretically and experimentally, tunable metasurfaces supporting sharp Fano-resonances inspired by optical bound states in the continuum. We explore the use of arsenic trisulfide (a photosensitive chalcogenide glass) having optical properties which can be finely tuned by light absorption at the post-fabrication stage. We select the resonant wavelength of the metasurface corresponding to the energy below the arsenic trisulfide bandgap, and experimentally control the resonance spectral position via exposure to the light of energies above the bandgap.

High Q-factor multiple Fano resonances for high-sensitivity sensing in all-dielectric metamaterials

We design and numerically investigate the high Q-factor, high modulation depth, and multiple Fano resonances based on a periodical all-dielectric asymmetric nanorod dimer in the near-infrared regime. It is demonstrated that, due to the excitation of the subradiant hybrid modes, five sharp Fano resonances can be achieved by breaking the symmetry of the dimer and can be flexibly tuned by varying the geometrical parameters. All five Fano resonances have a narrow line width, the maximal Q-factor exceeds 9700, and even the minimal Q-factor also reaches about 1090 in magnitude. Particularly, the modulation depth can reach nearly 100%. In addition, the maximal figure of merit reaches 5045. Considering the narrow line-width and significant near-field enhancement, five Fano resonances with large modulation depths in the proposed array are useful for lasing, nonlinear optics, and multiwavelength biosensing.

Metasurface Engineering through Bound States in the Continuum

Metasurfaces have attracted a lot of attention in recent years due to novel ways they provide for the efficient wavefront control and engineering of the resonant transmission. We discuss an approach allowing effectively control appearance of the sharp Fano resonances in metasurfaces associated with the {\it bound states in the continuum}. We demonstrate that by breaking the symmetry transversely, in the direction perpendicular to a metasurface with a complex unit cell, we can control the number, frequency, and type of high-$Q$ resonances originating from bound states in the continuum. As example, we demonstrate experimentally the metasurfaces with magnetic dipole and toroidal dipole responses governed by the physics of multipolar bound states.

High Q-factor multiple Fano resonances for high-sensitivity sensing in all-dielectric nanocylinder dimer metamaterials

We investigated the high Q-factor, high modulation depth, and multiple Fano resonances based on a periodical all-dielectric asymmetric nanocylinder dimer. It is demonstrated that due to excitation of subradiant hybrid modes five sharp Fano resonances can be achieved by breaking symmetry and can be flexibly tuned by geometry. All Fano resonances have a narrow line width, maximal Q-factor exceeds 5000 and even minimal Q-factor also reaches 870. Moreover, modulation depth can reach nearly 100%. In addition, the bulk refractive index sensitivity exceeds 530 nm/RIU, and maximal figure of merit reaches 3350. The proposed array is useful for nonlinear optics and biosensing.

High sensitivity and FOM refractive index sensing based on Fano resonance in all-grating racetrack resonators

A refractive index sensor based on all-grating racetrack resonators is proposed and demonstrated in this paper. The structure consists of two subwavelength grating (SWG) waveguide-based racetrack resonators, a straight SWG bus waveguide and a SWG waveguide-based open racetrack. A sharp Fano resonance can be obtained by the mutual coupling of the two SWG racetrack resonators. Theoretical analysis and numerical simulations show that the introduction of grating can enhance the interaction between the sensor and the measured liquid analyte, thus increasing the sensitivity of the device. Calculated sensitivity of 500 nm/RIU around wavelength 1550 nm is achieved, and the figure of merit is 2000 RIU−1. The extinction ratio of this device can reach 21.53 dB.

Multifunctional Metasurfaces Based on Direct Nanoimprint of Titania Sol–Gel Coatings

AbstractDielectric Mie resonators are taking momentum in the last years thanks to their peculiar properties in light management at visible and near‐infrared frequencies. However, their full exploitation demands for cheap materials and versatile fabrication methods, extendible over large surfaces and potentially C‐MOS compatible. Here, a sol–gel deposition and nanoimprint lithography method is used to obtain titania‐based Mie resonators over large areas (several square millimeters), showing that this platform can potentially be exploited for light management with different devices. First, their use for structural colors and efficient band pass filters covering the visible spectrum is demonstrated. Then, exploiting sharp Fano resonances in reflection, their potential for refractive index sensing is addressed obtaining a figure of merit of ≈20. Finally, when placing the resonators on porous silica, a large and reversible color tuning can be produced as a result of water adsorption within the substrate porosity. These results open the path to titania sub‐micrometric structures for applications as a multifunctional metamaterial for smart windows, displays, and all‐optical sensing.