Fano resonances based on multimode and degenerate mode interference in plasmonic resonator system

Multi-Wavelength Selective and Broadband Near-Infrared Plasmonic Switches in Anisotropic Plasmonic Metasurfaces

Anisotropic plasmonic metasurfaces have attracted broad research interest since they possess novel optical properties superior to natural materials and their tremendous design flexibility. However, the realization of multi-wavelength selective plasmonic metasurfaces that have emerged as promising candidates to uncover multichannel optical devices remains a challenge associated with weak modulation depths and narrow operation bandwidth. Herein, we propose and numerically demonstrate near-infrared multi-wavelength selective passive plasmonic switching (PPS) that encompasses high ON/OFF ratios and strong modulation depths via multiple Fano resonances (FRs) in anisotropic plasmonic metasurfaces. Specifically, the double FRs can be fulfilled and dedicated to establishing tailorable near-infrared dual-wavelength PPS. The multiple FRs mediated by in-plane mirror asymmetries cause the emergence of triple-wavelength PPS, whereas the multiple FRs governed by in-plane rotational asymmetries avail the implementation of the quasi-bound states in the continuum-endowed multi-wavelength PPS with the ability to unfold a tunable broad bandwidth. In addition, the strong polarization effects with in-plane anisotropic properties further validate the existence of the polarization-resolved multi-wavelength PPS. Our results provide an alternative approach to foster the achievement of multifunctional meta-devices in optical communication and information processing.

Analysis of the water-soluble vitamins based on MIM waveguide structure and Fano resonance

Fano resonance (FR) is extremely sensitive to extremely small changes in the surrounding environment. We first propose an optical nano-refractive index sensor based on Fano resonance, which is applied to the identification of water-soluble vitamins B1, B5 and B6 and the measurement of the concentration of vitamin B1. The sensor can be used to rapidly identify pure vitamins B1, B5, and B6 at a concentration of 1 g/50 mL at 25 °C based on the relationship between the wavelength shift in the FR line spectrum and the refractive index. This work shows that the sensitivity of the sensor can reach 1327.5 nm/RIU, the sensor can be used to rapidly identify vitamins B1, B5, and B6 through changes in refractive index under certain conditions. Moreover, rapid calculation of vitamin B1 solution concentration is achieved based on the relationship between different concentrations of vitamin B1 solution and their corresponding refractive indexes and wavelength shifts in their FR line spectrums, which is an important step for the application of the designed MIM waveguide structures to the fields of biology, chemistry, and medicine.

Plasmonic Sensing and Switches Enriched by Tailorable Multiple Fano Resonances in Rotational Misalignment Metasurfaces

Fano resonances that feature strong field enhancement in the narrowband range have motivated extensive studies of light-matter interactions in plasmonic nanomaterials. Optical metasurfaces that are subject to different mirror symmetries have been dedicated to achieving nanoscale light manipulation via plasmonic Fano resonances, thus enabling advantages for high-sensitivity optical sensing and optical switches. Here, we investigate the plasmonic sensing and switches enriched by tailorable multiple Fano resonances that undergo in-plane mirror symmetry or asymmetry in a hybrid rotational misalignment metasurface, which consists of periodic metallic arrays with concentric C-shaped- and circular-ring-aperture unit cells. We found that the plasmonic double Fano resonances can be realized by undergoing mirror symmetry along the X-axis. The plasmonic multiple Fano resonances can be tailored by adjusting the level of the mirror asymmetry along the Z-axis. Moreover, the Fano-resonance-based plasmonic sensing that suffer from mirror symmetry or asymmetry can be implemented by changing the related structural parameters of the unit cells. The passive dual-wavelength plasmonic switches of specific polarization can be achieved within mirror symmetry and asymmetry. These results could entail benefits for metasurface-based devices, which are also used in sensing, beam-splitter, and optical communication systems.

A Nanoscale Sensor Based on a Toroidal Cavity with a Built-In Elliptical Ring Structure for Temperature Sensing Application

In this article, a refractive index sensor based on Fano resonance, which is generated by the coupling of a metal-insulator-metal (MIM) waveguide structure and a toroidal cavity with a built-in elliptical ring (TCER) structure, is presented. The finite element method (FEM) was employed to analyze the propagation characteristics of the integral structure. The effects of refractive index and different geometric parameters of the structure on the sensing characteristics were evaluated. The maximum sensitivity was 2220 nm/RIU with a figure of merit (FOM) of 58.7, which is the best performance level that the designed structure could achieve. Moreover, due to its high sensitivity and simple structure, the refractive index sensor can be applied in the field of temperature detection, and its sensitivity is calculated to be 1.187 nm/℃.

Optical-fibre characteristics based on Fano resonances and sensor application in blood glucose detection

We propose an optical-fibre metal-insulator-metal (MIM) plasmonic sensor based on the Fano resonances of surface plasmon polaritons (SPPs). Its structure consists of a coupling fibre that connects C-shaped and rectangular cavities and a main fibre that contains a semi-circular resonator. When incident light passes through the main fibre, it excites SPPs along the interface between the metal and medium. The SPPs at the resonator induce Fano resonances, owing to the coupling effect. The results show that the designed optical-fibre MIM plasmonic sensor could flexibly tune the number of Fano resonances by adjusting the structure and geometric parameters to optimise the sensing performance. The full width at half maximum of the Lorentzian resonance spectra formed by the electric and magnetic fields reached 23 nm and 24 nm, respectively. The wavelength of the Fano resonance shifted as the refractive index changed; thus, the proposed sensor could realise the application of sensing and detection. The highest sensitivity achieved by the sensor was 1770 nm/RIU. Finally, we simulated the designed sensor to human blood-glucose-level detection, and observed that the resonance wavelength would increase with the increase of glucose concentration. Our study shows that optical fibres have broad application prospects in the field of electromagnetic switching and sensing.

Analogous plasmon-induced absorption based on an end-coupled MDM structure with area-cost-free cavities

We theoretically investigate an end-coupled metal-dielectric-metal (MDM) structure that achieves analogous plasmon-induced absorption (APIA) in an area-cost-free manner. First, a squared ring is set to end-couple with MDM input and output waveguides, generating three Lorentzian-like peaks in the spectrum. Then, two APIA windows as well as two Fano resonances can be induced via appropriately arranging two area-free cavities. Numerous numerical results demonstrate that the proposed structure has remarkable sensing and phase characteristics. Our proposed PIA-based MDM structure is promising in potential applications of bio-chemical sensing, slow light devices, optical switching, and chip-scale plasmonic devices.

High-Property Refractive Index and Bio-Sensing Dual-Purpose Sensor Based on SPPs

A high-property plasma resonance-sensor structure consisting of two metal-insulator-metal (MIM) waveguides coupled with a transverse ladder-shaped nano-cavity (TLSNC) is designed based on surface plasmon polaritons. Its transmission characteristics are analyzed using multimode interference coupling mode theory (MICMT), and are simulated using finite element analysis (FEA). Meanwhile, the influence of different structural arguments on the performance of the structure is investigated. This study shows that the system presents four high-quality formants in the transmission spectrum. The highest sensitivity is 3000 nm/RIU with a high FOM^* of 9.7 × 10⁵. In addition, the proposed structure could act as a biosensor to detect the concentrations of sodium ions (Na⁺), potassium ions (K⁺), and the glucose solution with maximum sensitivities of 0.45, 0.625 and 5.5 nm/mgdL⁻¹, respectively. Compared with other structures, the designed system has the advantages of a simple construction, a wide working band range, high reliability and easy nano-scale integration, providing a high-performance cavity choice for refractive index sensing and biosensing devices based on surface plasmons.

Near-infrared plasmonic sensing and digital metasurface via double Fano resonances

Plasmonic sensing that enables the detection of minute events, when the incident light field interacts with the nanostructure interface, has been widely applied to optical and biological detection. Implementation of the controllable plasmonic double Fano resonances (DFRs) offers a flexible and efficient way for plasmonic sensing. However, plasmonic sensing and digital metasurface induced by tailorable plasmonic DFRs require further study. In this work, we numerically and theoretically investigate the near-infrared plasmonic DFRs for plasmonic sensing and digital metasurface in a hybrid metasurface with concentric ϕ-shaped-hole and circular-ring-aperture unit cells. We show that a plasmonic Fano resonance, resulting from the interaction between a narrow and a wide effective dipolar modes, can be realized in the ϕ-shaped hybrid metasurface. In particular, we demonstrate that the tailoring plasmonic DFRs with distinct mechanisms of actions can be accomplished in three different ϕ-shaped hybrid metasurfaces. Moreover, the resonance mode-broadening and mode-shifting plasmonic sensing can be fulfilled by modulating the polarization orientation and the related geometric parameters of the unit cells in the near-infrared waveband, respectively. In addition, the plasmonic switch with a high ON/OFF ratio can not only be achieved but also be exploited to establish a single-bit digital metasurface, even empower to implement two- and three-bit digital metasurface characterized by the plasmonic DFRs in the telecom L-band. Our results offer a new perspective toward realizing polarization-sensitive optical sensing, passive optical switches, and programmable metasurface devices, which also broaden the landscape of subwavelength nanostructures for biosensors and optical communications.

Multiple Sharp Fano Resonances in a Deep-Subwavelength Spherical Hyperbolic Metamaterial Cavity

We theoretically study the multiple sharp Fano resonances produced by the near-field coupling between the multipolar narrow plasmonic whispering-gallery modes (WGMs) and the broad-sphere plasmon modes supported by a deep-subwavelength spherical hyperbolic metamaterial (HMM) cavity, which is constructed by five alternating silver/dielectric layers wrapping a dielectric nanosphere core. We find that the linewidths of WGMs-induced Fano resonances are as narrow as 7.4–21.7 nm due to the highly localized feature of the electric fields. The near-field coupling strength determined by the resonant energy difference between WGMs and corresponding sphere plasmon modes can lead to the formation of the symmetric-, asymmetric-, and typical Fano lineshapes in the far-field extinction efficiency spectrum. The deep-subwavelength feature of the proposed HMM cavity is verified by the large ratio (~5.5) of the longest resonant wavelength of WGM_1,1 (1202.1 nm) to the cavity size (diameter: 220 nm). In addition, the resonant wavelengths of multiple Fano resonances can be easily tuned by adjusting the structural/material parameters (the dielectric core radius, the thickness and refractive index of the dielectric layers) of the HMM cavity. The narrow linewidth, multiple, and tunability of the observed Fano resonances, together with the deep-subwavelength feature of the proposed HMM cavity may create potential applications in nanosensors and nanolasers.

Improved Refractive Index-Sensing Performance of Multimode Fano-Resonance-Based Metal-Insulator-Metal Nanostructures

This work proposed a multiple mode Fano resonance-based refractive index sensor with high sensitivity that is a rarely investigated structure. The designed device consists of a metal–insulator–metal (MIM) waveguide with two rectangular stubs side-coupled with an elliptical resonator embedded with an air path in the resonator and several metal defects set in the bus waveguide. We systematically studied three types of sensor structures employing the finite element method. Results show that the surface plasmon mode’s splitting is affected by the geometry of the sensor. We found that the transmittance dips and peaks can dramatically change by adding the dual air stubs, and the light–matter interaction can effectively enhance by embedding an air path in the resonator and the metal defects in the bus waveguide. The double air stubs and an air path contribute to the cavity plasmon resonance, and the metal defects facilitate the gap plasmon resonance in the proposed plasmonic sensor, resulting in remarkable characteristics compared with those of plasmonic sensors. The high sensitivity of 2600 nm/RIU and 1200 nm/RIU can simultaneously achieve in mode 1 and mode 2 of the proposed type 3 structure, which considerably raises the sensitivity by 216.67% for mode 1 and 133.33% for mode 2 compared to its regular counterpart, i.e., type 2 structure. The designed sensing structure can detect the material’s refractive index in a wide range of gas, liquids, and biomaterials (e.g., hemoglobin concentration).

High Sensitivity Plasmonic Sensor Based on Fano Resonance with Inverted U-Shaped Resonator

Herein, we propose a tunable plasmonic sensor with Fano resonators in an inverted U-shaped resonator. By manipulating the sharp asymmetric Fano resonance peaks, a high-sensitivity refractive index sensor can be realized. Using the multimode interference coupled-mode theory and the finite element method, we numerically simulate the influences of geometrical parameters on the plasmonic sensor. Optimizing the structure parameters, we can achieve a high plasmonic sensor with the maximum sensitivity for 840 nm/RIUand figure of merit for 3.9 × 105. The research results provide a reliable theoretical basis for designing high sensitivity to the next generation plasmonic nanosensor.

Ultrawide Bandgap and High Sensitivity of a Plasmonic Metal-Insulator-Metal Waveguide Filter with Cavity and Baffles

A plasmonic metal-insulator-metal waveguide filter consisting of one rectangular cavity and three silver baffles is numerically investigated using the finite element method and theoretically described by the cavity resonance mode theory. The proposed structure shows a simple shape with a small number of structural parameters that can function as a plasmonic sensor with a filter property, high sensitivity and figure of merit, and wide bandgap. Simulation results demonstrate that a cavity with three silver baffles could significantly affect the resonance condition and remarkably enhance the sensor performance compared to its counterpart without baffles. The calculated sensitivity (S) and figure of merit (FOM) in the first mode can reach 3300.00 nm/RIU and 170.00 RIU⁻¹. Besides, S and FOM values can simultaneously get above 2000.00 nm/RIU and 110.00 RIU⁻¹ in the first and second modes by varying a broad range of the structural parameters, which are not attainable in the reported literature. The proposed structure can realize multiple modes operating in a wide wavelength range, which may have potential applications in the on-chip plasmonic sensor, filter, and other optical integrated circuits.

Fano resonance based on D-shaped waveguide structure and its application for human hemoglobin detection

Fano resonance is a pervasive resonance phenomenon which can be applied to high sensitivity sensing, perfect absorption, electromagnetic-induced transparency, and slow-light photonic devices. In this paper, we propose a metal-insulator-metal (MIM) waveguide structure consisting of a D-shaped cavity and a bus waveguide with a silver-air-silver barrier. The Fano resonance can be achieved by the interaction between the D-shaped cavity and the bus waveguide. The finite element method is used to analyze the transmission characteristics and magnetic-field distributions of the structure in detail. Simulation results show the Fano resonance can be adjusted by altering the geometric parameters of the MIM waveguide structure or the refractive index of the D-shaped cavity. The maximum refractive index sensitivity of the structure can reach up to 1510 nm/RIU, and there is a good linear relationship between resonance wavelength and refractive index. Since it has good sensitivity and tunability, the MIM waveguide structure can be used in bio-sensing, such as human hemoglobin detection. We show its applicability for the detection of three different human blood groups as well.

Ultra-compact high-sensitivity plasmonic sensor based on Fano resonance with symmetry breaking ring cavity

Miniaturizing optical devices with desired functionality is a key prerequisite for nanoscale photonic circuits. Based on Fano resonance, an on-chip high-sensitivity sensor, composed of two waveguides coupling with a symmetry breaking ring resonator, is theoretically and numerically investigated. The established theoretical model agrees well with the finite-difference time-domain simulations, which reveals the physics of Fano resonance. Differing with the coupled cavities, the Fano resonance originates from the interference between symmetry-mode and asymmetry-mode in a single symmetry-broken cavity. The spectral responses and sensing performances of the plasmonic structure rely on the degree of asymmetry of cavity. In particular, the plasmonic sensor can detect the refractive index changes as small as 10^-5, and the figure of merit (FOM) of symmetry-breaking cavity structure is 17 times larger than that of symmetrical cavity system. Additionally, the sensitivity to temperature of ethanol analyte achieves 0.701 nm/^○C. Compared with the coupled cavities, the on-chip high-sensitivity sensor using a single cavity is more compact, which paves the way toward highly integrated photonic devices.

Dynamically tunable switch and filter in single slot cavity structure

A single slot cavity coupled with two waveguides has been researched in theory and simulation. The results comparison between theory and simulation shows they agree well. It is found that the lateral displacement S plays an important role in transmission properties. Moreover, increasing the width of the slot cavity results in the emergence of new resonant peaks. At the same time, the shift of the resonant peaks have been explained well. The slot cavity with Kerr nonlinear material can act as a dynamically tunable four channel switch and filter. The single slot cavity has the advantages of simple and compact structure, easy fabrication, and the excellent properties are helpful to control light in photonics circuits.

Tuning Multiple Fano Resonances for On-Chip Sensors in a Plasmonic System

This paper proposed a plasmonic resonator system, consisting of a metal-insulator-metal structure and two stubs, and a Fano resonance arose in its transmittance, which resulted from the coupling between the two stubs. On the basis of the proposed structure, a circle and a ring cavity are separately added above the stubs to create different coupled plasmonic structures, providing triple and quadruple Fano resonances, respectively. Additionally, by adjusting the geometric parameters of the system, multiple Fano Resonances obtained can be tuned. The proposed structure can be served as a high efficient refractive index sensor, yielding a sensitivity of 2000 nm/RIU and figure of merit (FOM) of 4.05×104 and performing better than most of the similar structures. It is believed that the proposed structure may support substantial applications for on-chip sensors, slow light and nonlinear devices in highly integrated photonic circuits.

Independently Tunable Fano Resonances Based on the Coupled Hetero-Cavities in a Plasmonic MIM System

In this paper, based on coupled hetero-cavities, multiple Fano resonances are produced and tuned in a plasmonic metal-insulator-metal (MIM) system. The structure comprises a rectangular cavity, a side-coupled waveguide, and an upper-coupled circular cavity with a metal-strip core, used to modulate Fano resonances. Three Fano resonances can be realized, which originate from interference of the cavity modes between the rectangular cavity and the metal-strip-core circular cavity. Due to the different cavity-cavity coupling mechanisms, the three Fano resonances can be divided into two groups, and each group of Fano resonances can be well tuned independently by changing the different cavity parameters, which can allow great flexibility to control multiple Fano resonances in practice. Furthermore, through carefully adjusting the direction angle of the metal-strip core in the circular cavity, the position and lineshape of the Fano resonances can be easily tuned. Notably, reversal asymmetry takes place for one of the Fano resonances. The influence of the direction angle on the figure of merit (FOM) value is also investigated. A maximum FOM of 3436 is obtained. The proposed structure has high transmission, sharp Fano lineshape, and high sensitivity to change in the background refractive index. This research provides effective guidance to tune multiple Fano resonances, which has important applications in nanosensors, filters, modulators, and other related plasmonic devices.

Tunable Nanosensor Based on Fano Resonances Created by Changing the Deviation Angle of the Metal Core in a Plasmonic Cavity

In this paper, a type of tunable plasmonic refractive index nanosensor based on Fano resonance is proposed and investigated. The sensor comprises a metal-insulator-metal (MIM) nanocavity with a center-deviated metal core and two side-coupled waveguides. By carefully adjusting the deviation angle and distance of the metal core in the cavity, Fano resonances can be obtained and modulated. The Fano resonances can be considered as results induced by the symmetry-breaking or geometric effect that affects the field distribution intensity at the coupling region between the right waveguide and the cavity. Such a field-distribution pattern change can be regarded as being caused by the interference between the waveguide modes and the cavity modes. The investigations demonstrate that the spectral positions and modulation depths of Fano resonances are highly sensitive to the deviation parameters. Furthermore, the figure of merit (FOM) value is calculated for different deviation angle. The result shows that this kind of tunable sensor has compact structure, high transmission, sharp Fano lineshape, and high sensitivity to the change in background refractive index. This work provides an effective method for flexibly tuning Fano resonance, which has wide applications in designing on-chip plasmonic nanosensors or other relevant devices, such as information modulators, optical filters, and ultra-fast switches.

Multiple Fano-Like MIM Plasmonic Structure Based on Triangular Resonator for Refractive Index Sensing

In this paper, we present a Fano metal-insulator-metal (MIM) structure based on an isosceles triangular cavity resonator for refractive index sensing applications. Due to the specific feeding scheme and asymmetry introduced in the triangular cavity, the resonator exhibits four sharp Fano-like resonances. The behavior of the structure is analyzed in detail and its sensing capabilities demonstrated through the responses for various refractive indices. The results show that the sensor has very good sensitivity and maximal figure of merit (FOM) value of 3.2 × 10⁵. In comparison to other similar sensors, the proposed one has comparable sensitivity and significantly higher FOM, which clearly demonstrates its high sensing potential.

Constructive-interference-enhanced Fano resonance of silver plasmonic heptamers with a substrate mirror: a numerical study

Plasmonic nanostructures with strong Fano resonance are of fundamental interest. Here, our systematic simulations show that rational positioning of a silver plasmonic heptamer above a highly reflective substrate mirror can significantly enhance its intrinsic Fano-resonance intensity. The silver nanodisk heptamer positioned at an appropriate distance above the reflective substrate enables 2.4 times field enhancement and 3.6 times deeper Fano-dip respectively compared to the heptamer directly placed on silicon oxide substrate. Besides, our results indicate that the Fano-dip position does not shift when the silver nanodisk heptamer gradually shifts away from the reflective substrate mirror (≥60 nm).