Metal-dielectric Waveguide Research Articles

Nanoplasmonic Directional Coupler Using Asymmetric Parallel Coupled MIM Waveguides

This article presents the design and analysis of a nanoplasmonic directional coupler using an asymmetric parallel-coupled line metal insulator metal (MIM) waveguide. The proposed plasmonic directional coupler is designed by applying three wire transmission line (TL) model based on the basic TL characteristic parameters of the related asymmetric parallel coupled line structures. To this end, the effective refractive index, propagation length, and characteristic impedance of the related coupled line MIM waveguide structure are obtained by using fullwave simulation software CST microwave studio suite. Specifically, the dual-band nature can be obtained by exchanging the asymmetric coupled line MIM waveguide section with two asymmetric even mode step impedance resonators (SIRs) by adjusting the gap width (g <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> ). Subsequently, a side-coupled resonator is designed and analyzed. Hence, the designed coupler is more useful to exchange several plasmonic devices such as power splitters and mixers in photonic integrated circuits (PICs) at a subwavelength scale.

High efficiency coupling to metal-insulator-metal plasmonic waveguides.

A periodic array of dual-Vivaldi antennas integrated with metal-insulator-metal (MIM) plasmonic waveguides was designed and investigated for its infrared light absorbance efficiency. Full-wave analysis was used to optimize MIM waveguides compatible with parallel and series connected DC leads without sacrificing radiation efficiency. Free-space to MIM waveguide in-coupling efficiency as high as 41% has been obtained in a sub-wavelength unit cell geometry at a wavelength of 1373 nm. Higher efficiency, up to 85%, is predicted with a modified design including a backplane reflector. A nanofabrication process was developed to realize test devices and far-field optical spectroscopy was used as experimental evidence for antenna-waveguide matching.

Tunable band-pass plasmonic filter and wavelength triple-channel demultiplexer based on square nanodisk resonator in MIM waveguide

This paper proposes a wavelength demultiplexer (WDM) in a metal-insulator-metal (MIM) waveguide based on square cascade resonators, which is numerically simulated using the finite difference time domain (FDTD) approach. WDM's basic construction is a plasmonic filter with two waveguides at the input and output (bass and drop) and a central cavity coupled horizontally to two square nanodisks. The proposed demultiplexer is formed by stacking three examples of these filters and square resonators with different dimensions and with a vertical cavity and a waveguide at the structure's input. The simulation results show that in the designed structure, the Full Width at Half-Maximum (FWHM) in some channels is around 10 nm. The proposed WDM has a cross talk effect of less than − 25 dB. In fact, when compared to structures at its level, this structure has a high sensitivity, a low cross talk effect, and a proper Figure of Merit (FoM). If the geometric parameters and insulation's refractive index (RI) are set appropriately, the transmission qualities are able to tune to desired level. The proposed structure has the potential to be utilised in optically integrated circuits, nanosensors, to develop into a 1 ×N demultiplexer, and ultracompact plasmonic devices.

Topological edge states at singular points in non-Hermitian plasmonic systems

We introduce non-Hermitian plasmonic waveguide-cavity systems with topological edge states (TESs) at singular points. The compound unit cells of the structures consist of metal-dielectric-metal (MDM) stub resonators side-coupled to an MDM waveguide. We show that we can realize both a TES and an exceptional point at the same frequency when a proper amount of loss is introduced into a finite three-unit-cell structure. We also show that the finite structure can exhibit both a TES and a spectral singularity when a proper amount of gain is introduced into the structure. In addition, we show that we can simultaneously realize a unidirectional spectral singularity and a TES when proper amounts of loss and gain are introduced into the structure. We finally show that this singularity leads to extremely high sensitivity of the reflected light intensity to variations of the refractive index of the active materials in the structure. TESs at singular points could potentially contribute to the development of singularity-based plasmonic devices with enhanced performance.

Narrow-spectrum enhanced multiparameter gas sensor based on Fano resonance in an asymmetric MDM waveguide

In this study, a highly sensitive and narrow–spectrum enhanced multiparameter gas sensor is proposed based on Fano resonance in an asymmetric metal–dielectric–metal (MDM) waveguide. A side-coupled MDM structure comprising an upright rectangular cavity is initially investigated under different gaps. An assumed junction model is used to analyze the physical mechanism of a through coupled system (gap = 0) with an ultralow minimum transmission coefficient and higher refractive index sensitivity compared to other cases. Becausethe transmission spectrum of through-coupled structure is relatively broad, a ring resonator was added on the upper side of straight waveguide to generate an ultranarrow resonant mode as a discrete state that interferes with the previous broad mode serving as a quasicontinuum state to afford Fano resonance. Consequently, its full width at half maximum decreases by 420% from 496 to 11.56 nm. The response rate and detection difficulty of detection of y-asymmetric plasmonic nanostructures used in this study are exceed those of x-asymmetric plasmonic nanostructures. A multiparameter gas sensor can be developed by filling hydrogen- and methane-sensitive films in corresponding cavities of the plasmonic nanostructure. The sensitivities of methane and hydrogen can reach −2.863 and −0.265 nm/%, respectively, after optimizing relevant structural parameters. The proposed method provides an effective approach for designing narrow-spectrum enhanced and multichannel nanophotonic devices.

Plasmonic Tamm states in periodic stubbed MIM waveguides: analytical and numerical study

We investigate both analytically and numerically the existence of localized surface modes, the so-called plasmonic Tamm states (PTSs), in a new and versatile platform based on a periodic array of metal-insulator-metal (MIM) stubs grafted along a MIM waveguide. By considering a semi-infinite structure in which we modify the length of the segment at the surface, we show the existence of surface states inside the bandgaps of the periodic structure and investigate the dependence of the localized modes as a function of the geometrical parameters and the boundary conditions applied at the surface. Three types of surface boundary conditions are considered, namely, two limiting cases of zero surface impedance (or perfect electric conductor), infinite surface impedance (or perfect magnetic conductor), and a third case where the structure is in contact with a real metal. In the latter case, we show that the existence of the interface state can be demonstrated based on topological arguments using the Zak phase. We also demonstrate that if a finite size comb-crystal is vertically grafted along a horizontal waveguide, the PTSs can be detected from the dips in the amplitudes of transmission and reflection coefficients as well as from the peaks in their delay times and the local density of states (LDOS). Our theoretical study is first performed analytically with the help of a Green’s function method, which allows the calculation of the dispersion relations of the bulk and surface modes and the LDOS, as well as the transmission and reflection coefficients of the plasmonic comb-like structure. Then, these results are confirmed by a numerical simulation utilizing a 2D finite element method. Besides providing a deep physical analysis of the PTSs, our work demonstrates the capability of the analytical method as a predictive approach in more complex structures. The proposed designs in this paper can be useful to realize highly sensitive plasmonic nanosensors.

All-Optical Logic Gates Using a Plasmonic MIM Waveguide and Elliptical Ring Resonator

All-optical logic gates OR, XOR, AND, and NOT based on two-dimensional (2D) plasmonic metal-insulator-metal (MIM) coupled with an elliptical ring resonator (ERR) are presented, simulated, and investigated by using the numerical method of the FEM (finite elements method). The results are compared and validated with the finite difference time domain (FDTD) method. The proposed logic gates are achieved with the same structure using the constructive and destructive optical interferences between a control signal and input signal(s). Their characterization was mainly done for two spectral regions, visible and near-infrared. A high-intensity contrast ratio (CR) between the logic states (“1” and “0”) can be achieved (28 dB) at these spectral regions. We introduce a new parameter, “gap-threshold ratio (GTR),” to characterize the gap between the maximum and minimum of the transmitted signal intensity for all logic gates. The suggested value of transmission threshold between logic “0” and logic “1” states is \(T_{th}=0.2\). A comparison of the two parameters, CR and GTR, with previous works shows that the proposed structure gives very good results for all logic gates configurations. The proposed all-optical logic gates configuration can be a key component in optical processing and telecommunication devices.

Dynamically tunable refractive index sensor based on Fano resonance with metal-insulator-metal-graphene nanotube hybrid structure

In practical applications, the performances of conventional metal-insulator-metal (MIM) waveguide structured optical devices cured during fabrication are not dynamically tunable. In order to address the problem that such devices are not dynamically tunable, based on the excellent optoelectronic properties of graphene materials, graphene nanotubes are induced into the metal-insulator-metal waveguide coupled circular resonant cavity structure, thus designing a dynamically tunable MIM-graphene nanotube hybrid structure refractive index sensor in this work. The finite element method (FEM) is used to numerically study the transmission characteristics, electric field distribution and magnetic field distribution of the system, and the theoretical analysis is performed by multimode interference coupled mode theory (MICMT) to verify its correctness. The results show that after adding graphene nanotube to the MIM waveguide coupled ring resonant cavity structure, a Fano resonance peak appears in this system, which originates from the coherent coupling between the TM&lt;sub&gt;10&lt;/sub&gt; cavity resonance mode and the graphene plasmonic electrical resonance mode. The sensor can dynamically tune the resonance wavelength and linewidth of Fano resonance in a wide wavelength range by changing the chemical potential of graphene, thus realizing the performance tuning of the refractive index sensor. Hence, the problem that the conventional plasma refractive index sensor is not dynamically tunable issolved. In addition, the influence of the geometrical parameters of the structure on the sensing performance of this system is also studied in detail. The sensor sensitivity increases up to 1250 nm/RIU and the quality factor rises up to 42.4 RIU&lt;sup&gt;–1&lt;/sup&gt; at the optimal structural parameters. Compared with the traditional metal-insulator-metal waveguide structure design, this device has many merits such as wide operating band range, easy processing and dynamic tunability, which is a guideline for designing the dynamically tunable high performance nano-photonic integrated devices.

Terahertz analysis of a highly sensitive MIM-SRR-TiO2 nanostructure for bio-sensor applications with the FDTD method

This work analyzes the transmission line characteristics of a plasmonic metal-insulator-metal (MIM) waveguide and a square ring resonator (SRR) for dual band applications. The MIM-SRR-based bio-sensor using 3-Aminopropyl triethoxy silane (APTES) and titanium dioxide ( T i O 2 ) as a precursor material are proposed and investigated in this paper. The MIM waveguide characteristics such as propagation length and effective refractive index have been obtained using the finite differential time domain (FDTD) solver of CST studio suite. The MIM-based SRR band-pass filter was designed and analyzed at 1300 nm (230.6 THz) and 1600 nm (187.37 THz) wavelengths. In this work, the bio-compatibility material T i O 2 is used as an agent of bio-molecules detection, and APTES is used as a linker between silicon dioxide and T i O 2 . The proposed MIM-SRR- T i O 2 -based bio-sensor is operated at 1320 nm (227.115 THz) and 1630 nm (183.922 THz). The proposed MIM-SRR-APTES- T i O 2 bio-sensor is operating at dual pass bands of 1415 nm (211.867 THz) and 1762 nm (170.143 THz). The proposed MIM-SRR-APTES- T i O 2 has a Q -factor of 22.353 and 20.655 and a sensitivity of 571.02 and 877.86 nm/RIU at 1415 and 1762 nm, respectively. APTES and T i O 2 are responsible for detecting cervical cancer, which is generally observed in woman. The scalability and multiplexing capability of semiconductor-based technology, silicon photonic ring resonators, are promising analytical tools for detecting various sensing applications. The overall size of the proposed bio-sensor is 1200 n m × 300 n m × 60 n m .

Multiple Fano resonance refractive index sensor based on a plasmonic metal-insulator-metal based Taiji resonator

We propose and numerically analyze a multiple Fano resonance device with high quality ( Q ) factor and high modulation depth based on a metal-insulator-metal (MIM) waveguide laterally coupled Taiji resonator in the near-infrared regime. A silver-air-silver groove (SASG) is embedded in the MIM waveguide. The so-called Taiji resonator is an S-shaped additional crossover branch embedded in a ring cavity to selectively couple counterpropagating cavity modes, therefore increasing the propagating modes relative to the traditional ring cavity. Up to septuple Fano peaks rise from the interference between narrow discrete modes generated from the Taiji resonator and broad continuum modes stemming from the SASG, which can be easily tuned by adjusting the different geometric parameters of the proposed structure. The maximal Q -factor is about 317 with a modulation depth of 24.1 dB in transmission. Thanks to a narrow resonance linewidth with strong subwavelength light confinement of multiple Fano resonances, an optical refractive index sensor with a sensitivity of 2016 nm/RIU and figure of merit of 345 is demonstrated. Furthermore, the sensor can be used for sensing biological parameters such as glucose solution concentration with maximal sensitivity of 0.245 nm · L/g. This simple proposed structure may provide a further step for the development of Fano resonance applications in nanosensing, lasing, and nonlinear optics.

Nanoantennas Inversely Designed to Couple Free Space and a Metal-Insulator-Metal Waveguide.

The metal–insulator–metal (MIM) waveguide, which can directly couple free space photons, acts as an important interface between conventional optics and subwavelength photoelectrons. The reason for the difficulty of this optical coupling is the mismatch between the large wave vector of the MIM plasmon mode and photons. With the increase in the wave vector, there is an increase in the field and Ohmic losses of the metal layer, and the strength of the MIM mode decreases accordingly. To solve those problems, this paper reports on inversely designed nanoantennas that can couple the free space and MIM waveguide and efficiently excite the MIM plasmon modes at multiple wavelengths and under oblique angles. This was achieved by implementing an inverse design procedure using a topology optimization approach. Simulation analysis shows that the coupling efficiency is enhanced 9.47-fold by the nanoantenna at the incident wavelength of 1338 nm. The topology optimization problem of the nanoantennas was analyzed by using a continuous adjoint method. The nanoantennas can be inversely designed with decreased dependence on the wavelength and oblique angle of the incident waves. A nanostructured interface on the subwavelength scale can be configured in order to control the refraction of a photonic wave, where the periodic unit of the interface is composed of two inversely designed nanoantennas that are decoupled and connected by an MIM waveguide.

A MIM Waveguide Structure of a High-Performance Refractive Index and Temperature Sensor Based on Fano Resonance

A plasmonic refractive index nanosensor structure consisting of a metal-insulator-metal (MIM) waveguide with two symmetrical rectangle baffles coupled with a connected-concentric-double rings resonator (CCDRR) is presented. In this study, its transmission characteristics were investigated using the finite element method (FEM). The consequences, studied via simulation, revealed that the transmission spectrum of the system presents a sharp asymmetric Fano profile due to the destructive interference between the wide-band mode of two rectangle baffles on the bus waveguide and the narrow-band mode of the CCDRR. The effects of the geometric parameters of the structure on the transmission characteristics were investigated comprehensively. A sensitivity of 2260 nm/RIU and figure of merit (FOM) of 56.5 were the best levels of performance that the designed structure could achieve. In addition, the system could act as a sensor for use for temperature sensing, with a sensitivity that could reach 1.48 nm/°C. The designed structure advances with technology with new detection positions and has good application prospects in other high-sensitivity nanosensor fields, for example, acting as a biosensor to detect the hemoglobin level in the blood.

A Nanoscale Structure Based on an MIM Waveguide Coupled with a Q Resonator for Monitoring Trace Element Concentration in the Human Body.

In this study, a nano-refractive index sensor is designed that consists of a metal–insulator–metal (MIM) waveguide with a stub-1 and an orthogon ring resonator (ORR) with a stub-2. The finite element method (FEM) was used to analyze the transmission characteristics of the system. We studied the cause and internal mechanism of Fano resonance, and optimized the transmission characteristics by changing various parameters of the structure. In our experimental data, the suitable sensitivity could reach 2260 nm/RIU with a figure of merit of 211.42. Furthermore, we studied the detection of the concentration of trace elements (such as Na+) of the structure in the human body, and its sensitivity reached 0.505 nm/mgdL−1. The structure may have other potential applications in sensors.

Refractive Index Sensing Based on Multiple Fano Resonances in a Split-Ring Cavity-Coupled MIM Waveguide

A metal–insulator–metal (MIM) waveguide consisting of a circular split-ring resonance cavity (CSRRC) and a double symmetric rectangular stub waveguide (DSRSW) is designed, which can excite quadruple Fano resonances. The finite element method (FEM) is used to investigate influences of geometric parameters on the transmission characteristics of the structure. The results show that Fano resonances are excited by the interference between the DSRSW and the CSRRC. Among them, the resonance wavelengths of the Fano resonances are tuned by the narrow-band discrete state excited by the CSRRC, and the resonance line transmittance and profiles are tuned by the wide-band continuous state excited by the DSRSW. The sensitivity (S) can be up to 1328.8 nm/RIU, and the figure of merit (FOM) can be up to 4.80 × 104. Based on these advantages, the structure has potential applications in sensing in the sub-wavelength range.

Detecting the temperature of ethanol based on Fano resonance spectra obtained using a metal-insulator-metal waveguide with SiO2 branches

Based on the transmission characteristics of surface plasmon polaritons (SPPs) in sub-wavelength structures, this paper proposes a metal-insulator-metal (MIM) waveguide structure composed of a main waveguide with glass (SiO2) branches (WWGB) coupled with an elliptical split-ring resonance cavity (ESRRC). WWGB has a broadband continuous transmission spectrum, while ESRRC has a narrow-band discrete transmission spectrum. The coupling and interference between the two can generate excited dual-Fano resonance, with sensitivities and figures of merits (FOM) of 800 nm/RIU, 1150 nm/RIU, and 9.88, 104.55, respectively. After adding SiO2 branches to both sides of the main waveguide, the FOM are enhanced to 28.57 and 127.78, representing increases of 189% and 22.15%, respectively. This structure can be applied as a temperature sensor. After filling the cavity of the to-be-tested material with 75% ethanol, as the temperature increases, the Fano resonance wavelength to drift, therefore, the corresponding temperature can be calculated by the Fano resonance wavelength. Experiments show that the proposed MIM waveguide has a maximum sensitivity of 1406.25 nm/RIU, an FOM of 156.25, and a temperature sensitivity of 0.45 nm/℃. Ultimately, the results demonstrate that incorporating SiO2 branches enhances the sensing characteristics of the MIM waveguide, after adding ethanol, the MIM can be applied to temperature sensors, with a high sensitivity of 1406.25 nm/RIU, thereby providing a new design strategy for producing high-performance waveguides.

High Sensitivity Plasmonic Sensor Based on Metal–Insulator–Metal Waveguide Coupled with a Notched Hexagonal Ring Resonator and a Stub

In this paper, a compact and highly sensitive refractive index plasmonic sensor, with a metal-insulator–metal waveguide coupled with a notched hexagonal ring resonator and the stub is proposed. Structural parameters of the sensor have a key role in the sensor’s sensitivity and transmission spectrum, which are investigated using the finite-difference time-domain method embedded in the commercial simulator R-Soft. The results yield a linear link with the refractive index of the material under testing and its wavelength resonances. Moreover, the maximum linear sensitivity is S = 2547 nm/RIU, its corresponding sensing resolution is 3.92 × 10−6 RIU. Therefore, this sensor can be implemented in high-performance nano-sensors and bio-sensing devices. In this proposed structure, the positions of transmission peaks can be easily manipulated, by adjusting the length and width of the stub.

Numerical Study of Propagation of Nonlinear Coupled Surface and Leaky Electromagnetic Waves in a Circular Cylindrical Metal–Dielectric Waveguide

Numerical Study of Propagation of Nonlinear Coupled Surface and Leaky Electromagnetic Waves in a Circular Cylindrical Metal–Dielectric Waveguide

Compact hybrid plasmonic slot waveguide sensor with a giant enhancement factor for surface-enhanced Raman scattering application.

In this paper, a surface-enhanced Raman scattering (SERS) sensor with a giant field enhancement factor based on the coupling of surface plasmon polaritons (SPPs) is designed and studied theoretically. The proposed sensor adopts a metal-dielectric layered hybrid slot waveguide structure, combining thin metal (gold) layers and silicon nitride strip waveguides. Unlike other similar sensors, the silicon nitride waveguide structure does not serve as an excitation signal channel, conventionally loaded with the guided modes, but as an auxiliary layer, making it easier to concentrate the light field in the slot. Therefore, the sensor has a higher enhancement factor compared to the pure metal or dielectric slot structure. The results exhibit that we can obtain a maximum enhancement factor exceeding 10^6 under the compact configuration of 510 × 300 × 225nm^3 at the wavelength of 785 nm. By analyzing the dependence of the sensor performance on the structural parameters, we show that the structure of such sensor can directly be applied to SERS spectroscopic analysis as well as integrated with micro-and nano-photonic platform to perform on-chip detection system.

Formation Laws of Direction of Fano Line-Shape in a Ring MIM Plasmonic Waveguide Side-Coupled with a Rectangular Resonator and Nano-Sensing Analysis of Multiple Fano Resonances

Plasmonic MIM (metal-insulator-metal) waveguides based on Fano resonance have been widely researched. However, the regulation of the direction of the line shape of Fano resonance is rarely mentioned. In order to study the regulation of the direction of the Fano line-shape, a Fano resonant plasmonic system, which consists of a MIM waveguide coupled with a ring resonator and a rectangle resonator, is proposed and investigated numerically via FEM (finite element method). We find the influencing factors and formation laws of the ‘direction’ of the Fano line-shape, and the optimal condition for the generation of multiple Fano resonances; and the application in refractive index sensing is also well studied. The conclusions can provide a clear theoretical reference for the regulation of the direction of the line shape of Fano resonance and the generation of multi Fano resonances in the designs of plasmonic nanodevices.

Refractive index sensing based on multiple Fano resonances in a plasmonic defective ring-cavity system

A type of coupled plasmonic resonant system using a metal–insulator -metal (MIM) waveguide is proposed and investigated for generating multiple Fano resonances in this paper. The multi-mode interference coupled-mode theory (MICMT) is mainly employed to analyze the system theoretically, and these results agree well with the results numerically obtained with the finite-difference time-domain (FDTD) method. In this proposed system, multiple asymmetrical Fano-resonant peaks are achieved in the spectra by employing two waveguides separated by a metallic baffle and coupled with a defective ring cavity at one side of the MIM waveguide. According to standing wave theory, we can easily manipulate these Fano peaks by adjusting the main parameters of the defective ring cavity, such as the radius and the notch range. As a result, the structures with three and nine ultra-sharp Fano resonances are obtained and discussed, and the sensitivities and figures of merit (FOMs) are also represented. Furthermore, the phase shifts and the group delays that are induced by the mode interactions are investigated. The designed structures show great potential for the high integrated optical circuits of chip-scale optical sensors, slow light devices or splitters.