Metallic Waveguide Structures Research Articles

Nanosensor Based on the Circular Ring with External Rectangular Ring Structure

This paper presents a novel nanoscale refractive index sensor, which is produced by using a metal–insulator–metal (MIM) waveguide structure coupled with the circular ring with an external rectangular ring (CRERR) structure with the Fano resonance phenomenon. In this study, COMSOL software was used to model and simulate the structure, paired with an analysis of the output spectra to detail the effect of constructional factors on the output Fano curve as measured from a finite element method. After a series of studies, it was shown that an external rectangular ring is the linchpin of the unsymmetrical Fano resonance, while the circular ring’s radius strongly influences the transducer’s capability to achieve a maximum for 3180 nm/RIU sensitivity and a FOM of 54.8. The sensor is capable of achieving sensitivities of 0.495 nm/mgdL−1 and 0.6375 nm/mgdL−1 when detecting the concentration of the electrolyte sodium and potassium ions in human blood and is expected to play an important role in human health monitoring.

Tunable uniform field enhancement in a subwavelength air pillar by photonic doping in epsilon-near-zero medium

In this study, a novel electric field compressor is proposed by doping a metal-air-metal pillar in epsilon-near-zero medium within a metallic waveguide, which effectively enhances the background electric fields in a sub-wavelength air pillar with high uniformity. The field enhancement factor can be analytically determined through theoretical derivations from Maxwell’s equations, reaching its maximum value by appropriately selecting the size of the air pillar. Furthermore, the proposed compressor can achieve a tunable electric field enhancement effect within the deep sub-wavelength air pillar by adjusting the height of the air pillar through movement of two metal pillars inserted into the waveguide. Both theoretical analysis and numerical simulations are employed to validate the performance of the electric field compressor, which exhibits a wide range of tunable field enhancement effect (i.e., continuously adjustable enhancement factor between 20 and 800) with uniformity below 1.5 within the deep sub-wavelength air pillar (wherein the air volume is smaller than λ0/10 × λ0/10 × λ0/16). Finally, a practical model is proposed for the realization of the electric field compressor in the microwave range, where specially placed metal pillars and wires are incorporated into a rectangular metallic waveguide structure. The field enhancement and tunable effects of this practical model have been verified through simulations.

The Application of Optical Sensors with Built-in Anchor-like Cavities in the Detection of Hemoglobin Concentration

This paper introduces a refractive index sensor based on Fano resonance, utilizing a metal–insulator–metal (MIM) waveguide structure with an Anchor-like cavity. This study utilizes the finite element method (FEM) for analyzing the propagation characteristics of the structure. The evaluation concentrated on assessing how the refractive index and the structure’s geometric parameters affect its sensing characteristics. The designed structure demonstrates optimum performance, achieving a maximum sensitivity of 2440 nm/RIU and an FOM of 63. Given its high sensitivity, this nanoscale refractive index sensor is ideal for detecting hemoglobin concentrations in blood, and the sensor’s sensitivity is 0.6 nm·g/L, aiding in clinical prevention and treatment.

Refractive index sensor based on a D-shaped resonator and a stub resonator and its slow light characteristics

In this paper, a metal–insulator–metal waveguide structure is proposed based on a D-shaped resonator and a stub resonator, and it can generate multiple Fano resonances. The magnetic field (Z-direction) distribution and transmission spectrum of the structure are analyzed by the finite element method (FEM). The theory results are performed with the multimode interference coupled mode theory (MICMT), and they show high agreement with the simulation outcomes. The calculations also discuss the effect of structural parameters on the transmission spectrum. Moreover, the structure’s sensing characteristics are analyzed in detail, achieving a maximum sensitivity and FOM* of 1300 nm/RIU and 181.44, respectively. In addition, the slow light properties of the structure are also investigated, whose group delay and group index can reach 0.24 ps and 48.93, respectively. The group delay time is dynamically regulated by controlling the number of D-shaped resonators to achieve the effect of a delay line. It can provide some reference value for multifunctional on-chip optical integrated devices based on MIM waveguide structure.

Mid-Infrared Gas Sensing Based on Electromagnetically Induced Transparency in Coupled Plasmonic Resonators.

The existence of surface plasmon polaritons in doped silicon micro-scale structures has opened up new and innovative possibilities for applications, such as sensing, imaging, and photonics. A CMOS-compatible doped Si plasmonic sensor is proposed and investigated. The plasmon resonance can be tuned by controlling the carrier density and dopant concentration. In this paper, we demonstrate that using silicon doped with phosphorus at a concentration of 5 × 1020 cm-3 can induce surface plasmon resonance in the mid-infrared region. Two ring resonators of two different radii based on metal-insulator-metal waveguide structures are studied individually. Then, the two ring resonators are integrated in the same device. When the two ring resonators are coupled and resonate at the same frequency; two distinct resonance spectral lines are generated with striking features that improve its potential use for sensing and modulation applications. The propagating plasmonic mode is studied, including its mode profile and bend loss. We evaluate the effectiveness of a microstructure gas sensor with dimensions of 15 μm × 15 μm by measuring its sensitivity and selectivity towards methane and ethane gases. Small alterations in the surrounding refractive index led to noticeable shifts in the resonance peak. The sensor achieved a sensitivity of 7539.9 nm/RIU at the mid-infrared spectral range around the 7.7 μm wavelength. Furthermore, by combining the resonators, we can achieve a smaller full width at half maximum (FWHM), which will ultimately result in greater sensitivity than using a single-ring resonator or other plasmonic resonator configurations. Once the sensitivity and selectivity of the sensor are measured, the FOM can be calculated by dividing the sensitivity by the selectivity of the sensor, resulting in an FOM of 6732.

Nanometer-Thin Stacks of MXenes (Ti3C2Tx and Ta4C3Tx) for Applications as Nonlinear Photonic Devices

Two-dimensional (2D) carbides of transition metals, so called “MXenes”, maintain excellent optical properties and unique layer stacking form via hydrogen bonds in contrast to other low-dimensional materials. These promising features have attracted increasingly research interest in the field of ultrafast photonics. In this work, titanium carbide (Ti3C2Tx) and tantalum carbide (Ta4C3Tx), as two promising MXene materials for photonic applications, are experimentally synthesized by the ultrasound-assisted liquid phase exfoliation technique. The morphology of prepared few-layer Ti3C2Tx and Ta4C3Tx has been systematically characterized by transmission electron microscopy and X-ray diffraction analysis. Density functional theory is performed to obtain their electronic band structures, demonstrating metallic properties and verifying their optical absorption at 1 μm. Their tempting optical modulation properties for multi-gigahertz pulsed laser emission are demonstrated by using them as saturable absorbers in a waveguide laser cavity. Particularly, high-performance Q-switched mode-locked lasers with/without laser mirrors are realized based on the hybrid waveguide laser configuration, delivering 1 μm laser pulses with durations as short as 30 ps. The results presented in this work show the great potential of metallic MXenes and waveguide structures for applications in functional photonic devices.

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/℃.

Metal–insulator–metal waveguide structure coupled with T-type and ring resonators for independent and tunable multiple Fano resonance and refractive index sensing

This study proposes a metal–insulator–metal (MIM) waveguide structure coupled with symmetric T-type resonators for exciting double Fano resonances. Furthermore, the proposed structure can generate an additional independent Fano resonance mode when one of the T-type resonators is replaced with a ring resonator. We employed the finite difference time domain method to simulate the transmission spectrum of the structure and the electric field distribution at resonant wavelengths. The formation mechanism of the Fano resonance is explained using the standing wave theory and multimode interference coupled mode theory. After studying the sensing characteristics of the designed structure, we achieved a maximum sensitivity of 1012 nm/RIU and a figure of merit of 5.57 × 104, which is higher than that of most of the structures reported recently. The proposed structure has broad application prospects in the field of nano sensing.

Pressure sensor based on multiple Fano resonance in metal–insulator–metal waveguide coupled resonator structure

This paper presents a plasmonic pressure sensor via an optical metal–insulator–metal (MIM) waveguide structure. The proposed pressure sensor is composed of a ring resonator, a stub, and a U-shaped resonator coupled to the bus waveguide. The finite-difference time-domain method (FDTD) is used to study the Fano resonance (FR) characteristics and magnetic field distributions. Results show that the Fano line shapes can be independently tuned by the geometrical parameters. When pressure is exerted on the U-shaped resonator layer, the deformation leads to the shift of the resonant wavelength. The calculated maximum pressure sensitivities are 10.96 and 10 .5 n m / M P a for FR 2, and FR 3, respectively ( M P a = 10 6 P a ). The good linear relationship between the pressure and resonance wavelength indicates that the proposed nano-scale structure can find potential applications in the fields of nano-biosensing and biomedical sensing.

Simulation of a Thermally Efficient Heat-Assisted Magnetic Recording Ridge Waveguide NFT on an AlN Heat Sink

A thermally efficient ridge waveguide near-field transducer (NFT) design on an AlN heat sink is presented. Through simulation, we found our design can achieve thermal efficiency (TE), defined as the ratio of peak temperature rise in the medium to that in the NFT as high as 2.93. The ridge waveguide NFT is placed on a heat sink of high thermal conductivity material AlN, while otherwise surrounded by SiO2 as the top cladding and cavity filling material. The relatively large metallic waveguide structure can dissipate heat effectively even in the presence of finite interface thermal conductance between the NFT and the AlN. Different NFT excitation methods are examined, with an impinging dielectric waveguide mode leading to the highest TE. Some challenges are identified in using excitation by resonant disk structures. We note that surface plasmon is confined inside the NFT cavity, while heat escapes through the outer surface of the NFT, separating the dielectric interfaces responsible for optical propagation and heat transfer. This allows for further reduction in thermal interface resistance by adding an interface adhesion layer like Cr at the NFT/AlN interface without sacrificing optical properties.

Design and characteristic analysis of an all-optical AND, XOR, and XNOR Y-shaped MIM waveguide for high-speed information processing.

All-optical logic gates are exceptionally suited for Boolean ultrahigh-speed operation and logical computing. This study presents a plasmonic model that uses a Y-shaped metal-insulator-metal waveguide structure that realizes the ultrafast all-optical AND, XOR, and XNOR gate operation that is developed at a footprint of 6.6µm×3.4µm with a wavelength of 1.55 µm. This construction relies on the notion of linear interference. The insertion loss and extinction ratio of the model are observed as 1.49 dB and 21.49 dB for AND, 1.03 dB and 18.97 dB for XOR, and 2.06 dB, and 10.92 dB for XNOR, respectively. The transmission efficiency, response time, and speed of the structure also are calculated and are used to improve the performance of any complex circuit in the future. The theoretical analysis of the proposed structure is carried out using the finite-difference time-domain method.

Analytical Study of Dirac Type Dispersion in Simple Periodic Waveguide Structures for Leaky-Wave Applications

In this work, we present a study on the existence of Dirac type dispersion in the simplest of periodic metallic waveguide structures. It is shown that periodic repetitions of two dissimilar waveguides (WGs) can be properly designed to lead to a Dirac type dispersion. A simple theory using circuit modeling is presented to find the condition for Dirac point operation. In addition, mode-matching followed by full-wave simulations validate that the band structure matches that of the theory and shows that a Dirac dispersion can be realized. A Dirac Leaky-Wave Antenna (DLWA) is then implemented using this simple arrangement in substrate-integrated-waveguide (SIW) technology. This DLWA has the closed broadside stopband feature, leading to continuous frequency beam scanning through broadside and a simple feeding network. Phase and leakage constants are adjusted to yield a directive fan-beam featuring continuous scanning in a wide range of angles. The presented DLWA has a wide impedance bandwidth with high efficiency and operates with peak gains of about 12.5 dBi without any significant gain variation throughout the frequency range from 13.5 GHz to 16.5 GHz. Measured results show excellent agreement with the simulated results, validating the proposed concepts.

Ultra-broadband long-wave infrared metasurface absorber based on Peano fractal curve

Fractals have emerged as a promising platform for realizing broadband absorption metasurfaces. In this paper, we propose an ultra-broadband absorber with high absorption operational in the long-wave infrared regime by means of a second-order Peano fractal curve (PFC) metasurface with a four-layer insulator–metal–insulator–metal (Ge/Cr/Ge/Cr) waveguide structure. The unique space-filling shape of the PFC enables the generation of multiple gap plasmon resonance modes, which expands the bandwidth of the absorber. In addition, the hybridization of these modes with cavity mode resonances and localized surface plasmon resonances enhances the average absorption of the device in the spectrum of operation, with an average absorption of 94.3% obtained for TM-polarized light with wavelengths between 12.3 µm and 32 µm in our simulations. A second-order quasi-PFC is designed for polarization-insensitive broadband absorption. Our findings establish general and systematic strategies for guiding the design of metamaterial absorbers with excellent broadband absorption.

Research on Fano Resonance Sensing Characteristics Based on Racetrack Resonant Cavity.

To reduce the loss of the metal–insulator–metal waveguide structure in the near-infrared region, a plasmonic nanosensor structure based on a racetrack resonant cavity is proposed herein. Through finite element simulation, the transmission spectra of the sensor under different size parameters were analyzed, and its influence on the sensing characteristics of the system was examined. The analysis results show that the structure can excite the double Fano resonance, which has a distinctive dependence on the size parameters of the sensor. The position and line shape of the resonance peak can be adjusted by changing the key parameters. In addition, the sensor has a higher sensitivity, which can reach 1503.7 nm/RIU when being used in refractive index sensing; the figure of merit is 26.8, and it can reach 0.75 nm/°C when it is used in temperature sensing. This structure can be used in optical integrated circuits, especially high-sensitivity nanosensors.

On-Chip Generation of Ultrafast Current Pulses by Nanolayered Spintronic Terahertz Emitters

We generate ultrafast current pulses by optical means with the aid of the inverse spin Hall effect. In ferromagnet/normal metal bilayer structures, intense spin current pulses are generated by optical pumping with intense femtosecond laser pulses. Subsequently, the spin current pulses are converted into a charge current pulse via the inverse spin Hall effect and launched into waveguide structures. The electrical signals are detected using a sampling oscilloscope. We propose to use these nanolayered emitters to act as on-chip THz sources and robust ultrafast photodetectors, which can be combined with metallic waveguide structures. In doing so, the on-chip control of THz fields becomes possible on the nanometer scale.

Independently tunable triple Fano resonances based on MIM waveguide structure with a semi-ring cavity and its sensing characteristics

In this paper, a metal-insulator-metal (MIM) waveguide structure consisting of a side-coupled rectangular cavity (SCRC), a rightward opening semi-ring cavity (ROSRC), and a bus waveguide is reported. The finite element method is used to analyze the transmission characteristics and magnetic-field distributions of the structure in detail. The structure can support triple Fano resonances, and the Fano resonances can be tuned independently by altering the geometric parameters of the structure. Moreover, the structure can be applied in refractive index sensing and biosensing. The maximum sensitivity of refractive index sensing is up to 1550.38 nm/RIU, and there is a good linear relationship between resonance wavelength and refractive index. The MIM waveguide structure has potential applications in optical on-chip nano-sensing.

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.

Optical properties of MIM plasmonic waveguide with an elliptical cavity resonator

Metal–insulator–metal waveguide structure, which has a fascinating feature to confine the signal far beyond the diffraction light is numerically investigated by the finite difference time domain and the finite element methods. In this study, the MIM waveguide is both coupled with a half-elliptical groove (HEG) and an elliptical cavity resonator (ECR), and it can support the propagation of light in the nanoscale regime at the visible and near-infrared ranges. The interaction between these last elements gives rise to Fano resonance modes. Thanks to its interesting characteristics, a high sensitivity value, a factor of merit and interesting value of the group index are obtained for the proposed structure. We show that the transmission of the Fano system and the group index can reach 90% and a value of 63, respectively. We also report an investigation of the influence of the both geometrical HEG and ECR’s parameters on optical properties. Hence, the proposed structure could find a potential for applications in the integrated optical circuits such as optical storage, ultrafast plasmonic switchers, high performance filters and slow light devices.

Reconfigurable Millimeter-Wave Components Based on Liquid Crystal Technology for Smart Applications

This paper presents recent development of tunable microwave liquid crystal (LC) components in the lower millimeter wave (mmW) regime up to the W-band. With the utilization of increasing frequency, conventional metallic waveguide structures prove to be impractical for LC-based components. In particular, the integration of the electric bias network is extremely challenging. Therefore, dielectric waveguides are a promising alternative to conventional waveguides, since electrodes can be easily integrated in the open structure of dielectric waveguides. The numerous subcategories of dielectric waveguides offer a high degree of freedom in designing smart millimeter wave components such as tunable phase shifters, filters and steerable antennas. Recent research resulted in many different realizations, which are analyzed in this paper. The first demonstrators of phased array antennas with integrated LC-based phase shifters are reviewed and compared. In addition, beam steering with a single antenna type is shown. Furthermore, the possibility to realize tunable filters using LC-filled dielectric waveguides is demonstrated.

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.