Metallic Resonators Research Articles

3D Printed Metallic Pillar Nanomechanical Resonators Decorated with TiO2 Nanotubes for Highly Sensitive Environmental Applications

AbstractMicro and nanomechanical devices offer enhanced sensing capabilities for detecting biological and chemical small molecules. However, miniaturization necessitates advanced fabrication processes and complex measurement systems, hindering routine sensor analysis. While alternative methods like 3D printing show promise, challenges such as low device resolution persist due to intrinsic damping of polymer inks. In this study, an array of micrometric pillar resonators is fabricated in Ti6Al4 V alloy using additive manufacturing based on laser powder bed fusion technology. These metallic nanomechanical resonators exhibit a very high quality factor with minimal difference between air and vacuum measurements due to low intrinsic damping. Furthermore, titania nanotubes grown on the pillars via anodic oxidation heighten sensitivity for molecular dye degradation evaluation. Leveraging the weak coupling phenomenon among the pillars in the array, these devices facilitate large‐scale parallelized measurements, here demonstrated with real‐time analysis of dye degradation process. This approach to creating mass sensing devices via metallic additive manufacturing can usher in a new generation of highly performing resonating sensor arrays, offering a cost‐effective and efficient alternative to traditional silicon microfabrication methods.

Analysis of broadband linear polarization-converting meta-materials and their sensing and detection functions

In this paper, we present a broadband perfect-reflective linear polarization-converting metamaterial, which achieves perfect-reflective linear polarization conversion over a broadband frequency range of 28.15 GHz–60.80 GHz, and the narrow-band perfect-polarization-converting peaks appearing at the high frequency of 67.121 GHz can be used for microwave solution concentration detection. The design consists of a surface metal resonator structure, a Roggers 5880 dielectric layer and a copper metal backing. The surface metal resonator is a combination of a circular open ring, a square open ring, and a centrally located cross-metal cross ring nested in a modified, highly anisotropic structure. The perfect polarization transition peak at the high frequency band can be used for the solution detection function, which can detect the concentration of salt solution, glucose solution, and alcohol solution. When the refractive index of the solution sample to be tested changes gradually from 1.0 to 1.4, the polarization conversion peak shows obvious frequency shift, and the peak polarization conversion rate is always kept above 99%. The polarization principle was analyzed using surface electromagnetic field distribution and related theories, and the sample structure was processed and tested. The designed super-surface polarization conversion structure has potential applications in the field of microwave detection and microwave communication.

A comprehensive study of optical resonances in metals, dielectrics, and excitonic materials in double interface structures

From an optical perspective, depending on the relationship between the real (n) and imaginary (k) parts of its refractive index, three broad categories of materials can be distinguished: metals (k ≫ n), dielectrics (n ≫ k), and materials in which n ≈ k (termed here excitonic materials). The modes and optical resonances that appear in a thin film bounded by two dielectrics with similar refractive index, what we call here a double interface structure, have been widely studied in the case of metals, but not for dielectrics, or materials with n ≈ k. In this work, we propose a new approach, based on employing the phase matching condition to correlate the resonances that appear in the wavelength versus incident angle color maps of the reflected power with the modal analysis of the cross section of the structure. This analysis is performed, using an attenuated total reflection (ATR) setup, for thin film materials that belong to each of the mentioned categories: a metal (gold, Au), a dielectric (titanium dioxide, TiO2), and a material with n ≈ k (chromium, Cr). The theoretical analysis is supported with experimental results. It is demonstrated that this method enables to identify any resonance at any wavelength or incident angle, being valid for all three types of materials. Therefore, it is considered the suggested approach will help the research in these materials and in the double interface structure in the optics and photonics field.

REFRACTIVE INDEX BASED DETECTION WITH A HIGH SENSITIVITY BIOSENSOR ENHANCED BY GRAPHENE

Over the past decade, optical sensors have made significant advances. An optical sensor examines the environmental impact through the change of an optical signal and offers advantages such as low cost and label-free detection. In this study, a sensor consisting of a single graphene layer and a slit positioned on the substrate is proposed. The strip gap made to improve the excitation of graphene plasmons allowed to achieve 96.2% high transmission resonance mode. This demonstrates the ability of the sensor surface to detect changing environmental conditions. The results show that the sensitivity of the sensor is 6282 nm/RIU when the sensor surface is exposed to analytes with different refractive indices. The use of a single graphene sheet eliminates the need for a metal resonator and achieves a higher sensitivity compared to some experiments recently published in the literature. Thus, the disadvantage of significant ohmic losses in metal resonators is avoided. Furthermore, a thorough discussion of various factors, including the modification of the strip gap width on the graphene layer and electrical tunability, led to the achievement of optimal sensitivity.

Patterned Laser-Induced graphene enabling a High-Performance gas sensing Split-Ring resonator

The outstanding potential of microwave resonator-based energy-efficient gas sensors has been overshadowed by their underperforming gas sensing capabilities, especially insignificant sensitivity to detect low-concentration volatile organic compounds (VOCs). This unmet challenge persists primarily due to predominant metallic resonator structures, which further limit their wearable applications and pose environmental concerns. This work presents a laser-induced graphene (LIG)-enabled split-ring resonator (SRR) sensor on a flexible polyimide substrate for the rapid and sensitive detection of gaseous VOCs. To implement the sensor, the conductive traces of the SRR were created using a computer-controlled CO2 laser at an optimized power level, thus inducing 48 µm thick, conductive (24 S/cm) graphene layers on a polyimide substrate. The SRR gas sensor, in which LIG with three-dimensional networks of porosity serves as conductive and even gas-sensitive traces, benefits from a significantly enhanced interaction between the SRR’s electromagnetic field and VOC gases. As a proof of concept, a prototype of the LIG-enabled SRR was implemented and mounted on a test fixture. The developed gas sensor operated at a resonant frequency of 1.402 GHz, which exhibited rapid (∼17 s) and noticeable shifts when exposed to 200 ppm of different VOCs (acetone, ethanol, methanol, toluene, and isopropyl alcohol). Additionally, the sensor demonstrated a linear correlation between the resonant frequency and acetone gas concentration (1 ppm-200 ppm), with a sensitivity of 188 KHz/ppm. These electromagnetic and gas sensing results suggest that polyimide-derived LIG traces can replace metal microstrip lines in the SRR structure, opening up possibilities for high-performance microwave resonator gas sensors, even suitable for flexible and wearable applications.

Polarization insensitive and wideband terahertz absorber using high-impedance resistive material of RuO2

This paper introduces a novel, cost-effective solution designed to achieve large absorption bandwidth within the THz spectrum employing a miniaturized, single-layer metamaterial structure. The designed structure features a single circular ring composed of an ohmic resistive sheet with notably higher sheet resistance than traditional metallic resonators. This distinctive design is implemented on a lossy dielectric polyimide substrate with a backing of metallic gold. Our developed absorbing structure demonstrates the capability to achieve a substantial absorption bandwidth ranging from 3.78 to 4.25 THz, maintaining a consistent absorption rate of over 90%. Moreover, we conducted an analysis to assess its absorption performance under various sheet resistance values within the top layer. Additionally, we characterized its angular stability and polarization insensitivity through oblique incident and polarization angle analysis. Finally, an RLC circuital and interference theory approach is adopted to justify its simulated results. The proposed absorber shows potential for a broad spectrum of applications, encompassing communication, imaging, and diverse integrated circuits operating within the THz band.

High brightness terahertz quantum cascade laser with near-diffraction-limited Gaussian beam

High-power terahertz (THz) quantum cascade laser, as an emerging THz solid-state radiation source, is attracting attention for numerous applications including medicine, sensing, and communication. However, due to the sub-wavelength confinement of the waveguide structure, direct beam brightness upscaling with device area remains elusive due to several mode competition and external optical lens is normally used to enhance the THz beam brightness. Here, we propose a metallic THz photonic crystal resonator with a phase-engineered design for single mode surface emission over a broad area. The quantum cascade surface-emitting laser is capable of delivering an output peak power over 185 mW with a narrow beam divergence of 4.4° × 4.4° at 3.88 THz. A high beam brightness of 1.6 × 107 W sr−1m−2 with near-diffraction-limited M2 factors of 1.4 in both vertical and lateral directions is achieved from a large device area of 1.6 × 1.6 mm2 without using any optical lenses. The adjustable phase shift between the lattices enables a stable and high-intensity surface emission over a broad device area, which makes it an ideal light extractor for large-scale THz emitters. Our research paves the way to high brightness solid-state THz lasers and facilitates new applications in standoff THz imaging, detection, and diagnosis.

High sensitivity metasurface sensor for estimating the complex permittivity of ionic liquids

In this paper, a sensor based on a metasurface structure for the complex permittivity of ionic liquids is designed. The structure consists of an F4B substrate with a metal resonance pattern, and a polystyrene foam box is used to contain the ionic liquids to achieve non-contact measurements. The simulation results show that without the presence of the ionic liquids, the sensor can achieve a significant reflection spectrum depth of −37 dB at 11.2 GHz, after adding the ionic liquids, the sensor can generate different responses depending on the types of ionic liquids in the frequency range of 8–9.3 GHz. Furthermore, by employing a characteristic matrix model inversion based on the relationships between the complex permittivity, resonance frequency, and quality factor variations, the standard deviations of the real and imaginary parts of the complex permittivity for seven ionic liquids are estimated to be 0.18 and 0.3, respectively. The estimated results show a good agreement with the standard values obtained from probe measurements. The sensor designed in this paper features a small size, simple structure, and low fabrication cost. With a volume of 0.216 milliliters of the sample, it achieves a high sensitivity of 462.87 MHz/ԑ′and frequency detection resolution of 391.57 MHz/ԑ′. It demonstrates excellent discriminatory detection capability for ionic liquids, providing a new direction for the application research of metasurfaces.

Femtosecond laser direct writing wedge metallic microcavities for terahertz sensing

Metallic resonators allow the manipulation of electromagnetic fields in subwavelength volumes for strong light-matter interaction, underpinning emerging flat-optics devices. Metamaterials designed with optimized effective mode volume (Veff) for stimulating and maximizing the utilization of high-density photons are highly desirable for sensing applications. Here, we demonstrate a bowtie-type terahertz (THz) metallic aperture metamaterial structure with low Veff for enhanced biosensing. The device leverages the inherent characteristic of the femtosecond laser fabrication process enabling wedge cavities down to 3 µm by tailoring the pulse energy, whose sensing performance is superior to trapezoidal ones. Such a substrate-free, microcavity metallic metamaterial sensor with extremely strong interactions achieves an experimentally normalized sensitivity of 0.654 RIU−1, and demonstrates the detection of L-proline down to 0.87 nmol. These findings provide new insights into the design and optimization of efficient THz metamaterials with tightly confined fields for ultrasensitive biomolecular detection.

Compact polarization-insensitive microwave metamaterial absorber with hepta-band characteristics

This paper presents an ultra-thin and compact metamaterial absorber (MMA) capable of achieving near-perfect absorption peaks across the C, X, Ku, and K frequency bands. The MMA structure features a modified metallic plus-shaped resonator surrounded by symmetric L-shaped resonators within a compact size of 13 × 13 mm2. The absorber exhibits seven absorption peaks at different resonant frequencies including 4.23, 6.48, 10.62, 12.92, 14.03, 17.39, and 18.11 GHz. With a thickness of 0.0225λ and a compact size of 0.1833λ at the lowest frequency, the absorber offers remarkable thinness and compactness. Different characteristics of the absorber, such as normalized impedance, surface current distribution, and electric field distribution, are also examined. The polarization-insensitive behavior of the MMA is assessed through absorption and reflection responses under different polarization and incident angles. The Equivalent Circuit Model (ECM) of the metamaterial absorber is also designed to accurately represent the MMA unit cell across all resonant frequencies. Experimental validation of the proposed MMA confirms its performance consistency with simulation results. The proposed MMA design holds potential for applications in defense, detection, and sensing. The sensing ability of the MMA is analyzed using simulations at different refractive index values.

Tunable Fano resonance in a novel compact metal–insulator–metal structure

A novel compact scheme to realize tunable Fano resonance is proposed and investigated theoretically and numerically. The scheme is based on two slot cavities in a metal–insulator–metal structure. The model and formation mechanism of Fano resonance in this structure are studied. A new method based on four-mode temporal coupled-mode theory is used to analyze model of the structure with two slot cavities. Compared with previous studies, this method only considers the interaction between modes within two cavities rather than considering the energy coupling between them. The tunability and slow light phenomenon in the new structure are also studied. It is believed that research in this article can provide a new method to achieve Fano resonance. Furthermore, it is helpful to establish the Fano resonance model and reveal the formation mechanism of Fano resonance.

Selective Photocatalytic CO2 Reduction Through Plasmonic Z‐Scheme Ag‐Bi2O3‐ZnO Heterostructures

AbstractCombination of plasmonic noble metal with semiconductors offers a promising and potential solar energy harvesting and conversion route. Herein, plasmonic Z‐scheme Ag‐Bi2O3‐ZnO (AgBZ) heterostructure is designed and synthesized via solution combustion and photo‐deposition method. Metallic Ag nanoparticles (NPs) were deposited over Bi2O3‐ZnO heterostructure; thus, they exhibit strong visible light absorption owing to surface plasmon resonance (SPR). The photocatalytic efficiency of the synthesized catalysts is investigated by the photocatalytic CO2 reduction to fuels under simulated solar light irradiation. The present system has shown an outstanding photocatalytic CO2 reduction and excellent selectivity of CO. The evolved rate of CO fuel, the amount of CO produced per unit time, over optimized Ag‐Bi2O3‐ZnO (3AgBZ) heterostructures was (96.74 μmol g−1/h) which was approximately 12 times larger than that of Bi2O3‐ZnO (0AgBZ) and 49‐fold than the quantity of evolved CO fuels over pure ZnO photocatalyst. The percentage of evolved CO to CH4 fuels over optimized Ag‐Bi2O3‐ZnO (3AgBZ) heterostructures was about 50 : 1compared to that of 2 : 3 over Bi2O3‐ZnO (0AgBZ). The great improvement and the high selectivity would be ascribed to the synergistic effect of metallic surface plasmon resonance and the Z‐scheme charges transfer system. This work would open a window to construct a single photocatalyst with different charge separation systems and excellent CO2 reduction selectivity. Finally, the mechanism for the enhanced photocatalytic performance by the synergistic effect was described and discussed.

Purity Detection of Some Liquids by Using Reflection Values Based on Metamaterial

The aim of this work is to design and fabricate a type of sensor based on Metamaterials. This structure determines the purity of Methanol and Ethanol mixture in the water by using the Octagonal form of a resonator and sample holder. The proposed structure has been employed in the 8-12 GHZ frequency band. The important thing in the work is the changes of the waveform at the resonance frequency. The output waveform of materials (reflection coefficient S11 or transmission coefficient S12) must be changed in the liner figure by considering the dielectric coefficient. We use copper for the metal layer and resonator and Isola IS680 (3.2DK) (lossy) for substrate layer. We simulate one unit cell of this Meta-material sensor by CST microwave software and then achieve the results and evaluate them. Both the numerical and experimental tests, give the same outcomes and results and they will be in good agreement with each other. The proposed structure can be used in many applications where purity and determining of some materials might be necessary.

Huygens’ metasurface: From anomalous refraction to reflection

Huygens' metasurfaces (HMS) without any reflection or transmission are widely studied due to the high-efficiency manipulation of electromagnetic waves. In this work, three-layered metallic Huygens' metasurfaces are proposed and the geometry-dependences of anomalous refraction and reflection are investigated. The surface impedance and admittance are independently tailored by changing the length of metallic resonators, which is dictated by the introduction of the intermediate layer. Huygens’ metasurfaces can be geometrically engineered to realize anomalous refraction with an efficiency of 62% and anomalous reflection with an efficiency of 60%, respectively. It is anticipated that the proposed metasurface with tunable materials could achieve reconfigurable terahertz devices.

The Strong Coupling Effect between Metallic Split-Ring Resonators and Molecular Vibrations in Polymethyl Methacrylate.

We propose and study a nanoscale strong coupling effect between metamaterials and polymer molecular vibrations using metallic split-ring resonators (SRRs). Specifically, we first provided a numerical investigation of the SRR design, which was followed by an experimental demonstration of strong coupling between mid-infrared magnetic dipole resonance supported by the SRRs fabricated on a calcium fluoride substrate and polymethyl methacrylate molecular vibrations at 1730 cm-1. Characterized by the anti-crossing feature and spectral splitting behaviors in the transmission spectra, these results demonstrate efficient nanoscale manipulation of light-matter interactions between phonon vibrations and metamaterials.

Tuning the plasmonic resonance in TiN refractory metal

Plasmonic coatings can absorb electromagnetic radiation from visible to far-infrared spectrum for the better performance of solar panels and energy saving smart windows. For these applications, it is important for these coatings to be as thin as possible and grown at lower temperatures on arbitrary substrates like glass, silicon, or flexible polymers. Here, we tune and investigate the plasmonic resonance of titanium nitride thin films in lower thicknesses regime varying from ~ 20 to 60 nm. High-quality crystalline thin films of route-mean-square roughness less than ~ 0.5 nm were grown on a glass substrate at temperature of ~ 200 °C with bias voltage of − 60 V using cathodic vacuum arc deposition. A local surface-enhanced-plasmonic-resonance was observed between 400 and 500 nm, which further shows a blueshift in plasmonic frequency in thicker films due to the increase in the carrier mobility. These results were combined with finite-difference-time-domain numerical analysis to understand the role of thicknesses and stoichiometry on the broadening of electromagnetic absorption.

Monolithically integrated wide field-of-view metalens by angular dispersionless metasurface

Meta-lenses, which are made by sub-wavelength metallic or dielectric elements, have attracted widespread attentions due to its unprecedented electromagnetic wave manipulation abilities and novel applications. Dissimilar to previous metalens doublet by all-dielectric metasurfaces in the optical band, an innovative and more robust approach by angular dispersionless meta-atom to directly eliminate aberrations in low spectrum is proposed. As a proof of concept, an elaborately selected singlet metallic resonator for simultaneously high transmission, full phase coverage and the ignored angular dispersion is presented in the microwave. The designed metalens with only one single phase gradient profile can focus wide field-of-view (FOV) incident waves of ± 40° with both a large numerical aperture of 0.56 (NA = 0.56) and high focusing efficiency of 53 % on average at microwave regime. The metalens is also fabricated by a single-step printed circuit board technique and the measured high-performance focusing are also achieved for the entire FOV. The presented on-chip and integrated wide FOV metalens provides a simple yet cost-effective method to develop wide-angle metalenses especially in low frequency band where plasmonic metasurfaces are commonly adopted. Besides, its excellent wide-angle focusing performances can satisfy diverse applications in modern radar engineering such as imaging, sensing, communications and detection, etc.

Multiscale Supercrystal Meta-atoms.

Meta-atoms are the building blocks of metamaterials, which are employed to control both generation and propagation of light as well as provide novel functionalities of localization and directivity of electromagnetic radiation. In many cases, simple dielectric or metallic resonators are employed as meta-atoms to create different types of electromagnetic metamaterials. Here, we fabricate and study supercrystal meta-atoms composed of coupled perovskite quantum dots. We reveal that these multiscale structures exhibit specific emission properties, such as spectrum splitting and polaritonic effects. We believe that such multiscale supercrystal meta-atoms will provide novel functionalities in the design of many novel types of active metamaterials and metasurfaces.

Highly sensitive Borophene-metal-Si based multilayered Terahertz frequency spectrum based refractive index sensor

We presented the numerical investigation of a multilayered borophne-metal-Si-based refractive index sensor for the wide range of the THz frequency. The proposed structure is worked for the frequency range of 1 to 15 THz. The structure is formed to identify reflectance variation, resonating frequency and other physical parameters over the broad frequency spectrum. The overall structure is simulated using FEM (Finite element method) computational techniques with a periodic boundary condition-based two-port model. The resonance effect of the structure is also investigated for the different shapes of the top metal resonator structure, which significantly influences the overall frequency shift. The proposed structure is investigated for the X and Y polarized input incident condition for the entire frequency band where the oblique angle incident stability is observed up to 80°. The proposed structure offers the maximum variation in sensitivity up to 3.5 THz/RIU (∼ 11600 nm/RIU) for X-polarized and 5.5 THz/RIU (∼10600 nm/RIU) for Y-polarized incident wave conditions. We have applied the artificial neural network algorithm (ANN) to predict the overall behaviour of the structure from the data points generated in the simulated results. We used the Relu optimizer to train the model, generating promising results for our collected data. The machine learning model gives RMSE = 0.049422, MAE = 0.018531, MSE = 0.00328 and R2 = 0.93768 for the testing data set. Similarly, the model generated the minimum RMSE values = 0.045955, MAE = 0.017392, MSE = 0.00295, and R2 = 0.97673 for the training data set for 2500 epochs. The proposed results in the manuscript give the future scope to design borophene a wide range of refractive index (RI) sensor designs used in biosensors, gas sensors and other environment sensors where the refractive index range is between 1 and 2.4.

High gain and high-efficiency compact resonator antennas based on spoof surface plasmon polaritons

Abstract We proposed a high gain high-efficiency compact metallic resonator antenna operating at even-mode meander line spoof surface plasmon polaritons (MLSSPPs). The high radiation efficiency is caused by the bulk of fields crowded in lossless air near the antenna rather than in lossy dielectric as in conventional dielectric resonator antennas (DRAs). The proposed antenna also exhibits compact size because of its high effective refractive index. A reliable equivalent circuit model is proposed for the design of the resonator antenna with basic mode of half wavelength resonant mode. As an example, a meander-line SSPP antenna is designed, fabricated and measured. Both the simulated and measured results show the advantages of high efficiency and compact volume. In addition, the antenna achieves higher gain and wider relative bandwidth per wavelength cube volume compared with its counterparts. This method provides a good alternative for designing DRAs.