Metallic Split-ring Resonators Research Articles

Detection of food additives based on an integrated self-injected metasurface microfluidic sensor

Advanced sensing equipment exhibits high sensitivity and reliability in detecting food additives, enabling the practical assessment of the safety of processed foods. Currently, chemical detection methods are commonly utilized for identifying food additives. However, these approaches tend to be intricate and time-consuming. In this study, we designed and fabricated an integrated terahertz microfluidic sensor, which achieves high sensitivity by incorporating a metasurface within the microfluidic chip. The metasurface comprises metal wires and split-ring resonators, with three optional sensing sites within the frequency domain of 0.1–1.2 THz, thereby enhancing the reliability of the sensor. Additionally, the use of a self-injection micropump improves the stability of the liquid flow rate, preventing experimental errors caused by manual injection. Utilizing this sensor, we conducted concentration sensing experiments on potassium sorbate and sodium benzoate solutions, successfully identifying sugar-containing and sugar-substituted beverages with high sensitivity and rapid sensing speed. The average sensitivity of the sensor is 152.8 GHz·RIU−1. The results of this study provide a feasible method for the development of microfluidic metasurface sensors.

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.

Toroidal metamaterial-assisted terahertz biosensor with high sensitivity

In this paper, a toroidal metamaterial for achieving highly sensitive sensors in the terahertz (THz) frequency range is proposed. Such a subwavelength structure consists of a pair of mirror-symmetric metal split-ring resonators and a single metal wire. By exciting the structure with vertically incident electromagnetic waves, high-Q transmission peaks are generated at 2.40 and 2.44[Formula: see text]THz. Near-field analysis reveals that the reverse-flowing currents in the metal split-ring resonators and the metal wire create a pair of magnetic dipoles connected end to end, resulting in a vortex-like magnetic field known as a toroidal dipole. Calculations of multipole scattering energy further confirm that the rapidly enhanced toroidal dipole is the primary factor responsible for the high-Q transmission peaks. This toroidal metamaterial can be used to design refractive index sensors with a sensitivity of up to 20[Formula: see text]GHz/RIU, enabling the detection of subtle changes in the surrounding environment’s parameters. Our strategy paves an alternative route to the development of highly sensitive sensors for biological and chemical molecules in the THz frequency range.

Anapole state revealed by cloaking metallic cylinders with split ring resonators

This paper proposes the experimental demonstration of an anapole-based cylindrical electromagnetic cloaking scheme. An anapole state is excited by arranging around a cylindrical metallic target vertical split-ring resonators, forming an equivalent surface admittance boundary condition able to suppress the scattering. Using Mie formalism and multipole scattering theory, we identify the actual reason behind the cloaking operation, characterizing the anapole condition by the scattering contributions from toroidal and electric dipole moments. Numerical results are verified using full-wave simulation softwares and subsequently validated with back-scattering measurements inside an anechoic chamber.

Origin of plasmonic Fano resonance in metal-hole/split-ring-resonator metamaterials disclosed by temporal coupled-mode theory.

In plasmonic Fano resonance, the interaction between a discrete plasmonic mode and a continuum of plasmonic mode gives rise to an asymmetric line shape in the scattering or absorption spectrum, enabling a wide range of applications such as sensing, switching, and slow light devices. Here, we establish a theoretical solution in the framework of temporal coupled-mode theory (TCMT) to study the three-dimensional (3D) and two-dimensional (2D) Fano resonances induced by strong coupling between metal hole (MH) and split ring resonator (SRR) array. We first separately analyze the transmission spectra of the MH array and SRR array under different polarized light excitation. We further investigate the electromagnetic field and charge density distribution corresponding to the resonant modes at the peak or valley wavelength of the transmission spectrum and figure out the electric/magnetic dipole feature of these resonance modes. We then establish a theoretical solution by TCMT for Fano resonances arising from the coupling of these modes. The calculated transmission spectrum is closely matching with the numerically simulated transmission spectrum for these Fano resonances in the MH-SRR array, which effectively elucidates that the asymmetry of the Fano resonances is caused by the coupling between bright and dark plasmonic modes involved in the two structures. Our results can help to understand the profound physics in such coupled plasmonic systems.

The Simulation of the Terahertz Modulator by CMOS Process

The paper introduced the simulation of the terahertz modulator in complementary metal-oxide-semiconductor (CMOS) process. The modulator is composed of a metal split-ring resonator (SRR), CMOS, semiconductor dielectric layer and silicon substrate. The modulator can make different electromagnetic response to the transmitted terahertz wave between the connection state and the disconnection state of the gap in the SRR, which could be achieved by connecting CMOS in the gap. At 0.31THz, the simulation results show that the amplitude modulation depth of the modulator reached 28.8%. When the simulation keeped the modulator system in resonating state, the transmission coefficient was about 0.0018, while the conductive had reached 0.2895. If the design can pass the experimental verification in the future, it can make some references for further exploration of the high-speed and high modulation depth of the terahertz amplitude modulator.

Thermally controllable electromagnetically induced transparency in a coupled terahertz metamaterial

We investigate the thermally controllable electromagnetically induced transparency (EIT) effect in a planar terahertz (THz) metamaterial configuration comprising metallic semi-circular split ring resonators (MSCSRR) integrated with indium antimonide (InSb). In transmission configuration, a transparency window is observed by placing a horizontal cut wire (HCW) in close proximity to the MSCSRR. It develops by frequency detuning and strong near-field coupling between both the bright resonators. The transparency window is found to be controllable with temperature along with the structural parameters of the unit cell. The blue shift of 0.028 THz is observed when the temperature increases from 260 K to 320 K. Moreover, the group delay of the designed EIT structure can also dynamically tune by varying temperature of InSb. The shift from 3.19 ps to 1.98 ps is observed when the temperature increases from 260 K to 320 K. We used a coupled harmonic oscillator model to thoroughly understand the coupling between resonators and interpret the transparency effect. Our findings can be closely predicted from the model. The present study can be significant for the development of active and controllable terahertz photonic components such as slow light systems, buffers, etc.

Experiment on terahertz metasurfaces of metal split ring resonators with bound state in the continuum

A bound state in the continuum (BIC) is a wave that coexists with a continuous wave but remains localized. In the terahertz band, BIC can design devices with an ultra-high mass factor (Q factor), which is of great value for terahertz science and technology, so we designed a terahertz BIC metasurface structure composed of metal split ring resonators (SRRs). According to the symmetric protection principle of superlattice mode, the leakage process of BIC states to the far field is studied by changing the gap width of SRR. By introducing multiple SRRs and changing their arrangement, we obtain three superlattice modes and BIC states. The leakage of BIC states into the far field is observed experimentally, which means that observable quasi-BIC patterns are formed. We verify a feasible method that allows for flexible design and implementation of BIC.

Terahertz Metasurface with Bound States in Continuum Based on Metal Split Ring Resonator

Terahertz Metasurface with Bound States in Continuum Based on Metal Split Ring Resonator

Plasmon resonance in split-ring resonator based heterostructures

We herein propose a plasmonic nanostructure that is composed of heterogeneous metallic split-ring resonator (SRR) trimers with built-in crosses. We studied its optical transmission properties by using finite difference time domain (FDTD) method. Results showed that, the resonant modes can be changed when the nanostructures are composed of different types of metals. For the heterogeneous metallic SRR trimers, the resonant modes were in the near-infrared region blue shift, but the modes remained the same when the air slit width was increased. All the dips red-shifted as dielectric constant of the SRRs medium was increased. The resonant modes in the visible region red shifted when the arms of metallic cross length were increased, but locations of resonance dips in the near-infrared region remained unchanged. Magnetic and electric field distributions for certain wavelengths were also investigated.

Simple terahertz metasurface with broadband and efficient functionality.

Pancharatnam-Berry (PB) metasurfaces have demonstrated mighty capability to manipulate electromagnetic (EM) waves, and exhibited potential applications for devices with broadband and efficient functionality. However, it remains a challenge to simultaneously achieve broadband and efficient wavefront manipulation for terahertz (THz) components with simple profiles. Herein, we introduce a simple ultra-thin PB metasurface with superior properties in the THz region. The structure is composed of a simple metallic C-Shaped Split Ring Resonator (CSRR) patterned on a flexible polyimide support layer. It is verified that the circular transmission efficiency is close to the theoretical limit of the single-layer metasurface in the range of 0.6 - 1.2 THz. Furthermore, we design metasurfaces based on the PB meta-atoms with spatially rotated orientation to achieve beam steering and superposition of vortex waves. The results are basically in line with expectations, validating the good performances of our proposal. This simple and easily deployable metasurface will give rise to more possibilities for the design of THz functional devices.

Surface plasmon polaritons enhanced magnetic plasmon resonance for high-quality sensing

We investigate the metamaterials consisting of top layer U-shaped metallic split-ring resonators, the middle thin dielectric layer, and the bottom thick metal film, which can realize high-quality refractive index sensing. When propagating surface plasmon polaritons are introduced, two ultra-narrow-band hybridized modes will appear, whose full width at half maximum can reach up to 2.5 nm. Because of the ultra-narrow linewidth and huge magnetic field enhancement at hybridized modes, the metamaterials display very high sensitivity (S = 912 nm RIU−1, S* = 95.64 RIU−1) and large figure of merit (FOM = 364.8, FOM*= 160 740.34), which has great potential in label-free biomedical sensing.

Dynamic Tunable Meta-Lens Based on a Single-Layer Metal Microstructure

Ultra-thin focusing meta-lenses based on the metasurface structure with adjustable focal length show important applicant value in compact systems, especially in on-chip terahertz spectroscopy, imaging systems, and communication systems. A stretchable substrate, dynamic focusing meta-lens based on the cross-polarized metal C-shaped split ring resonators (SRRs) is designed and investigated. At the operation frequency of 0.1 THz, the operation characteristics of the unit cell structure and the formed meta-lens are investigated. The phase of the unit cell structures can be modulated by changing the rotation angle, width, and symmetry axis of the C-shaped metal SRRs. When the terahertz wave is incident vertically, the focusing performance can be achieved based on the specific arrangement of the metasurface unit cells. By stretching the flexible substrate of the meta-lens, the dynamic focusing effect can be realized. When the substrate stretches from 100% to 120%, the focal length changes from 59.8 mm to 125.2 mm, the dynamic focusing range is 109.4% of the minimum focal length, and the focusing efficiency changes between 5.5% and 10.5%.

Terahertz high-Q magnetic dipole resonance induced by coherent Fano interactions

High Q-factor resonance holds great promise for bio-chemical sensing and enhanced light–matter interaction. However, terahertz (THz) magnetic resonances usually demonstrate low Q-factors, resulting in huge energy radiation loss particularly in high frequency bands. Here, we show that high Q-factor magnetic dipole resonance at THz frequencies can be achieved by exploiting the coherent Fano interactions with strong field enhancements in an array composed of single metallic split-ring resonators, working at Wood–Rayleigh anomalies. It can give rise to ultrahigh Q-factor beyond 104 in the THz regime. Experimentally, the measured Q-factor of dominant magnetic dipole resonance can achieve no less than a level of ∼261 by Lorentzian fitting to the experimental data. In addition, a high Q-factor of the fundamental-order magnetic dipole resonance is demonstrated beyond 30. High-Q magnetic dipole resonance is closely associated with ultralow-damping and negative permeability in the THz band. The measurements of magnetic dipole resonances are in good agreement with the theoretical analyses. Our scheme suggests a feasible route to suppress radiative loss for enhanced THz field-matter interaction.

Programmable manipulation of terahertz beams by hybrid graphene-metal coding metasurfaces

Recently, traditional tunable coding metasurfaces integrated with active components or materials have exhibited not only many functionalities, but also reconfigurable responses, which can execute dynamic beam control. However, these metasurfaces are difficult to realize effective and active terahertz wavefront manipulation due to the inherent defects of the active components or materials, such as high loss, polarization dependence, slow response, and so on, dramatically hindering the potential practical applications of terahertz technologies and devices. To implement programmable manipulation of terahertz beams, here, we propose the reflective-type hybrid graphene-metal coding metasurfaces whose meta-atoms are composed of the different C-shaped metallic split-ring resonators integrated with the graphene, thus, obtaining different coding states by rotating structure and doping graphene. Based on these merits, a couple of coding metasurfaces can effectively achieve various functionalities, including single- and multi-beam deflections, diffusive scattering, and vortex beam, via changed coding patterns. Moreover, these functionalities can further realize dynamic reconfiguration by tuning Fermi energy of graphene. Therefore, the proposed coding metasurfaces would show great potential application prospects in terahertz sensing, display, telecommunication, and imaging.

WiFi Energy-Harvesting Antenna Inspired by the Resonant Magnetic Dipole Metamaterial.

WiFi energy harvesting is a promising solution for powering microsensors and microsystems through collecting electromagnetic (EM) energies that exist everywhere in modern daily lives. In order to harvest EM energy, we proposed a metamaterial-inspired antenna (MIA) based on the resonant magnetic dipole operating in the WiFi bands. The MIA consists of two metallic split-ring resonators (SRRs), separated by an FR4 dielectric layer, in the broadside coupled configuration. The incident EM waves excite surface currents in the coupled SRRs, and the energy is oscillating between them due to near-field coupling. By varying the vertical distance of the two SRRs, we may achieve impedance matching without complicated matching networks. Collected EM energy can be converted to DC voltages via a rectifier circuit at the output of the coupling coil. Measured results demonstrate that the designed MIA may resonate at 2.4 GHz with a deep-subwavelength form factor (14 mm×14 mm×1.6 mm). The WiFi energy-harvesting capability of the proposed MIA with an embedded one-stage Dickson voltage multiplier has also been evaluated. A rectified DC voltage is approximately 500 mV when the MIA is placed at a distance of 2 cm from the WiFi transmit antenna with a 9 dBm transmitting power. The proposed compact MIA in this paper is of great importance for powering future distributed microsystems.

Tunable Compact Metamaterial-Based Double-Negative/Near-Zero Index Resonator for 6G Terahertz Wireless Applications.

This paper introduces the tunability performance, concept, and analysis of a unique and miniaturized metamaterial (MTM) unit cell covering the upcoming 6G applications. The proposed metamaterial consists of two metallic star-shaped split-ring resonators (SRR). It has a line segment placed in the middle of the structure, which can feature tunable characteristics. The proposed design provides dual resonances of transmission coefficient S21 at 0.248 and 0.383 THz with a significant operating frequency span of 0.207–0.277 and 0.382–0.390 THz, respectively. Moreover, wide-range achievement, negative permittivity, double-negative (DNG) refractive index, and near-zero permeability characteristics have been exhibited in two (z and y) principal wave propagation axes. The resonance frequencies are selective and modified by adjusting the central slotted-strip line length. Furthermore, the metamaterial is constituted on a polyimide substrate while the overall dimensions are 160 × 160 μm2. A numerical simulation of the proposed design is executed in CST microwave studio and has been compared with advanced design software (ADS) to generate the proposed MTM’s equivalent circuit, which exhibits a similar transmission coefficient (S21).

Design of triple-/six-spectral perfect absorber using multiple resonance responses of metallic split-ring resonator

Multiple-spectral absorber is a kind of functional device with important applications in optoelectronic technology areas. However, the implementation of multiple-spectral absorber usually requires many metallic elements relying on design methods of stacking or coplanar, leading to complex structure design, time-consuming and laborious fabrication procedures. Simplified design of multiple-spectral absorber is given and suggested in this paper. It is revealed that the triple-spectral absorption is realized by merely using single metallic split-ring resonator (SRR). Combined effect of fundamental mode LC resonance, quadrupole resonance and high-order response of SRR gives rise to the triple-spectral absorption. What is more important is that six-spectral absorption can be achieved via embedding one more SRR on the basis of original SRR. Under the aid of the near-field maps, the formation mechanism of six-spectral absorption is discussed. Compared with previous metamaterial absorbers, the suggested devices have the obvious advantages of simple structure design, low construction cost, and thus can have large potential for applications.

Terahertz metasurface with multiple BICs/QBICs based on a split ring resonator.

Bound state in the continuum (BIC) refers to the trapped state in the radiation continuum of a system. In the terahertz band, BIC provides a unique and feasible method to design devices with ultra-high quality factor (Q factor) and to achieve intense terahertz-matter interaction, which is of great value to terahertz science and technology. Here, multiple BICs protected by the resonance symmetry in the terahertz metasurface consisting of metallic split ring resonators (SRR) is demonstrated. The evolution from the BIC to the quasi-BIC (QBIC) is induced by changing the gap width of the SRRs. The proposed BICs are experimentally demonstrated and analyzed by the coupled mode theory along with the numerical simulation. It is found that the leakage behavior of these QBICs is strongly affected by the intrinsic Ohmic loss in the SRRs while it is quite robust to the tilted incidence.

Low-Loss Dual-Band Transparency Metamaterial with Toroidal Dipole.

In this paper, a low-loss toroidal dipole metamaterial composed of four metal split ring resonators is proposed and verified at microwave range. Dual-band Fano resonances could be excited by normal incident electromagnetic waves at 6 GHz and 7.23 GHz. Analysis of the current distribution at the resonance frequency and the scattered power of multipoles shows that both Fano resonances derive from the predominant novel toroidal dipole. The simulation results exhibit that the sensitivity to refractive index of the analyte is 1.56 GHz/RIU and 1.8 GHz/RIU. Meanwhile, the group delay at two Fano peaks can reach to 11.38 ns and 12.85 ns, which means the presented toroidal metamaterial has significant slow light effects. The proposed dual-band toroidal dipole metamaterial may offer a new path for designing ultra-sensitive sensors, filters, modulators, slow light devices, and so on.