Microwave Split-ring Resonator Research Articles

Wireless neuromodulation at submillimeter precision via a microwave split-ring resonator.

A broad spectrum of electromagnetic waves has been explored for wireless neuromodulation. Transcranial magnetic stimulation, with long wavelengths, cannot provide submillimeter spatial resolution. Visible light, with its short wavelengths, suffers from strong scattering in the deep tissue. Microwaves have centimeter-scale penetration depth and have been shown to reversibly inhibit neuronal activity. Yet, microwaves alone do not provide sufficient spatial precision to modulate target neurons without affecting surrounding tissues. Here, we report a split-ring resonator (SRR) that generates an enhanced microwave field at its gap with submillimeter spatial precision. With the SRR, microwaves at dosages below the safe exposure limit are shown to inhibit the firing of neurons within 1 mm of the SRR gap site. The microwave SRR reduced seizure activity at a low dose in both in vitro and in vivo models of epilepsy. This microwave dosage is confirmed to be biosafe via histological and biochemical assessment of brain tissue.

Two-Layer Inkjet-Printed Microwave Split-Ring Resonators for Detecting Analyte Binding to the Gold Surface.

This work focuses on demonstrating the working principle of inkjet-printed Au nanoparticle (NP) two-layer Gigahertz (2.6 GHz) microwave split-ring resonators (SRRs) as a novel platform for the detection of analytes on flexible substrates. In contrast to the standard fabrication of split-ring resonator biosensors using printed circuit board technology, which results in a seven-layer system, the resonators in this work were fabricated using a two-layer system. A ground plane is embedded in the SRR measurement setup. In this method, a microwave electromagnetic wave is coupled into the Au SRR via an inkjet-printed Cu-NP stripline that is photonically sintered. This coupling mechanism facilitates the detection of analytes by inducing resonance shifts in the SRR. In this study, the functionality of the printed sensors was demonstrated using two different Au functionalization processes, firstly, with HS-PEG7500-COOH, and, secondly, with protein G with an N-terminal cysteine residue. The sensing capabilities of the printed structures are shown by the attachment of biomolecules to the SRR and the measurement of the resulting resonance shift. The experiments show a clear shift of the resonance frequency in the range of 20-30 MHz for both approaches. These results demonstrate the functionality of the simplified printed two-layer microwave split-ring resonator for use as a biosensor.

Design of infrared optical absorber using silver nanorings array made by a top-down process

This paper presents the numerical simulation and fabrication of a metasurface composed of silver nanorings with a split-ring gap. These nanostructures can exhibit optically-induced magnetic responses with unique possibilities to control absorption at optical frequencies. The absorption coefficient of the silver nanoring was optimized by performing a parametric study with Finite Difference Time Domain (FDTD) simulations. The absorption and scattering cross sections of the nanostructures are numerically calculated to assess the impact of the inner and outer radii, the thickness and the split-ring gap of one nanoring, as well as the periodicity factor for a group of four nanorings. This showed full control on resonance peaks and absorption enhancement in the near infrared spectral range. The experimental fabrication of this metasurface made of an array of silver nanorings is achieved by e-beam lithography and metallization. Optical characterizations are then carried out and compared to the numerical simulations. In contrast to usual microwave split-ring resonator metasurfaces reported in literature, the present study shows both the realization by a top-down process and modelling performed in the infrared frequency range.

Investigating the thickness-effect of free-standing high aspect-ratio TiO2 nanotube layers on microwave-photoresponse using planar microwave resonators

One-dimensional TiO2 nanotube (TNT) layers are a promising candidate for UV detection due to their distinctive anisotropic geometry which is effective for light harvesting and rapid carrier transport. Here, the photosensitivity efficiency of TNT layers with various thicknesses of 15, 50, 80, and 110 µm was utilized at a microwave frequency regime by modeling and experimentally. A planar microwave split ring resonator (PMSRR) was designed and fabricated to operate at ∼8 GHz to study TNT layers by monitoring the scattering parameter (S21) of the PMSRR under a constant UV irradiation power of ∼96.4 µW/cm2. According to the results, the 80 µm thick TNT layers demonstrated the highest resonant amplitude variation for the customized PMSRR. The change of the resonant amplitude was mainly attributed to the conductivity variation contributed by perturbation of trapped electron concentration, as the dominant factor under UV illumination, and their electromagnetic wave interaction. The main advantage of the proposed method of PMSRR for microwave photosensitivity monitoring over the conventional direct current (DC) conductivity measurements is to eliminate the effect of contact resistance between the TNT layers and metal electrodes utilizing the contactless aspect of wave interactions with the TNT layers at microwave regime to perform electrode-less measurements.

Microwave Split Ring Resonator Sensor for Determination of the Fluids Permittivity With Measurement of Human Milk Samples

AbstractIn this paper, the human milk dielectric constant by a microwave sensor has been evaluated. The proposed sensor is result of coupling between a simple microstrip line and a split ring resonator and its resonance frequency is 6 GHz. When up to 30 μl of milk are placed on the sensing area on the sensor, it causes the resonance frequency to shift. Six samples of human milk were measured by a network analyzer, then the dielectric constant of the samples were determined, and three similar samples were measured by the built‐in sensor. The proposed microwave sensor has Q‐factor of 38 and its sensitivity is 0.17%. The implemented sensor has acceptable performance compared to previous sensors and it can be used to determine the normal behavior of human milk with a simple method and for this work, it requires to small value of sample for measurement.

A Nonintrusive Flow Rate Sensor Based on Microwave Split-Ring Resonators and Thermal Modulation

Microwave sensors offer noncontact solutions for flow rate measurements, central to various applications in pharmaceutics, chemical analysis, and biomedicine. The challenge, however, remains for microwave flow rate sensing of uniform liquids, where different flow rates do not lead to any permittivity or conductivity variations. In this work, we propose a noncontact, nonintrusive microwave sensor, capable of measuring the flow rate of uniform liquids. This sensor comprises a microwave split-ring resonator (SRR) integrated with a temperature modulation technique. The sensor operates based on the change in the dielectric properties of the liquid inside the channel, induced by local delivery of thermal energy. Different flow rates affect the exposure time of the liquid to thermal energy, which alters its permittivity and loss tangent, consequently changing the response of the SRR. The SRR with the integrated fluidic channel is designed to operate at 4.6 GHz with a resonant amplitude of &#x2212;6.8 dB and a <inline-formula> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula>-factor (&#x2212;3 dB) of 98. By monitoring the <inline-formula> <tex-math notation="LaTeX">$S_{21}$ </tex-math></inline-formula> (dB) response, flow rates ranging from 1 to 40 mL/h are differentiated. The <inline-formula> <tex-math notation="LaTeX">$S_{21}$ </tex-math></inline-formula> (dB) response&#x2019;s achieved sensitivity to different flow rates is <inline-formula> <tex-math notation="LaTeX">$\vert 0.02\vert $ </tex-math></inline-formula> dB/mL/h. The proof of concept is successfully evaluated using water and isopropanol as two standard liquids. The results indicate the potential of the proposed nonintrusive flow rate sensing method in various applications, such as drug delivery and chemical sample preparation.

High-Resolution, Sensitivity-Enhanced Active Resonator Sensor Using Substrate-Embedded Channel for Characterizing Low-Concentration Liquid Mixtures

Sensitivity and resolution are important parameters that pose a significant challenge in using microwave microfluidic sensors to monitor low-concentration binary liquid mixtures. This work presents a microwave split-ring resonator (SRR) sensor equipped with a substrate-embedded fluidic channel and an active feedback loop to enhance sensitivity and resolution, respectively. The substrate-embedded channel eliminated the impact of the liquid-carrying tubes’ wall and maintained a significant interaction between the liquid in the channel and the electromagnetic (EM) field confined between the SRR and ground plane. Therefore, embedding the channel inside the substrate of the SRR sensor, operating at 2.63 GHz, enhanced the sensitivity by 60%. The constructive energy from the active feedback loop boosted the quality factor of the passive SRR by over <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$100\times $ </tex-math></inline-formula> (from 116 to 13 000), which significantly enhanced the resolution of the sensor. As a proof of concept, two prototypes of the sensor having different substrate-embedded channels with cross-sectional diameters of 1 and 0.762 mm were fabricated and used to detect low concentrations of ethanol (0–0.08 vol%) and salt (0–200 mM) in water. Embedding the channel inside the substrate of the active SRR sensor achieved a linear correlation ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$R^{2} = 0.99$ </tex-math></inline-formula> ) between the resonant frequency and the concentration of ethanol and salt in water with high sensitivity (19.95 MHz/vol% for ethanol and 4.35 kHz/mM for salt) and resolution (0.001 vol% for ethanol and 2.298 mM for salt). The achieved results demonstrate the feasibility of using the active SRR microwave sensor with a substrate-embedded channel for monitoring ethanol in fermentation broths and electrolytes in human sweat.

Reconfigurable Modular Platform for Prolonged Sensing of Toxic Gases in Particle Polluted Environments

The prolonged sensing of toxic gases in polluted particles and harsh environments is a challenging task that is also in high demand. In this work, the proof of principle of a sensitive, low-cost, and low-maintenance reconfigurable platform for filter-free and continuous ammonia (NH3) sensing in polluted environments is simulated. The platform can be modified for the detection of various toxic gases and includes three main modules: a microfluidic system for in-line continuous dust filtering; a toxic gas adsorption module; and a low-frequency microwave split-ring resonator (SRR). An inertia-based spiral microfluidic system has been designed and optimized through simulation for the in-line filtration of small particles from the intake air. Zeolite Y is selected as the adsorbent in the adsorption module. The adsorption module is a non-metallic thin tube that is filled with zeolite Y powder and precisely fixed at the drilled through-hole into the 3D microwave system. For the sensing module, a low-frequency three-dimensional (3D) split-ring resonator is proposed and optimally designed. A microwave resonator continuously monitors the permittivity of zeolite Y and can detect small permittivity alterations upon the presence of ammonia in the intake air. The microwave resonator is optimized at a frequency range of 2.5–3 GHz toward the detection of ammonia under different ammonia concentrations from 400 to 2800 ppm. The microwave simulation results show a clear contrast of around 4 MHz that shifts at 2.7 GHz for 400 ppm ammonia concentration. The results show the proof of principle of the proposed microfluidic-microwave platform for toxic gas detection.

TiO2 nanotube-integrated microwave planar resonator sensor for ultraviolet transmission-based liquid characterization

This work presents a planar microwave resonator sensor equipped with vertically self-aligned TiO2 nanotubes to enable real-time and contactless liquid characterization. The microwave sensor operates based on detecting a resonator response as it changes under exposure to UV light. This was used for liquid characterization by quantifying the transmission of ultraviolet (UV) light through a liquid sample and its subsequent absorbance by the TiO2 nanotube membrane. A microwave split ring resonator (SRR) was designed and simulated to operate with a resonant frequency of 8.73 GHz, resonant amplitude of -12 dB, and a quality factor of 103. The SRR was fabricated alongside an anatase-TiO2 nanotube membrane with a nanotube length of 20 μm, a wall thickness of 10 nm, and a tube diameter of 120 nm. The membrane was placed within the ring gap of the resonator to absorb any transmitted UV light which in turn would alter its dielectric properties. In this way, the designed sensor measured the ultraviolet transmission of a solution of S1813 photoresist in ethylene glycol (EG) and related the solvent concentration to a change in the transmission profile, without requiring contact or near-field proximity between the resonator sensor and the solution. Measuring different concentrations of S1813 photoresist in ethylene glycol, ranging from 0 to 100 v/v%, resulted in accurately calculated sensitivities of 0.18 MHz/%, 0.029 dB/%, and 0.45/% in resonant frequency, resonant amplitude, and quality factor, respectively. The achieved integration of TiO2 nanotube membranes for liquid characterization by analyzing microwave property variation may offer unique avenues for real-time liquid and UV detection and enable more comprehensive liquid and chemical analysis in comparison to conventional microwave sensor devices.

Rapid and real-time monitoring of bacterial growth against antibiotics in solid growth medium using a contactless planar microwave resonator sensor

Infection diagnosis and antibiotic susceptibility testing (AST) are pertinent clinical microbiology practices that are in dire need of improvement, due to the inadequacy of current standards in early detection of bacterial response to antibiotics and affordability of contemporarily used methods. This paper presents a novel way to conduct AST which hybridizes disk diffusion AST with microwave resonators for rapid, contactless, and non-invasive sensing and monitoring. In this research, the effect of antibiotic (erythromycin) concentrations on test bacterium, Escherichia coli (E. coli) cultured on solid agar medium (MH agar) are monitored through employing a microwave split-ring resonator. A one-port microwave resonator operating at a 1.76 GHz resonant frequency, featuring a 5 mm2 sensitive sensing region, was designed and optimized to perform this. Upon introducing uninhibited growth of the bacteria, the sensor measured 0.005 dB/hr, with a maximum change of 0.07 dB over the course of 15 hours. The amplitude change decreased to negligible values to signify inhibited growth of the bacteria at higher concentrations of antibiotics, such as a change of 0.005 dB in resonant amplitude variation while using 45 µg of antibiotic. Moreover, this sensor demonstrated decisive results of antibiotic susceptibility in under 6 hours and shows great promise to expand automation to the intricate AST workflow in clinical settings, while providing rapid, sensitive, and non-invasive detection capabilities.

Patterned PEDOT:PSS-enabled organic planar microwave resonator sensors

The demand for low-cost and environmentally friendly electronics has required novel materials that have high electrical performance while being mechanically stable and sustainable. Capitalizing on the water-dispersibility and high electrical conductivity of intrinsically conductive polymers, we first devise an all organic, environmentally friendly, and reformable microwave split-ring resonator (SRR) using poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), and second optimize its performance in solid and gas sensing applications. The conducting polymer layer is obtained by the solution-processing of PEDOT:PSS with dimethyl sulfoxide (DMSO) and is subsequently able to substitute metal traces used in conventional SRR structures. The performance of the fabricated organic microwave resonators (OMRs) in various sizes demonstrates the scalability of these resonators to be calibrated in a wide range of operational resonant frequencies, similar to their metal-based counterparts while being low-cost, environmentally friendly, and compatible with industrial-scale production. Moreover, by coupling two identical D-PEDOT:PSS resonators, an enhanced notch profile with an amplitude of −48.47 dB at the resonant frequency of 4.76 GHz is achieved, which is four times more sensitive than a single polymeric SRR at the same resonant frequency. The sensing capability of the polymeric microwave resonators is investigated by using solid standard materials, wherein the change in resonant frequency and resonant amplitude is observed in accordance with the permittivity of the test samples. Furthermore, the D-PEDOT:PSS OMRs developed in this study shows great potential when subjected to relative humidity sensing applications as a proof of concept. The performance of the OMRs is investigated for a wide relative humidity range of 3 % to 90 %, which demonstrates a sensitivity of 28 mdB and 300 kHz per unit relative humidity change on the resonant amplitude and the resonant frequency, respectively. The presented D-PEDOT:PSS split ring resonator paves the way for a novel generation of OMRs to be further developed in flexible and wearable microwave sensor applications in a wide variety of scenarios.

Optical magnetic response without metamaterials

One of the main achievements of metamaterials research has been the development of structured matter exhibiting optical magnetism: first, in an array of microwave split-ring resonators and, soon after, in plasmonic and dielectric metamaterials at THz to visible frequencies. We show here that metamaterial structuring is not necessary to achieve optical magnetic response. Indeed, such a response is an essential characteristic of homogeneous dielectric thin films—Fabry–Pérot resonances, for example, depend on interference among electromagnetic multipoles including the magnetic dipole.

Graphene oxide/polyaniline-based microwave split-ring resonator: A versatile platform towards ammonia sensing

Ammonia gas sensors have always received significant attention as robust platforms for emission control, food safety, and monitoring human exhaled breath for the early diagnosis of diseases such as dysfunction of the kidney and liver. This study explores the development of a microwave-based split-ring resonator (SRR) sensor with enhanced sensitivity to detect ammonia gas at low concentrations. The sensor is based on a nanocomposite fabricated by incorporating 10 wt% of graphene oxide (GO) into polyaniline (PANI) via the in-situ polymerization of aniline monomers over the surface of the GO sheets. The addition of GO to PANI results in a high sensitivity of 0.038 dB ppm−1 for low concentrations (1–25 ppm) and 0.0045 dB ppm−1 for high concentrations (> 25 ppm) of ammonia gas, in a 150–400 s time interval at room temperature. The prepared sensor can selectively sense ammonia gas in the presence of other higher concentrations of hazardous gases and a wide range of relative humidity levels (15–90%). The response signal is repeatable after 30 days with less than 0.32% deviation. The developed low-cost and robust sensor has the potential to monitor ammonia gas in various applications, including medical, environmental, food, and agricultural sectors.

Selective real-time non-contact multi-variable water-alcohol-sugar concentration analysis during fermentation process using microwave split-ring resonator based sensor

Real-time fermentation process monitoring is extremely important in process control attaining high quality products in many food industries. Although microwave resonators offer non-contact, high sensitivity, real-time sensing capabilities at extremely low cost with reusable structures, the lack of selectivity even in controlled environments, limit their applications to single variable systems or binary sensing. Since fermentation is a more complex process with at least three variables that could change simultaneously, the traditional microwave sensing techniques fail to measure the volumetric fraction of each of the components. This paper presents a novel technique enabling microwave resonators to perform volumetric fraction analysis of complex dynamically varying liquids while keeping all the interesting aforementioned characteristics for microwave resonator-based sensors. Multiple simulation and experimental results verify the capabilities of the proposed technique in real-time monitoring of ethanol, water, and sugar concentrations during the fermentation process.

Non-contact real-time water and brine concentration monitoring in crude oil based on multi-variable analysis of microwave resonators

Non-contact analysis of multi-component two-phase mixtures including immiscible liquids and solutions simultaneously by a single sensing structure with a wide dynamic range is addressed in this paper. The mixture of interest in this work comprises oil and water with variable salinity extended from 0 to 35 (g/kg). As low overall concentration of water as 1% is successfully measured through the innovative measurement procedure of microwave split-ring resonators. The enabling multi-variable analysis technique in this work is based on employing multiple features which are the frequency shifts of various harmonics of the resonator instead of just using one resonance frequency as the traditional microwave resonators-based sensors. Higher numbers of features provide information for the development of more equations enabling analyzing of more complex mixtures. Experimental results verify the developed approach with the resulting low Mean-Square- Errors in the range of 10−5 for the water-brine-oil mixture.

High-Dynamic-Range Chipless Microwave Resonator-Based Strain Sensor

Microwave split-ring resonators are utilized as sensors in a wide variety of applications due to their remarkable features, such as extremely low cost, high sensitivity, and relatively high quality factor. In this article, another application is enabled according to a recently demonstrated chipless tag-reader structure providing the possibility of simplifying the sensor structure from a “multilayer structure” consisting of a dielectric substrate sandwiched between two metallic layers to a single-layer structure formed from a conductive material. This capability is specifically important for strain sensing applications as it brings the possibility of utilizing low stiff conductive materials instead of copper (which is the primary material used in microwave application) while keeping the reader structure with high-quality microwave application-specified substrates intact. With the explained approach in this work, a low tensile silver–aluminum silicone elastomer conductive material is considered for the tag providing a very high tensile dynamic range. According to the whole sensing system structure, as high elongation range of as 20% and the high sensitivity in the range of 25 MHz/1% of strain is achieved. Multiple simulations and experimental results support the idea of the novel microwave strain sensor proposed in this work.

Gold-on-glass microwave split-ring resonators with PDMS microchannels for differential measurement in microfluidic sensing.

This paper describes a microwave resonator incorporating microfluidic lab-on-chip sensor system capable of performing simultaneous differential measurement based sensing of liquid samples. The resonators are split-ring resonator shapes made of gold on glass substrates. Directly bonded on glass substrates are polydimethylsiloxane microchannels. Sensor system design incorporates a pair of identical resonators, one of which performs reference reading from the background. Tracking the difference of the responses of both resonators simultaneously, rather than a single one, is used to obtain a more linear and noise-free reading. The sensor system was produced with conventional fabrication techniques. It is compatible with low-cost, simple, easy to handle sensing applications. Results indicate that reliable differential measurement was possible owing to a well-matched pair of sensors with a response error as low as 0.1%. It was also demonstrated that differential measurement capability enables sensing with improved linearity. Measurements were performed with glucose solutions in the range of 3.2-16.1 mM, achieving a sensitivity of 0.16 MHz/mM.

Room-Temperature Mid-Infrared Detection via Resonant Microwave Circuits

We demonstrate a room-temperature mid-wave infrared ( $\text {MWIR} ) photoconductive detection architecture utilizing a compact, Ku -band microwave split-ring resonator (SRR) loaded with a micrometer-scale indium arsenide antimonide (InAsSb) pixel. The resonant microwave photoconductor (RMPC) does not require the fabrication of ohmic contacts or the need for dc current collection. We compare the RMPC architecture with a traditional dc photoconductor (DCPC) and evaluate the conditions in which the overall sensitivity of the RMPC exceeds that of the DCPC. Time-domain simulations suggest that the resonator can shape and suppress the fundamental Johnson thermal noise of the semiconductor photon-absorbing element, resulting in decreased total noise and thus increased detectivity, with the SRR circuit itself being the primary generator of thermal noise. For this material system, we calculate that the RMPC has a factor of three improvement in the overall Johnson-limited detectivity ( ${D}^\ast $ ) when compared to the standard DCPC and discuss both the situations for which this improvement could be significantly larger as well as practical system noise considerations.

Wideband Tunable Modified Split Ring Resonator Structure Using Liquid Metal and 3-D Printing

Microwave split ring resonators (SRRs) are limited in modern designs to give a fixed resonant response based on their dimensions and material makeup. By designing an adjustable liquid metal component integrated in a highly coupled modified SRR, a frequency-tunable resonator structure was developed. The liquid metal integrated resonators were simulated to show that by varying the amount of liquid metal introduced to the system, the resonant frequency was continuously adjusted from 3.4 to 2.3 GHz, with a consistent resonant amplitude and quality factor of −22 dB and 75, respectively. The frequency-configurable sensor was fabricated with simple 3-D printing methods and inexpensive materials to control the liquid metal, resulting in a compact device with frequency tunability from 3.4 to 2.35 GHz. The fabricated resonator sensor had a resonant amplitude and quality factor of −24 dB and 70, respectively – with less than 10% variation along the entire frequency tuning process. Experimental analysis of the sensor showed continuous variation as the volume of liquid metal was increased within the straight channel, with a consistent resonant frequency sensitivity to liquid metal of 375 MHz/cm. The device enables measurement of a microwave frequency spectrum with user-desired resolution.

Weibull analysis of atmospheric pressure plasma generation and evidence for field emission in microwave split-ring resonators

The generation of atmospheric pressure microplasmas using microwave resonators is promising for many applications due to the possibility of high electron densities and low electrode degradation. In particular, such plasmas may help enable reconfigurable metamaterials operating from GHz to THz. Since plasma metamaterials may require the generation of tens to hundreds of plasmas, it is important to find ways to reduce the power required for plasma breakdown. Here, we study gold and silver microwave split-ring resonators (SRRs) with a variety of materials near the interelectrode gap (Cu, CuO nanowires, aluminum oxide). We focus on those fabricated using a traditional thick film technique, screen-printing, and using fs- and ns-laser ablation. The use of laser ablation allows us to explore small interelectrode gap sizes (7–100 μm) and the use of different lasers and laser parameters enables us to produce a variety of microstructures. We utilize Weibull statistics to examine breakdown in atmospheric pressure Ar with and without deep ultraviolet illumination of SRRs. Fabrication methods and materials are shown to influence both Q-factor of the SRRs and breakdown voltage independently. It is found that superior performance in terms of breakdown voltage and consistency in breakdown is related to Weibull modulus. The power requirement for breakdown varied as widely as an order of magnitude depending on fabrication method and material used for the SRRs. Furthermore, we consider the performance differences seen between various resonators and relate this to microstructure/material which suggests that field-emission may play a role in providing the seed electrons required for breakdown. This need for seed electrons appears to be especially important for gap sizes of 40 μm and smaller.