Symmetric Resonator Research Articles

Broadband terahertz metamaterial absorber: design and fabrication.

We proposed and experimentally demonstrated a broadband terahertz (THz) metamaterial absorber based on a symmetrical L-shaped metallic resonator. The absorber structure produces two absorption peaks at 0.491 and 0.73 THz, with the absorption rates of 98.6% and 99.6%, respectively. Broadband absorption was obtained from 0.457 to 1 THz, achieving a >90% absorption bandwidth of 0.543 THz. By analyzing the distributions of the electric and magnetic field at the two resonance frequencies, electric and magnetic dipole resonances were proposed to explain the broadband absorption mechanism. Furthermore, various widths and lengths of the symmetrical L-shaped metallic resonator on the absorption characteristics were investigated. Moreover, the broadband absorption characteristic can be maintained with an incident angle of up to 45° for transverse-electric and 30° for transverse-magnetic polarization. Finally, we experimentally observed a >70% broadband absorption characteristic from 0.42 to 1 THz. This proposed absorber has the potential for bolometric imaging, modulating, and spectroscopy in the THz region.

A Temperature-Compensated Differential Microstrip Sensor for Microfluidic Applications

The paper proposes a temperature-compensated sensor for microfluidic application. The sensor consists of a splitter/combiner section with two symmetrical complementary split-ring resonators (CSRR). A polydimethylsiloxane substrate is attached with the microfluidic channel aligned to the meander slot of the CSRR. A split-ring resonator (SRR) with peripheral circuit is added to detect environmental temperature. The water-methanol mixtures with various water fractions are measured at temperature ranging from 20°C to 50°C. The environmental temperature is firstly measured by the resonant frequency shift produced by SRR. Then, the variations in the resonant frequency and quality factor of the differential splitter/combiner section, as well as the temperature, are used as the input data for training the neural network. In comparison with previous designs, the proposed sensor is capable of not only suppressing the influences of environmental factors, but also retrieving the permittivity and loss tangent of liquid sample at specific temperature, thereby identifying the liquid accurately.

Supercattering Channels of Nonspherical structurers

A superscattering structure is an efficient energy-mapping device that of particular importance for various electromagnetic experiment methods, with potential sensing and energy harvesting applications. We study in this work the scattering cross-section of outgoing channels in the irreducible and singular basis for an arbitrary shape scatterer. The superscattering status is shown to occur within a single outgoing channel of an optimized cluster of cylinders, a forbidden mechanism in spherically symmetric Mie resonators.

Dual-Spectral Plasmon-Induced Transparent Terahertz Metamaterial with Independently Tunable Amplitude and Frequency

A bifunctional tunable metamaterial composed of pattern metal structure, graphene, and strontium titanate (STO) film is proposed and studied numerically and theoretically. The dual plasmon-induced transparency (PIT) window is obtained by coupling the bright state cut wire (CW) and two pairs of dark state dual symmetric semiring resonators (DSSRs) with different parameters. Correspondingly, slow light effect can also be realized. When shifting independently, the Fermi level of the graphene strips, the amplitudes of the two PIT transparency windows and slow light effect can be tuned, respectively. In addition, when independently tuning the temperature of the metamaterial, the frequency of the dual PIT windows and slow light effect can be tuned. The physical mechanism of the dual-PIT was analyzed theoretically by using a three-harmonic oscillator model. The results show that the regulation function of the PIT peak results from the change of the oscillation damping at the dark state DSSRs by tuning conductivity of graphene. Our design presents a new structure to realize the bifunctional optical switch and slow light.

Polarization and Incidence Angle Independent Low-Profile Wideband Metamaterial Electromagnetic Absorber Using Indium Tin Oxide (ITO) Film

We use indium tin oxide (ITO), one of the representative resistive materials, for the implementation of a metamaterial electromagnetic (EM) absorber with a high absorbance in a wide frequency range. Highly symmetrical split ring resonators made of ITO film are deposited on the polyethylene terephthalate with transparent and flexible features, and such a configuration causes the proposed absorber to be insensitive to polarization and incidence angles. The proposed absorber, with a profile of only 0.171λ, exhibits a wideband absorbance of 7.2 GHz to 27 GHz, with a 90% absorption criterion. A prototype is built, and all the computed expectations from the full-wave EM simulations in this work are verified experimentally.

High-Sensitive Multi-Gas Detection Based on MDM Waveguide With Symmetric Dual Side-Coupled Ring-Resonators

A high-sensitive multi-parameter gas sensor composed of symmetric dual side-coupled ring resonators in a metallic-dielectric-metallic (MDM) structure is investigated by the finite difference time domain (FDTD) method. The dual side-coupled resonators have lower transmission dip and faster response due to the superposition of the outgoing fields’ amplitude of the symmetric dual resonators, which is very beneficial for detection. Two independently tunable transmission dips are achieved by expanding the ring resonator in ${x}$ -direction, making it possible for dual-parameter sensing. The electric and magnetic distribution characteristics at the two resonant wavelengths are analyzed. Another plasmonic structure expanded along both sides in a central resonator-straight waveguide way is also investigated, and the former structure expanded in ${x}$ -direction works better in detection difficulty and structural utilization. After optimizing the radius and position of ring resonators, the refractive index sensitivities of cavity A and B can reach 2427 nm/RIU and 2431.9 nm/RIU, respectively. The methane and hydrogen sensitivities are −9.213 nm/% and −1.65 nm/% with an extremely high linearity, after coating the methane and hydrogen gas-sensitive film in cavity A and B respectively. This investigation provides an effective way for the design of multi-parameter high-sensitive independently tunable sensors.

Sub-gap Fano resonances in a topological superconducting wire with on-site Coulomb interactions

We consider theoretically a 1D-semiconducting wire with strong Rashba interaction in proximity with s-wave superconductor, driven into topological phase by external magnetic field. Additionally, we take into account on-site Coulomb interactions inside the wire. The system is modelled by a tight binding Hamiltonian with Rashba hopping term and induced s-wave superconductivity. Calculations are performed utilizing recursive Green’s function method, and Coulomb interactions are treated selfconsistently within Hubbard I approximation. For the Hubbard levels residing within p-wave superconducting gap, particle–hole symmetric four-resonance structure develops in the density of states, apart from Majorana resonance. One pair of particle–hole symmetric resonances is created by the discrete II-Hubbard levels of the particular site, and the second pair of Hubbard sub-bands originates from recursive summation over the sites of the wire. Quantum interference between both types of pairs of states creates in-gap charge-conjugated Fano resonances with opposite asymmetry factors. We demonstrate that when quantum interference is dominated by two-particle tunneling, the Majorana resonance is strongly diminished, while it is not altered when single-particle tunneling dominates in interference process. We also discuss some consequences for experimental distinction of true Majorana states, and show that on-site Coulomb interactions support the appearance of topological phase.

Three-dimensional imaging of electron spin resonance-magnetic resonance force microscopy at room temperature

In this study, we demonstrated the three-dimensional (3D) imaging by magnetic resonance force microscopy (MRFM) based on electron spin resonance (ESR) measurements at room temperature. For a microsample containing radicals, the 3D force distribution was obtained using a custom-made Si nanowire and a permanent magnetic particle. Calculation using precise values of the magnetic field distribution is required to define an accurate response function for the 3D deconvolution processing of the spin density distribution. A symmetric resonance magnetic field produces good periodic force maps using a spherical micromagnet, which simplifies the deconvolution processing with resonant slice systems. In addition, the 3D imaging method was processed in the wavenumber space by a Fourier transform that used a simple convolution with noise parameters in the response function. After the reconstruction of the distribution of electron spins (radicals), the shape of the sample agreed with that of the optical image; thus, the accuracy of the internal density structure was verified. We believe that the combination of a Si nanowire and a spherical magnetic particle used for magnetic resonance detection is a good candidate for Fourier transform 3D deconvolution in multiple MRFM applications.

Miniaturized quad‐band filter with improved selectivity using split ring resonators and metallic strips

A miniaturized bandpass filter of quad-band response with high selectivity is reported in this paper. The developed filter utilizes two sets of symmetric open-ended split-ring resonators (SRR), L-shaped and C-shaped strips along with rectangular stubs. Initially, open-ended SRR is connected with a C-shaped metallic strip to serves as a dual passband filter. Then by adding another SRR along with L-shape metallic strip and rectangular stub, it was converted into triple band and quad-band respectively. The operating frequency of the proposed filter structure can be controlled with the variation in the length ratio of resonators. In addition, transmission zero can be increased because of coupling between source/load and resonators which improves selectivity. The quad-band bandpass filter operates at 0.6/1.1/2.25/3.1 GHz, along with 3-dB fractional bandwidths of 17.9% (0.56-0.67 GHz), 17.7% (1.03-1.23 GHz), 5.8% (2.20-2.33 GHz), and 13.3% (2.88-3.29 GHz). It offers compactness with an electrical footprint area of 0.11λg × 0.10λg (23.5 mm × 21.0 mm) without feedline, where λg denotes the guided wavelength at 0.6 GHz. The measured and simulated outcomes were in virtuous correlation.

Dual-band metasurface for broadband asymmetric transmission with high efficiency

In this paper, a dual-band asymmetric transmission metasurface for linearly polarized wave is designed, analyzed, and verified experimentally for operation in S- and C-bands. The metasurface is established by a bi-anisotropy structure with three metallic layers separated by dielectric spacers. Symmetric rectangular split-ring resonators are adopted in the top and bottom metallic layers while modified metal strip with stepped widths is utilized in the middle. Both asymmetric transmission capability and operation bandwidth are remarkably improved. According to simulation, the design yields an asymmetric transmission parameter above 0.8 within two operation bands: 2.53–2.56 and 3.80–4.71 GHz, fractional bandwidths of 1.18% and 21.39% can be deduced, respectively. The oblique incidence property is further analyzed, which demonstrates robustness response to incidence up to 50°. Two operation bands, 2.53–2.56 and 3.87–4.78 GHz, are also validated through measurement. The corresponding bandwidths 1.18% and 21.04% show tiny discrepancy related to simulated results, which validate the reliability of the design. Comparisons with other recent published results show that this structure provides relative low profile, wider operation band with high asymmetric transmission efficiency. The proposed metasurface could be applied for electromagnetic interference suppression and polarization control for modern radar and satellite communication systems and can be scaled to operate for higher frequencies with considerable bandwidth and efficiency.

Dynamically stable single frequency ring resonator from diode pumped Nd:YAG modules with 55.6 W of output power

A Nd:YAG rod single-frequency ring laser based on side-pumped commercial modules is presented. Thermally induced birefringence compensation was applied in a dynamically stable resonator providing 55.6 W of continuous, linearly polarized, TEM00 output. The particular case of a symmetric ring resonator containing one or two focusing rods and a pair of curved mirrors was analyzed and a design technique is presented, allowing for easy, continuous shaping of the stability limits by changing only the distances in the resonator.

Reconfigurable and scalable 2,4-and 6-channel plasmonics demultiplexer utilizing symmetrical rectangular resonators containing silver nano-rod defects with FDTD method

Reconfigurable and scalable plasmonics demultiplexers have attracted increasing attention due to its potential applications in the nanophotonics. Therefore, here, a novel method to design compact plasmonic wavelength demultiplexers (DEMUXes) is proposed. The designed structures (two, four, and six-channel DEMUXes) consist of symmetrical rectangular resonators (RRs) incorporating metal nano-rod defects (NRDs). In the designed structures, the RRs are laterally coupled to metal–insulator-metal (MIM) waveguides. The wavelengths of the output channels depend on the numbers and radii of the metal NRDs in the RRs. The results obtained from various device geometries, with either a single or multiple output ports, are performed utilizing a single structure, showing real reconfigurability. The finite-difference time-domain (FDTD) method is used for the numerical investigation of the proposed structures. The metal and insulator used for the realization of the proposed DEMUXes are silver and air, respectively. The silver’s permittivity is characterized by the well-known Drude model. The basic plasmonic filter which is used to design plasmonic DEMUXes is a single-mode filter. A single-mode filter is easier to cope with in circuits with higher complexity such as DEMUXes. Also, different structural parameters of the basic filter are swept and their effects on the filter’s frequency response are presented, to provide a better physical insight. Taking into account the compact sizes of the proposed DEMUXes (considering the six-channel DEMUX), they can be used in integrated optical circuits for optical communication purposes.

High-temperature lithium atomic magnetometry by symmetric hyperfine coherent population trapping resonances

A glass lithium vapor cell is an atomic sensor operating at high temperature. The longitudinal magnetic-field strength is measured by coherent population trapping (CPT) in ground-state hyperfine levels. Two linearly polarized light beams are polarized orthogonally to each other. The difference of optical frequencies is tuned to generate a dark state. Additionally, a few light modes of equidistant frequency spacing are generated by modulating the difference frequency. The symmetry of the light modes and the energy levels enables simultaneous measurement of the hyperfine splitting frequency of the ground state and the magnetic-field strength. Tuning the optical frequency difference to the central CPT resonance, a sensitivity of 15 p T / H z is achieved. The time response and the sensitivity of the CPT magnetometry are studied by changing the cell temperature, the laser power, the frequency separation of the light modes, and the Zeeman frequency.

Active Control of Nanodielectric-Induced THz Quasi-BIC in Flexible Metasurfaces: A Platform for Modulation and Sensing.

A bound state in the continuum (BIC) is a nonradiating state of light embedded in the continuum of propagating modes providing drastic enhancement of the electromagnetic field and its localization at micro-nanoscale. However, access to such modes in the far-field requires symmetry breaking. Here, it is demonstrated that a nanometric dielectric or semiconductor layer, 1000times thinner than the resonant wavelength (λ/1000), induces a dynamically controllable quasi-bound state in the continuum (QBIC) with ultrahigh quality factor in a symmetric metallic metasurface at terahertz frequencies. Photoexcitation of nanostrips of germanium activates ultrafast switching of a QBIC resonance with 200% transmission intensity modulation and complete recovery within 7 ps on a low-loss flexible substrate. The nanostrips also form microchannels that provide an opportunity for BIC-based refractive index sensing. An optimization model is presented for (switchable) QBIC resonances of metamaterial arrays of planar symmetric resonators modified with any (active) dielectric for inverse metamaterial design that can serve as an enabling platform for active micro-nanophotonic devices.

A microstrip lowpass filter with sharp roll-off using arrow-shaped resonators and high-impedance open stubs

In this paper, an efficient microstrip low pass filter (LPF) using arrow-shaped resonators has been proposed. For attenuating undesired harmonics in stop band, stepped-impedance attenuators have been applied. To suppress components in frequency band of 6–10 GHz, two single-base high-impedance stubs have been exploited. To analyze the proposed structure accurately and define the role of each element in the filter, L-C equivalent circuit has been presented. Using this L-C equivalent model, poles and zeros of transfer function can be calculated and discussed. The structure design is based on reducing the coupling effect of elements on each other and neglecting these effects in obtaining the transfer function. Two symmetric arrow-shaped resonators with high-length basis provide 3-dB cut-off frequency as 1.447 GHz. The proposed filter has flat pass band with insertion loss as −0.08 dB and wide stop band with relative stop-band bandwidth (RSB) equal to 1.728. The figure of merit (FOM) is 113,355 which is much interesting compared to other similar works in this field as shown in simulation results.

Electromagnetically Induced Transparency-Like Terahertz Graphene Metamaterial With Tunable Carrier Mobility

In this work, a graphene-based planar terahertz electromagnetically induced transparency-like (EIT-like) metamaterial, composed of two horizontally symmetric mono-layer graphene micro-ring resonators (MRR) and a vertical mono-layer graphene micro-strip resonator (MSR), has been proposed. The transparent window was observed in the y polarization direction. By changing the carrier mobility of the graphene, the transparent window could be opened and closed while the position of the EIT-like window remained unchanged. The physical mechanism of the EIT-like effect was explained by the distribution of the electric field on E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>z</i></sub> and the three-level <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Lambda $ </tex-math></inline-formula> -type system. The theoretical fitting results based on the Lorentz oscillator model and the S-parameter inversion method were consistent with the numerical simulation results. In addition, the performance of the metamaterial for detecting the refractive index of the surrounding medium was analyzed. Numerical simulation showed that when the FOM exceeded 8.0, the sensitivity was 1.6 THz/RIU. Finally, as the graphene carrier mobility increased, the group delay of the device increased. The delay time reached 1.49 ps, and the group index was as high as 400. The proposed design provides a feasible method for the development of optical switches, biochemical molecular detection, and slow light equipment.

Tenors Not Sopranos: Bio-Mechanical Constraints on Calling Song Frequencies in the Mediterranean Field-Cricket

Male crickets and their close relatives bush-crickets (Gryllidae and Tettigoniidae, respectively; Orthoptera and Ensifera) attract distant females by producing loud calling songs. In both families, sound is produced by stridulation, the rubbing together of their forewings, whereby the plectrum of one wing is rapidly passed over a serrated file on the opposite wing. The resulting oscillations are amplified by resonating wing regions. A striking difference between Gryllids and Tettigoniids lies in wing morphology and composition of song frequency: Crickets produce mostly low-frequency (2–8 kHz), pure tone signals with highly bilaterally symmetric wings, while bush-crickets use asymmetric wings for high-frequency (10–150 kHz) calls. The evolutionary reasons for this acoustic divergence are unknown. Here, we study the wings of actively stridulating male field-crickets (Gryllus bimaculatus) and present vibro-acoustic data suggesting a biophysical restriction to low-frequency song. Using laser Doppler vibrometry (LDV) and brain-injections of the neuroactivator eserine to elicit singing, we recorded the topography of wing vibrations during active sound production. In freely vibrating wings, each wing region resonated differently. When wings coupled during stridulation, these differences vanished and all wing regions resonated at an identical frequency, that of the narrow-band song (∼5 kHz). However, imperfections in wing-coupling caused phase shifts between both resonators, introducing destructive interference with increasing phase differences. The effect of destructive interference (amplitude reduction) was observed to be minimal at the typical low frequency calls of crickets, and by maintaining the vibration phase difference below 80°. We show that, with the imperfect coupling observed, cricket song production with two symmetric resonators becomes acoustically inefficient above ∼8 kHz. This evidence reveals a bio-mechanical constraint on the production of high-frequency song whilst using two coupled resonators and provides an explanation as to why crickets, unlike bush-crickets, have not evolved to exploit ultrasonic calling songs.

Observation of the perturbed eigenvalues of PT-symmetric LC resonator systems

We address both theoretically and experimentally the influence of asymmetric perturbation on the eigenvalues of parity-time (PT) symmetric resonator systems under the symmetric gain-loss arrangement, based on an inductively coupled inductor-capacitor-resistor (LCR) pair. The perturbed eigenvalues have been theoretically presented, numerically simulated, and experimentally measured. It shows that the asymmetric perturbation breaks PT-symmetry, leading to complex eigenvalues, which is different from the broken PT-symmetric phase with complex-conjugate eigenvalues. We have analyzed the perturbed frequency responses in all phases. At the exceptional points (EP), the resulting eigenvalues splitting is proportional to the square root of perturbation, showing the advantage of being highly sensitive to asymmetric perturbation. Meanwhile, the smaller the perturbation, the higher the sensitivity. The perturbation effect of PT-symmetric systems may be utilized to detect small signal changes in LC passive wireless sensors.

A Low-Noise High-Order Mode-Localized MEMS Accelerometer

This paper reports a precision mode-localized accelerometer operating in a higher-order flexural mode. The accelerometer consists of two symmetric resonators coupled by a central rigid coupler to generate an ultra-weak coupling factor. A reduced noise floor is observed when the resonators operate in the higher-order flexural mode compared to the basic lower-order mode. The mode-localized accelerometer working in the fifth-order mode demonstrates an input-referred bias instability of 130 ng and noise floor of 85 ng/ √Hz, which are the best results obtained for accelerometers employing the mode localization paradigm to date. These results indicate that the performance of the mode-localized sensors can be improved by operating at a higher working frequency if the coupling factor and quality factor do not drop significantly. [2020-0365].

Piecewise asymmetric exponential potential under-damped bi-stable stochastic resonance and its application in bearing fault diagnosis

It is difficult to extract weak signals in strong noise background, therefore a piecewise asymmetric exponential potential under-damped bi-stable stochastic resonance (PAEUBSR) system is proposed. First, the theoretical analysis of the steady-state probability density (SPD), mean first passage time (MFPT) and output signal-to-noise ratio (SNR) are derived under the adiabatic approximation theory. At the same time, the influence of different system parameters on system performance is explored. Then the PAEUBSR system is applied to the fault signal diagnosis of different types of bearings, and the parameters are optimized through the adaptive genetic algorithm (AGA). The test results are compared with the exponential potential over-damped symmetric bi-stable stochastic resonance (EOSBSR) system and the exponential potential under-damped symmetric bi-stable stochastic resonance (EUSBSR) system. Finally, the detection results on two sets of bearing fault data show that the PAEUBSR system has better effects on the enhancement and detection of bearing fault signals. This provides good theoretical support and application value for this system in subsequent theoretical analysis and practical engineering applications.