Higher Resonant Modes Research Articles

Are the Horizontal-to-Vertical Spectral Ratios of Earthquakes and Microtremors the Same?

ABSTRACT We consider the similarities and differences between earthquake and microtremor horizontal-to-vertical spectral ratios (eHVSR and mHVSR, respectively) using a dataset of 161 sites in southern California. Quantitative comparisons are made in terms of the eHVSR and mHVSR lognormal median curves, as well as the frequencies and amplitudes associated with the fundamental- and higher-mode resonances where present. The results show only 58% of the eHVSR–mHVSR pairs agree in terms of their median curve and only 25% of the eHVSR–mHVSR pairs agree in terms of shared resonances, which increases to 68% if flat HVSRs are considered equivalent. Furthermore, while the shared resonances match very well in terms of frequency (root mean square error, RMSE, <0.11 Hz), the amplitudes of those resonances do not agree (RMSE >1.6). These findings demonstrate that while eHVSR and mHVSR agree at some sites, they are not equivalent at all sites. To investigate if the agreement between eHVSR and mHVSR could be related to features of the microtremor data, earthquake recordings, and/or the site conditions, three machine learning (ML) models at varying levels of interpretability are presented. The ML models—which include multivariate logistic regression, gradient-boosted trees, and support vector machines—show only partial success at using site-specific data to predict whether eHVSR and mHVSR will likely agree in terms of their median curve (accuracy of 78%) and number of resonances (accuracy of 84%). Therefore, we conclude that while eHVSR and mHVSR can be quite similar in terms of resonant frequencies at some sites, they are not identical at all sites. Furthermore, preliminary evidence shows that the agreement of eHVSR and mHVSR can be predicted a priori given features of the microtremor measurements, earthquake recordings, and site conditions, although a larger dataset will be necessary for developing a robust predictive model.

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.

Tunability-selective lithium niobate light modulators via high-Q resonant metasurface.

Herein, we propose and demonstrate an efficient light modulator by intercalating the nonlinear thin film into the optical resonator cavities, which introduce the ultra-sharp resonances and simultaneously lead to the spatially overlapped optical field between the nonlinear material and the resonators. Differential field intensity distributions in the geometrical perturbation-assisted optical resonator make the high quality-factor resonant modes and strong field confinement. Multiple channel light modulation is achieved in such layered system, which enables the capability for tunability-selective modulation. The maximal modulation tunability is up to 1.968 nm/V, and the figure of merit (FOM) reaches 65.6 V-1, showing orders of magnitude larger than that of the previous state-of-the-art modulators. The electrical switch voltage is down to 0.015 V, the maximal switching ratio is 833%, and the extinction ratio is also up to 9.70 dB. These features confirm the realization of high-performance modulation and hold potential for applications in switches, communication and information, augmented and virtual reality, etc.

Design and experimental study of a planar piezoelectric actuator with resonant actuation

Planar piezoelectric actuators, which can achieve high-precision motions over a wide range, have been widely applied in high-end equipment fields. However, the existing planar piezoelectric actuators can no longer meet the high actuation characteristics required for the rapidly development of these fields, which include large travel, high resolution, fast speed, planar three degrees of freedom (3-DOF) and a compact structure. A planar piezoelectric actuator with resonant actuation is proposed and tested in this work. The actuator consists of four symmetrically configured uniform bending-bending hybrid piezoelectric units, which coordinate to achieve large travel and planar 3-DOF characteristics. A vibration isolation mechanism adopting thin beam structure with low stiffness in coupling directions is designed to achieve vibration isolation among the units. The actuator can operate in high frequency resonance mode and low frequency stepping mode, allowing for the achievement of high-speed and high-resolution characteristics. A prototype is fabricated and tested. A compact structure measuring 53 mm × 53 mm × 13 mm and weighing only 30.2 g has been achieved. The maximum linear and rotary speeds have been tested to be over 400 mm/s and 13 rad/s, respectively. The linear and rotary resolutions are 0.42 μm and 12.9 μrad, respectively. The proposed actuator achieves characteristics of large travel, high resolution, fast speed, 3-DOF and a compact structure.

Transient study of droplet oscillation characteristics driven by an electric field

Electrowetting technology, a microfluidic technology, has attracted more and more attention in recent years and has broad prospects in terms of microdroplet drive. In this paper, the dynamic contact angle theory is used to develop a numerical model to predict the droplet dynamic contact behavior and internal flow field under electrowetting. In particular, based on the established computational model of droplet force balance, the dynamic process of a droplet under electrowetting is analyzed, including the perspective of pressure variation and force balance inside the droplet. The results show that when the alternating current frequency increases from 50 Hz to 500 Hz, the amplitude of the oscillation waveform after droplet stabilization is 0.036 mm, 0.016 mm, 0.013 mm and 0.002 mm, while the relevant droplet oscillation period T is 11 ms, 4 ms, 2 ms and 1 ms, respectively. It is also found that the initial phase angle does not affect the droplet oscillation amplitude. In addition, the pressure on the droplet surface under alternating current electrowetting increases rapidly to the maximum value with resonant waveform oscillation, and the droplet will present different resonance modes under voltage stimulation. The higher the resonance mode is, the smaller the droplet oscillation amplitude is and the streamline at the interface will present an eddy current, in which the number of vortices matches the resonance mode. A high resonance mode corresponds to a small droplet amplitude, while there are more vortices with a smaller size.

Reconfigurable Millimeter-Wave Tri-Band Antenna Based on VO2-Films-Embedded Co-Aperture Metasurface Structures

A novel reconfigurable millimeter-wave tri-band antenna using co-aperture metasurface structures embedded with electrically controlling vanadium dioxide (VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) films is proposed. A co-aperture metasurface structure is designed by loading a large square ring surrounded with eight small square patches in the same aperture. By embedding the VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> film into the large ring, two lower frequencies of 22 and 28 GHz can be excited and switched when the film operates in metallic and insulating states respectively. Meanwhile, an extra higher-mode resonance at 40 GHz is produced by the small patches. Benefit from the reasonable co-aperture layout, the three frequency bands exhibit good independent tunability. Furthermore, the tri-band metasurface structure is used as radiation element to construct a reconfigurable antenna. To realize good feeding coupling at three bands with large frequency ratio for the antenna, a new kind of combined slot is proposed and etched on the transverse surface of the feeding waveguide. The combined slot contains a loop slot, a dumbbell-shaped slot and two U-shaped slots, corresponding to low, medium and high frequencies respectively. In addition, a simple coplanar DC bias circuit with little effect on radiation is designed. As results, when the VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> film is in insulating state, the antenna can simultaneously cover two bands of 27.7-29.8 GHz and 38.8-40 GHz with the maximum gain of about 8.7 dBi and 9.5 dBi respectively; when the film is activated to metallic state, it can also work in two bands of 21.1-21.6 GHz and 38.6-40 GHz with the maximum gain of about 7.2 dBi and 9.3 dBi. Compared with other reported VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -based antennas, the proposed antenna can work in three different bands with adjustable frequency ratio and superior radiation performance.

Asymmetrical Plasmon Distribution in Hybrid AuAg Hollow/Solid Coded Nanotubes

Morphological control at the nanoscale paves the way to fabricate nanostructures with desired plasmonic properties. In this study, we discuss the nanoengineering of plasmon resonances in 1D hollow nanostructures of two different AuAg nanotubes, including completely hollow nanotubes and hybrid nanotubes with solid Ag and hollow AuAg segments. Spatially resolved plasmon mapping by electron energy loss spectroscopy (EELS) revealed the presence of high order resonator-like modes and localized surface plasmon resonance (LSPR) modes in both nanotubes. The experimental findings accurately correlated with the boundary element method (BEM) simulations. Both experiments and simulations revealed that the plasmon resonances are intensely present inside the nanotubes due to plasmon hybridization. Based on the experimental and simulated results, we show that the novel hybrid AuAg nanotubes possess two significant coexisting features: (i) LSPRs are distinctively generated from the hollow and solid parts of the hybrid AuAg nanotube, which creates a way to control a broad range of plasmon resonances with one single nanostructure, and (ii) the periodicity of the high-order modes are disrupted due to the plasmon hybridization by the interaction of solid and hollow parts, resulting in an asymmetrical plasmon distribution in 1D nanostructures. The asymmetry could be modulated/engineered to control the coded plasmonic nanotubes.

Tunable High-Q Factor Substrate for Selectively Enhanced Raman Scattering

Most Surface-enhanced Raman scattering (SERS) substrates enhance all the Raman signals in a relative broad spectral range. The substrates enhance both the interested and background signals together. To improve the identification of target molecules from numerous background ones, substrates with multi high-quality (Q) factor resonance wavelengths can be designed to achieve the selective enhancement of specific Raman transitions. When the resonance frequencies are modulated to match the excitation and Raman scattering frequencies, the detection of the target molecule can be more effective. In this paper, we design a tunable high-Q SERS substrate with periodic silver bowtie nanoholes on silica spacer and silver film. The substrate possessed three high-Q and high electric field resonance modes, which resulted from the interaction of the localized surface plasmon resonance (LSPR) of the bowtie nanoholes, the surface plasmon polariton (SPP) of the period bowtie nanoholes and the Fabry–Perot (FP) resonance between the bowtie and silver film bottom. The interaction between these resonance modes resulted in not only a higher quality (Q) factor, but also a higher electric field, which can be employed to realize a potential substrate in high-sensitivity and selective-detection fields.

Piezoelectric-AlN resonators at two-dimensional flexural modes for the density and viscosity decoupled determination of liquids

A micromachined resonator immersed in liquid provides valuable resonance parameters for determining the fluidic parameters. However, the liquid operating environment poses a challenge to maintaining a fine sensing performance, particularly through electrical characterization. This paper presents a piezoelectric micromachined cantilever with a stepped shape for liquid monitoring purposes. Multiple modes of the proposed cantilever are available with full electrical characterization for realizing self-actuated and self-sensing capabilities. The focus is on higher flexural resonances, which nonconventionally feature two-dimensional vibration modes. Modal analyses are conducted for the developed cantilever under flexural vibrations at different orders. Modeling explains not only the basic length-dominant mode but also higher modes that simultaneously depend on the length and width of the cantilever. This study determines that the analytical predictions for resonant frequency in liquid media exhibit good agreement with the experimental results. Furthermore, the experiments on cantilever resonators are performed in various test liquids, demonstrating that higher-order flexural modes allow for the decoupled measurements of density and viscosity. The measurement differences achieve 0.39% in density and 3.50% in viscosity, and the frequency instability is below 0.05‰. On the basis of these results, design guidelines for piezoelectric higher-mode resonators are proposed for liquid sensing.

Resolving Mechanical Properties and Morphology Evolution of Free‐Standing Ferroelectric Hf0.5Zr0.5O2

Advances in creating polar structures in atomic‐layered hafnia‐zirconia (HfxZr1−xO2) films not only augurs extensive growth in studying ferroelectric nanoelectronics and neuromorphic devices, but also spurs opportunities for exploring novel integrated nanoelectromechanical systems (NEMS). Design and implementation of HfxZr1−xO2 NEMS transducers necessitates accurate knowledge of elastic and electromechanical properties. Up to now, all experimental approaches for extraction of morphological content, elastic, and electromechanical properties of HfxZr1−xO2 are based on solidly mounted structures, highly stressed films, and electroded architectures. Unlike HfxZr1−xO2 layers embedded in electronics, NEMS transducers require free‐standing structures with highly contrasted mechanical boundaries and stress profiles. Here, a nanoresonator‐based approach for simultaneous extraction of Young's modulus and residual stress in free‐standing ferroelectric Hf0.5Zr0.5O2 films is presented. High quality factor resonance modes of nanomechanical resonators created in predominantly orthorhombic Hf0.5Zr0.5O2 films are measured using nondestructive optical transduction. Further, the evolution of morphology during creation of free‐standing Hf0.5Zr0.5O2 structures is closely mapped using X‐ray diffraction measurements, clearly showing transformation of ferroelectric orthorhombic to nonpolar monoclinic phase upon stress relaxation. The extracted Young's modulus of 320.0 ± 29.4 GPa and residual stress of σ = 577.4 ± 24.1 MPa show the closest match with theoretical calculations for orthorhombic Hf0.5Zr0.5O2.

Enhancing higher-order eigenmodes of AFM using bridge/cantilever coupled system

The wide application of multi-frequency atomic force microscopy (AFM) places higher demands on the higher-order modes response of the cantilever. The response of the higher modes however is generally weaker than that of the fundamental mode in air. Researchers have proposed many methods, most of which involve cantilever modification, to enhance higher-order eigenmodes response. These previous results are proved to be effective, but the microfabrication is expensive. In this article, we propose a novel model based on bridge/cantilever coupled system to enhance the higher-order modes response of AFM cantilever. The segmented beam model provides a new thinking to explain the appearance of undesired peaks in mode analysis of cantilever. Through theoretical analysis and simulation, we find that higher resonance modes are enhanced by tuning the bridge to match the high resonances of the single clamped cantilever. The length, thickness of the coupled system and the location of excitation can affect the enhancement. In summary, this model provides a new way to improve higher mode response for multi-frequency and other high bandwidth applications of AFM.

A neural network for prediction of high intensity resonance modes in magnetic multilayers

The use of magnetic materials as building blocks for frequency applications makes it possible to fabricate micrometer and nanometer high frequency devices. Moreover, devices with multiple high intensity modes for multiband devices can be designed by using magnetic multilayers. However, as the number of layers increases the multilayer becomes more complex, making it very difficult to find optimal configurations due to a big number of possible configurations. Fortunately, over the past decade a surge in the applicability and accessibility of machine learning algorithms and neural networks has been observed, which allow to analyse big quantities of data in search of complex patterns not always evident to humans. In this work, a theoretical model is used to generate approximately 10 × 106 data points, which in turn are used to train a neural network to calculate the number of high intensity resonance modes of three ferromagnetically coupled magnetic layers with an accuracy of over 99.8%. The neural network is then used to identify a configuration of the multilayer which provides the maximum number of high-intensity modes, and comparisons with the theoretical model are presented. Finally, the correlations between parameter were calculated over 600 million of data points, and clear guidelines for obtention of two high intensity resonance modes were identified. These results provide a simple way to find a configuration of the trilayer that have a high number of high intensity modes, thus greatly simplifying the design process of magnetic multi-band frequency devices.

Shape Optimization of Rectangular Plates for Vibration-Based Fatigue Testing

Abstract In vibration-based high cycle fatigue testing, a base-excited plate is driven at a high frequency resonant mode until failure. In one vibration-based method involving a cantilevered square plate, a mode often referred to as the “two-stripe” mode is sometimes used because it exists at high frequencies and produces large uniaxial bending stresses along the free edge that are suitable for fatigue testing. The purpose of this work is to precisely investigate how the dimensions of a more generally rectangular plate influence performance when driven at the two-stripe mode. Included are the results of many thousands of modal analysis simulations. From these simulations, general trends with respect to resonant frequencies, frequency isolation, and stress fields in the plate are examined. Results of select geometries were then experimentally validated using a 1000 lb shaker. It is generally shown that, compared with square plates, rectangular plates with 1.37 length-to-width ratio exhibit more favorable stress distributions and frequency isolation. Recommendations are also given for how to quickly select preferable plate dimensions when planning a test based around the operating frequencies of the test setup.

Low-voltage controlling high order plasmonic modes based on graphene/metal electrodes

To improve the compatibility of the nanophotonic device with the electronic device, a kind of hybrid system based on graphene/metal electrode is demonstrated and characterized by both coupled mode theory and numerical simulation. Our results show that, assisted by metallic slits, multiple high order plasmonic resonant modes, including even and odd ones, are efficiently excited in graphene ribbons. The slits perforated in a metal electrode act as magnetic dipole sources and break the structural symmetry in individual graphene ribbons. Due to nano-sized space between graphene and metal gate electrodes, low voltages are required to dynamically control plasmonic modes of such hybrid system. At last, based on Salisbury effect, a low-voltage controlling multi-band absorber are successfully designed. The proposed structure would find potential applications in the field of integrated photonics which are compatible with electronic technology, specifically in low-voltage controlled plasmonic devices such as tunable multiband mid-infrared absorbers, modulators, biosensors, photonic memory cells etc.

Spectral Inversion for Seismic Site Response in Central Oklahoma: Low-Frequency Resonances from the Great Unconformity

ABSTRACTWe investigate seismic site response by inverting seismic ground-motion spectra for site and source spectral properties, in a region of central Oklahoma, where previous ground-motion studies have indicated discrepancies between observations and ground-motion models (GMMs). The inversion is constrained by a source spectral model, which we computed from regional seismic records, using aftershocks as empirical Green’s functions to deconvolve site and path effects. Site spectra across the region exhibit multiple, strong, low-frequency (f&amp;lt;2 Hz) resonances. Modeling of vertically propagating SH waves reproduces the mean amplitudes and frequencies of the site spectra and requires a deep (∼1–2 km) impedance contrast. Comparison of regional seismic velocity models and geologic profiles indicates that the seismic impedance contrast is, or is in proximity to, the Great Unconformity, which marks the interface between Precambrian basement rocks and overlying Paleozoic sedimentary rocks. Depth to Precambrian basement increases to the southwest across the study region (∼1500–4500 m), and the fundamental frequencies of the site spectra are anticorrelated with basement depth. The first higher-mode resonance also exhibits dependence on basement depth; although modeling suggests that the second higher mode should depend on basement depth, site spectra do not support this. The low-frequency resonances in central Oklahoma are not represented in the GMMs used in current seismic hazard analyses for tectonic earthquakes, though approaches to account for such features are under consideration in other regions of the central and eastern United States. Given the broad spatial extent of the Great Unconformity underlying eastern North America, it is likely that similar effects on seismic site response also occur in other areas. This study highlights the impact of regional geologic structure on earthquake ground motions and reiterates the need for modeling regional effects to improve ground-motion predictions and seismic hazard assessments.

Theoretical and numerical investigations on resonant characteristics of two piezoelectric plates coupled with a finite fluid field

The vibration characteristics of two piezoelectric cantilevered plates in air and water are investigated in this study. The two-piezoelectric-plate system can provide different vibration characteristics compared to the previous studies in which only one cantilever plate is used because the anti-phase motion can be evoked in this newly designed system. First, the vibration characteristics of the plates coupled with a bounded compressible inviscid fluid are derived by theoretical analysis. The governing equation of the piezoelectric plate and the superposition method are used to analyze the vibration displacement of the piezoelectric plate, and the acoustic wave equation is used to derive the fluid equation. The frequency eigenfunction of the coupled system is derived by the boundary conditions of hydrostatic equilibrium. After solving the characteristics equation, the resonant frequencies and corresponding mode shapes for assumptions about both compressible and incompressible fluids and the velocity and pressure fields of the fluid can be obtained. Next, finite element analysis (FEA) is used to verify the theoretical results. It is shown from this study that the theoretical predictions of resonant frequencies and the corresponding mode shapes have a good agreement with those of the FEA for water and compressible fluid assumption of air. The unique high pressure value in the outer or middle layer in an air-coupled condition does not occur in water because water’s compressibility is relatively smaller than that of air. The influence of the depth and properties of the fluid on the behavior of the coupled solid–fluid is also investigated. For anti-phase motion, the frequency variation with change in the depth of the middle layer is similar to that of the outer layer for both air- and water-coupled conditions. For in-phase motion, the variation in frequency with the change in the depth of the middle layer is negligible. The particular modes in fluid compounded by modes in vacuum are analyzed on the distortion of mode shapes at higher resonant modes. These results can provide complete information on the vibration characteristics of a solid–fluid coupled system and will contribute to the evolution of hydroacoustic devices in which two piezoelectric plates are used.

Resonance mode control by superposing external sound on the sound in standing-wave type thermoacoustic system

The technique to control the resonance mode necessary for enhancing the energy conversion efficiency of thermoacoustic system is discussed. A standing-wave type thermoacoustic system is set in the condition that the self-excited vibration with 3/2 wavelength of the total tube length takes place by adjusting the position of the stack. The sound wave of the fundamental mode is then externally superposed on the sound wave excited in the system. As a result, the resonance of the system is shifted to the fundamental mode from the higher mode. Furthermore, the resonance control is supposed to be realized by the reason that thickness of the viscous boundary layer is reduced and that the self-excited vibration is enabled at the lower entropy generation comparing with the higher-mode resonance. These results suggest that the technique to use the superposing sound wave is valid for enhancing the energy conversion efficiency by the resonance control.

Second flexural and torsional modes of vibration in suspended microfluidic resonator for liquid density measurements

Suspended microfluidic resonators enable detection of fluid density and viscosity with high sensitivity. Here, a two-legged suspended microchannel resonator that probes pico-litres of liquid is presented. The higher resonant modes (flexural and torsional) were explored for increased sensitivity and resolution. Unlike other reported microchannel resonators, this device showed an increase in the quality factor with resonant frequency value. The performance of the resonator was tested by filling the channel with three liquids, one at a time, over a density range of 779−1110 kg m−3 and a viscosity range of 0.89−16.2 mPa s. The highest resolution obtained was 0.011% change in density. Measurements with torsional mode showed an improvement of about six times in sensitivity and about fifteen times in resolution compared to the first flexural mode. When the empty channel was filled with liquids of different viscosity, the quality factor of the first flexural mode remained overall constant with a variation below 3.3% between the fluids, and confirming the inherent property of suspended microchannel resonators. However, it significantly decreased for second flexural and torsional modes. No noticeable difference was observed in the quality factor between different liquid viscosities for all modes.

Infrared localized surface plasmon dark multipole modes generated on subwavelength corrugated metal disks

Terahertz (THz) radiation is very promising for chemical and biological sensing. One of the ways to realize a THz sensor is exciting multipole localized surface plasmon resonances (LSPRs) on subwavelength corrugated metal disks. In this paper, numerical modeling and analysis of LSPRs on a periodic array of such structures were made. At normal incidence of the incoming wave, only dipole resonances can be excited. An additional C-shaped resonator placed in the vicinity of the disk produces dark multipole modes (quadrupole, hexapole, octupole, and decapole ones) due to hybridization of these modes with the brigth dipole mode of the C resonator. Besides, multipole modes can be excited at oblique illumination of the structure. With increase in the incident angle, the multipole resonances become stronger. The high Q-factor resonance modes observed for PEC disks are wider or disappeared in case of Drude gold permittivity.

Rotated fourfold U-shape metasurface for polarization-insensitive strong enhancement of mid-infrared photodetection.

Metasurface with thin planar resonant elements offers great capability in manipulating electromagnetic waves and their interaction with semiconductors. Split-ring resonator (SRR), as the basic building block, has been extensively investigated for myriad applications owing to its multiple electric and magnetic resonant modes. In this work, we report a rotated fourfold U-shape SRR metasurface for polarization-insensitive strong enhancement of mid-infrared photodetection. The integrated photodetector consists of a rotated fourfold SRR array and an InAsSb based heterojunction photodiode. A photosensitivity enhancement factor as high as 11 has been achieved by adoption of superimposed high order magnetic and electric resonant modes in the SRR metasurface. This work provides a promising pathway for exploring high performance polarization-insensitive photodetection in different electromagnetic wave ranges.