High Frequency Modes Research Articles

Active nonreciprocal metamaterial using a spatiotemporal modulation control strategy

A class of active nonreciprocal metamaterial (ANMM) is presented which consists of an acoustic duct with periodically placed active diaphragms that are controlled by a spatiotemporal modulation strategy. The acoustic nonreciprocities can be realized by modulating a system's properties spatiotemporally. Such an approach has been extensively employed by many investigators to break the reciprocity in acoustic and elastic metamaterials. However, our proposed ANMM distinguishes itself from the above-mentioned methods by introducing actively tunable space-time modulated feedback gain of the controllers. The controller is implemented in an analog manner to enable fast response at high modulation frequencies. By discretizing a 1D acoustic duct into multiple acoustic unit cavities, we introduced a time-varying gain with a phase difference between adjacent acoustic cavities. Directional band gaps of the modulated system are numerically analyzed as the asymmetric acoustic wave propagation can be realized by converting the acoustical energy from the fundamental mode to higher order modes. In addition, nonreciprocal behavior of the proposed ANMM was experimentally demonstrated using a waveguide with periodically placed condenser microphones (sensors) and speakers (actuators).

Can modeling soil heterogeneity in 2D site response analyses improve predictions at vertical array sites?

This study uses a database of 21 vertical borehole arrays in California to examine whether a two-dimensional (2D) site response analysis framework that accounts for soil heterogeneity via spatially correlated random fields can explain misfits observed from prior one-dimensional (1D) ground response modeling. The main hypothesis is that the overprediction of ground motion at site modal frequencies, consistently observed in many site response validation studies, is caused by soil heterogeneity and 2D/three-dimensional (3D) wave propagation effects that cannot be captured by 1D analyses. We apply classical “within” boundary conditions for borehole input motion along with equivalent incident wave motions derived using a framework developed here to help elucidate the effects of the down-going wave on observed fundamental mode resonances. Results from 2D and 1D analyses are compared to observations using a transfer-function-based taxonomy and residuals of other intensity measures (IMs) including response spectra. The uncertainty in predicted IMs is estimated from the many realizations of 2D models. This 2D approach was found capable of scattering seismic waves and producing transfer function variability resembling the observed event-to-event variability in empirical transfer functions (ETFs). For several sites that exhibit less down-going wave effects in ETFs (i.e. flatter peaks) and/or higher variability in ETFs, median transfer functions from 2D analyses provide a significantly better estimate of the median ETF than conventional 1D deterministic analyses, especially at fundamental modes. In contrast, for some of the sites that are well represented by 1D methods (e.g. Wildlife Liquefaction Array and Treasure Island), 2D methods with generic levels of spatial variability may over-represent the heterogeneity and consequently underpredict amplifications at higher mode frequencies.

High bandwidth frequency modulation of an external cavity diode laser using an intracavity lithium niobate electro-optic modulator as output coupler

We present a novel approach to high bandwidth laser frequency modulation. A lithium niobate chip is used as an intracavity electro-optic modulator in a tunable cateye external cavity diode laser. The modulator is conveniently integrated with the cateye output coupler, providing a unique approach to high bandwidth frequency stabilization and linewidth narrowing. The intracavity modulator feedback was successfully operated below 1 V and achieved superior frequency noise suppression compared to conventional feedback through diode injection current modulation. A closed loop bandwidth of 1.8 MHz was demonstrated, and the laser linewidth reduced to around 1 Hz as measured by the heterodyne measurement.

Rapidly frequency-tuneable, in-vacuum, and magnetic levitation chopper for fast modulation of infrared light.

We present an in-vacuum mechanical chopper running at high speed and integrated into a magnetic levitating motor for modulating optical beams up to 200kHz. The compact chopper rotor allows fast acceleration (10kHzs-1 as standard) for rapid tuning of the modulation frequency, while 1mm diameter slots provide high optical throughput for larger infrared beams. The modulation performances are assessed using a reference visible laser and the high brightness, broadband, infrared (IR) beam of synchrotron radiation at the MIRIAM beamline B22 at Diamond Light Source, UK. For our application of IR nanospectroscopy, minimizing the temporal jitter on the modulated beam due to chopper manufacturing and control tolerances is essential to limit the noise level in measurements via lock-in detection, while high modulation frequencies are needed to achieve high spatial resolution in photothermal nanospectroscopy. When reaching the maximum chopping frequency of 200kHz, the jitter was found to be 0.9% peak-to-peak. The described chopper now replaces the standard ball-bearing chopper in our synchrotron-based FTIR photothermal nanospectroscopy system, and we demonstrate improved spectroscopy results on a 200nm thickness polymer film.

High-frequency modes in a magnetic buckyball nanoarchitecture

Artificially fabricated three-dimensional magnetic nanostructures have recently emerged as a new type of magnetic material with the potential of displaying physical properties absent in thin-film geometries. Interconnected nanowire arrays yielding three-dimensional versions of artificial spin-ices are of particular interest within this material category. Despite growing interest in the topic, several properties of these systems are still unexplored. Here, we study, through micromagnetic simulations, the high-frequency dynamic modes developing in buckyball-type magnetic nanoarchitectures. We obtain a characteristic excitation spectrum and analyze the corresponding mode profiles and their magnetic field dependence. The magnetic resonances are localized at different geometric constituents of the structure and depend on the local magnetic configuration. These features foreshow the potential of such systems for reprogrammable magnonic device applications with geometrically tunable frequencies.

Simultaneous Phase Detection of Multi-Wavelength Interferometry Based on Frequency Division Multiplexing

Simultaneous phases detection in multi-wavelength interferometry (MWLI) is essential for high-accuracy dynamic absolute distance measurement in high-end equipment manufacturing. To address this problem, a simultaneous phase detection scheme based on frequency-division multiplexing (FDM) technique is proposed in this paper. High-frequency sinusoidal phase modulation with non-overlapping frequencies were applied to multiple lasers individually using electro-optic modulators (EOMs), and interference phases of multiple wavelengths were extracted in parallel using phase generated carrier (PGC) demodulation technique from the FDM interference signal. Principles of FDM multi-wavelength interferometry and simultaneous phase detection scheme were illustrated. A FDM dual-wavelength interferometer was constructed to verify the performance of the proposed phase detection scheme by simulation analysis and experiments. Results of simulations show that the resolution, accuracy and nonlinear error of the two demodulated phases are all better than 0.01°. Experiments of nanometer displacement measurement further verified the good performance of the phase detection scheme and its feasibility in dynamic measurement.

Improved Uniform Error Bounds on Time-Splitting Methods for Long-Time Dynamics of the Nonlinear Klein--Gordon Equation with Weak Nonlinearity

We establish improved uniform error bounds on a second-order Strang time-splitting method which is equivalent to an exponential wave integrator for the long-time dynamics of the nonlinear Klein--Gordon equation (NKGE) with weak cubic nonlinearity, whose strength is characterized by $\varepsilon^2$ with $0 < \varepsilon \leq 1$ a dimensionless parameter. Actually, when $0 < \varepsilon \ll 1$, the NKGE with $O(\varepsilon^2)$ nonlinearity and $O(1)$ initial data is equivalent to that with $O(1)$ nonlinearity and small initial data, the amplitude of which is at $O(\varepsilon)$. We begin with a semidiscretization of the NKGE by the second-order time-splitting method and derive a full-discretization by the Fourier spectral method in space. Employing the regularity compensation oscillation technique which controls the high frequency modes by the regularity of the exact solution and analyzes the low frequency modes by phase cancellation and energy method, we carry out the improved uniform error bounds at $O(\varepsilon^2\tau^2)$ and $O(h^m+\varepsilon^2\tau^2)$ for the second-order semidiscretization and full-discretization up to the long time $T_\varepsilon = T/\varepsilon^2$ with $T$ fixed, respectively. Extensions to higher-order time-splitting methods and the case of an oscillatory complex NKGE are also discussed. Finally, numerical results are provided to confirm the improved error bounds and to demonstrate that they are sharp.

Modulated dilatometry as a tool for simultaneous study of vacancy formation and migration

Abstract A model is presented to derive both vacancy formation and migration characteristics from length change measurements upon modulated time-linear heating. The length variation with linear heating yields access to the equilibrium concentration of thermal vacancies. The modulation amplitude and the phase shift between modulated temperature and length change is determined by the ratio of equilibration rate and modulation frequency which yields access to the vacancy migration characteristics. The contribution from thermal lattice expansion is obtained from a reference measurement at high modulation frequencies. Compared to static isothermal equilibration measurements after temperature jumps, the processes are monitored under quasi-equilibrium conditions avoiding obstacles associated with fast temperature changes. Furthermore, in contrast to the static isothermal case where the equilibration rate is obtained from the time-exponential decay, its determination from the amplitude and phase shift of modulation offers higher precision. The method is suitable for materials with high thermal vacancy concentrations and low vacancy diffusivities, among which is the important class of intermetallic compounds with B2-structure.

An Experimental Approach for the Determination of the Mechanical Properties of Base-Excited Polymeric Specimens at Higher Frequency Modes

Structures made of the thermoplastic polymer polyether ether ketone (PEEK) are widely used in dynamically-loaded applications due to their high-temperature resistance and high mechanical properties. To design these dynamic applications, in addition to the well-known stiffness and strength properties the vibration-damping properties at the given frequencies are required. Depending on the application, frequencies from a few hertz to the ultrasonic range are of interest here. To characterize the frequency-dependent behavior, an experimental approach was chosen and applied to a sample polymer PEEK. The test setup consists of a piezoelectrically driven base excitation of the polymeric specimen and the non-contact measurement of the velocity as well as the surface temperature. The beam’s bending vibrations were analyzed by means of the Timoshenko theory to determine the polymer’s storage modulus. The mechanical loss factor was calculated using the half-power bandwidth method. For PEEK and a considered frequency range of 1 kHz to 16 kHz, a storage modulus between 3.9 GPa and 4.2 GPa and a loss factor between 9 × 10−3 and 17 × 10−3 were determined. For the used experimental parameters, the resulting mechanical properties were not essentially influenced by the amplitude of excitation, the duration of excitation, or thermal degrad.ation due to self-heating, but rather slightly by the clamping force within the fixation area.

From Networked to Isolated: Observing Water Hydrogen Bonds in Concentrated Electrolytes with Two-Dimensional Infrared Spectroscopy.

Superconcentrated electrolytes have emerged as a promising class of materials for energy storage devices, with evidence that high voltage performance is possible even with water as the solvent. Here, we study the changes in the water hydrogen bonding network induced by the dissolution of lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) in concentrations ranging from the dilute to the superconcentrated regimes. Using time-resolved two-dimensional infrared spectroscopy, we observe the progressive disruption of the water-water hydrogen bond network and the appearance of isolated water molecules interacting only with ions, which can be identified and spectroscopically isolated through the intermolecular cross-peaks between the water and the TFSI- ions. Analyzing the vibrational relaxation of excitations of the H2O stretching mode, we observe a transition in the dominant relaxation path as the bulk-like water vanishes and is replaced by ion-solvation water with the rapid single-step relaxation of delocalized stretching vibrations into the low frequency modes being replaced by multistep relaxation through the intramolecular H2O bend and into the TFSI- high frequency modes prior to relaxing to the low frequency structural degrees of freedom. These results definitively demonstrate the absence of vibrationally bulk-like water in the presence of high concentrations of LiTFSI and especially in the superconcentrated regime, while additionally revealing aspects of the water hydrogen bond network that have been difficult to discern from the vibrational spectroscopy of the neat liquid.

Evaluation of SAR and Optical Image Fusion Methods in Oil Palm Crop Cover Classification Using the Random Forest Algorithm

This paper presents an evaluation of land cover accuracy, particularly regarding oil palm crop cover, using optical/synthetic aperture radar (SAR) image fusion methods through the implementation of the random forest (RF) algorithm on cloud computing platforms using Sentinel-1 SAR and Sentinel-2 optical images. Among the fusion methods evaluated were Brovey (BR), high-frequency modulation (HFM), Gram–Schmidt (GS), and principal components (PC). This work was developed using a cloud computing environment employing R and Python for statistical analysis. It was found that an optical/SAR image stack resulted in the best overall accuracy with 82.14%, which was 11.66% higher than that of the SAR image, and 7.85% higher than that of the optical image. The high-frequency modulation (HFM) and Brovey (BR) image fusion methods showed overall accuracies higher than the Sentinel-2 optical image classification by 3.8% and 3.09%, respectively. This demonstrates the potential of integrating optical imagery with Sentinel SAR imagery to increase land cover classification accuracy. On the other hand, the SAR images obtained very high accuracy results in classifying oil palm crops and forests, reaching 94.29% and 90%, respectively. This demonstrates the ability of synthetic aperture radar (SAR) to provide more information when fused with an optical image to improve land cover classification.

Prediction and validation of the strongly modulated forced response of two beams undergoing frictional impacts

We consider two cantilevered beams undergoing frictional impacts at the free end. The beams are designed to be of similar geometry so that they have distinct but close natural frequencies. Under harmonic base excitation near the primary resonance with the higher-frequency fundamental bending mode, the system shows a strongly modulated non-periodic response. The purpose of this work is to analyze to what extent the non-periodic vibro-impact dynamics can be predicted. To this end, we use a recently developed modeling and simulation approach. The approach relies on component mode synthesis, the massless boundary concept and an appropriate time stepping scheme. Unilateral contact and dry friction are modeled as set-valued laws and imposed locally within the spatially resolved contact area. A linear model updating is carried out based on the natural frequencies and damping ratios identified in the regime without impacts. The nonlinear simulation of the steady-state response to forward and backward stepped sine excitation is compared against measurements. The results are in very good agreement, especially in the light of the uncertainty associated with the observed material loss in the contact region and the nonlinear behavior of the clamping.

Choices of the Critical Frequency for φ in TL-φ Algorithms When Applied to Multi-Degree of Freedom Systems

TL-φ algorithms are newly developed explicit structure-dependent integration algorithms utilized for solving the temporally discretized equations of motion. In contrast to the existing algorithms, the most significant improvement of TL-φ algorithms is in diminishing the amount of period errors by introducing a precorrection coefficient φ into the integration parameters of TL-φ algorithms, which is related to the critical frequency of a system. In the previous work, the fundamental frequency of the system is deemed to be the critical frequency, so that φ is a constant scaling corresponding to the fundamental frequency for both single-degree-of-freedom (SDOF) and multi-degree-of-freedom (MDOF) systems. However, for a MDOF system, the first mode may not contribute to the total response more than other ones under a given external excitation, calculating φ only by the fundamental frequency will underestimate the contribution of the higher-frequency modes to structural dynamics. In this paper, choices of the critical frequency for φ when applying TL-φ algorithms to MDOF systems are investigated thoroughly. By considering the initial structural properties of the system and the frequency characteristics of the external excitation simultaneously, a calculation criterion of φ for MDOF systems under specific external excitations is proposed. Four numerical examples with different initial structure properties and loading conditions are designed, and the results demonstrate that the proposed criterion can be potentially used to solve structural dynamic problems of MDOF systems with a more desirable numerical dispersion performance.

Optical amplification in a charge density wave phase of a quasi-two-dimensional material

We theoretically study the quenching of a charge-density wave phase in a model system of a quasi-two-dimensional material, which includes both electron-hole and electron-phonon interactions leading, respectively, to excitonic and coherent-phonon contributions. We discuss how interaction processes affect anomalous expectation values and present a microscopic dynamical picture of the quenching of the phase. We use projection techniques to illustrate the time-dependent appearance of additional bands and anomalous expectation values. We propose an optical amplification effect with a high modulation frequency in the midinfrared regime.

Regularized variational formulation for nonlinear dynamics of viscoplastic plates

In this work we present novel approach to Reissner–Mindlin plates, targeting the nonlinear dynamics applications and in particular robust scheme for improved performance in vibration and wave propagation computations, where the standard finite element isoparametric interpolations no longer provide the best approximation property. The first key ingredient of the present development is regularized variational formulation, recast in terms of stress resultants (bending moments and shear forces), which allows any convenient interpolations. The discrete approximation is here constructed by using the lowest order Raviart–Thomas vector space employed to discretize bending moment and shear force fields, providing the continuity of the stress resultants projections across the boundary between adjacent plate elements. The latter is of particular interest for dynamic analysis for it can capture smooth approximation for wave propagation phenomena. This particular space discrete approximation is further combined with the most appropriate time-integration schemes capable of enforcing either energy conservation or decay of high frequency modes, which delivers the schemes with higher stability and robustness in long-term computations. The developments are carried out for viscoplastic constitutive behavior of plate material, where the corresponding evolution equations are obtained by appealing to the penalty-like form of the principle of maximum plastic dissipation. The choice of viscoplasticity (rather than plasticity) is crucial in that allows development of second-order scheme by integrating simultaneously the momentum balance and viscoplastic strains evolution equations. Several numerical simulations are given to illustrate a very satisfying performance of the proposed hybrid-stress discrete approximation for thick plate element and of the energy conserving/decaying schemes.

Effects of substitutional Ga on the phonons of δ -phase Pu based on density functional theory calculations

The delta phase of Pu is stabilized by Ga doping, but the mechanism of this stabilization remains an open question. Density functional theory calculations focused on how Ga doping affects the phonons sheds some light on the phonons' contribution to the stabilization. The calculated phonon modes of Ga-doped delta phase Pu fall into two distinct types: localized, high frequency Ga-dominated phonon modes, and Pu-dominated modes at lower frequencies. Increasing the Ga concentration has an effect on the Pu-dominated phonon modes opposite to that of compression: higher-frequency modes soften, and lower-frequency modes stiffen. The latter provides an indication that the stabilization mechanism is not due to a thermodynamic contribution from the phonons. Furthermore, the stiffened phonon modes include candidate modes that describe possible pathways into low-temperature phases, suggesting that doping with Ga could impede such pathways.

Modal Analysis of Separation Bubble Unsteadiness in Conical Shock Wave/Turbulent Boundary Layer Interaction

The frequency properties and the typical flow features within the interaction zone in conical shock-wave/turbulent boundary-layer interactions (CSBLIs) are analyzed by performing proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) on a time-resolved direct numerical simulation database. The streamwise velocity and pressure fluctuations are processed according to their energy and frequency, respectively. As is the case of planar shock-wave/boundary-layer interactions, fluctuations in some bands of frequencies are more energetic downstream of the flow separation zone. The energetic low-frequency modes of the streamwise velocity fluctuations are reminiscent of Görtler-like vortices, whereas those of pressure fluctuations are connected with the “breathing” motion of the separation bubble. Higher-frequency modes, on the other hand, are related to wrinkling of the impinging conical shock. Although they bear resemblance with those in planar shock/turbulent boundary-layer interactions, it was found that the flow structures herein are less coherent owing to spanwise inhomogeneity of the impinging shock strength. The POD and DMD results of CSBLI support the notion that Görtler-like vortices might be an intrinsic mechanism of the interaction zone, which is probably responsible for the low-frequency unsteadiness of CSBLI. The present results may serve as a basic guidance for the design of fluidic control strategies of shock-induced flow separation.

二量体分子における強誘電性スメクチック-A相の誘電緩和挙動

The dielectric relaxation behavior is investigated in the ferroelectric smectic-A phase, which is a mixture of schiff base dimeric molecules with different alkyl chain lengths. The ferroelectric smectic-A phase shows the Goldstone mode due to its high dielectric strength (relative permittivity excluding the contribution of ion diffusion in the low frequency region). The dielectric strength increases dramatically with increasing the cell thickness. By applying the DC bias fields, the Goldstone mode of the ferroelectric smectic-A phase is quenched, and this phenomenon is explained by the suppression of the dipoles' fluctuation due to the DC bias fields. The Goldstone mode and the high frequency mode of the SmAPF phase follow the standard Arrhenius equation in the vicinity of the phase transition with the isotropic phase.

STAT4 and COL1A2 are potential diagnostic biomarkers and therapeutic targets for heart failure comorbided with depression

BackgroundHeart failure (HF) and depression are common disorders that markedly compromise quality of life and impose a great financial burden on the society. Although increasing evidence has supported the closely linkage between the two disorders, the comorbidity mechanisms remain to be fully illuminated. We performed a bioinformatics network analysis to understand potential diagnostic biomarkers and therapeutic targets for HF comorbided with depression. MethodsWe downloaded the datasets of HF and depression from the Gene Expression Omnibus (GEO) database and constructed co-expression networks by Weighted Gene Co-Expression Network Analysis (WGCNA) to identify key modules. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis were performed on the common genes existing in the HF and depression related modules. Then, we employed the STRING database to construct the protein-protein interaction (PPI) network and detected the hub genes in the network. Finally, we validated the expression difference of hub genes from additional datasets of HF and depression. ResultsFunctional enrichment analysis indicated that platelet activation, chemokine signaling and focal adhesion were probably involved in HF comorbided with depression. PPI network construction indicated that HF comorbided with depression is likely related to 5 hub genes, including STAT4, CD83, CX3CR1, COL1A2, and SH2D1B. In validated datasets, STAT4 and COL1A2 were especially involved in the comorbidity of HF and depression. ConclusionOur work indicated a total of 5 hub genes including STAT4, CD83, CX3CR1, COL1A2, and SH2D1B, in which STAT4 and COL1A2 especially underlie the comorbidity mechanisms of HF and depression. These shared pathways might provide new targets for further mechanistic studies of the pathogenesis and treatment of HF and depression.

Analyzing the Potential Impact of Docked Bikeshare on Transit First-Last Mile Accessibility in East Los Angeles

We model how docked bikeshare can improve transit accessibility in two Los Angeles neighborhoods. We measure the fraction of study area population within 5- and 10-minute walk/bike commutes from high frequency transit (≤ 10-minute morning peak headway). We show that bikeshare can increase the number of residents within 5-minutes / 10-minutes of high frequency transit by 12% / 13% (deploying 74 docked bikeshare stations) and by 42% / 66% (deploying 296 bikeshare stations.) The results for within the 5-minute travel time are robust to siting specification changes, while the 10-minute results are more sensitive to specification.