Antisymmetric Modes Research Articles

Ultraprecise relative energies in the (2 0 0) vibrational band of H2 16O

The technique of Noise-Immune Cavity Enhanced Optical Heterodyne Molecular Spectroscopy (NICE-OHMS) is employed to detect rovibrational transitions of at wavelengths of 1.4 m. This intracavity-saturation approach narrows down the typical Doppler-broadened linewidths of about 600 MHz to the sub-MHz domain. The locking of the spectroscopy laser to a frequency-comb laser and the assessment of collisional and further line broadening effects result in transition frequencies with an absolute uncertainty below 10 kHz. The lines targeted for measurement are selected by the spectroscopic-network-assisted precision spectroscopy (SNAPS) approach. The principal aim is to derive precise and accurate relative energies from a limited set of Doppler-free transitions. The 71 newly observed lines, combined with further highly accurate literature transitions, allow the determination of the relative energies for all of the 59 rovibrational states up to J = 6 within the vibrational parent of , where J is the overall rotational quantum number and , , and are quantum numbers associated with the symmetric stretch, bend, and antisymmetric stretch normal modes, respectively. An experimental curiosity of this study is that for strong transitions an apparent signal inversion in the Lamb-dip spectra is observed; a novelty reserved to the NICE-OHMS technique.

Bound states in the continuum from a symmetric mode with a dominant toroidal dipole resonance

Bound states in the continuum (BICs), both the symmetry-protected and accidental ones, can be realized in the band of antisymmetric mode in a nanoparticle chain as long as the particles radiate exactly out of phase pairwisely. However, for the symmetric mode, BICs cannot form in this way. Here, we show that accidental BICs can be generated from the symmetric mode, as exemplified by a toroidal dipole mode-induced BIC in a trimerized system. This type of BIC is fully studied by an eigenmode analysis as well as multipole decomposition. It is also demonstrated that the BICs correspond to certain phase singularity of the system, indicating the robust existence of the BICs ensured by their topological nature. The combination of BICs and toroidal dipole resonance may find applications in lasing and subwavelength waveguiding.

Dipolar optical plasmon in thin-film Weyl semimetals

In a slab geometry with large surface-to-bulk ratio, topological surface states such as Fermi arcs for Weyl or Dirac semimetals may dominate their low-energy properties. We investigate the collective charge oscillations in such systems, finding striking differences between Weyl and conventional electronic systems. Our results, obtained analytically and verified numerically, predict that time-reversal symmetry-broken Weyl semimetal thin films will host a single $\ensuremath{\omega}\ensuremath{\propto}\sqrt{q}$ plasmon mode that results from collective, antisymmetric charge oscillations between the two surfaces, in stark contrast to conventional 2D bilayers as well as Dirac semimetals with Fermi arcs, which support antisymmetric acoustic modes along with a symmetric optical mode. These modes lie in the gap of the particle-hole continuum and are thus spectroscopically observable and potentially useful in plasmonic applications.

Imaging coherent phonons and precursor dynamics in LaFeAsO with 4D ultrafast electron microscopy

We used 4D ultrafast electron microscopy (UEM) to directly image femtosecond photoinduced structural dynamics in single-crystal LaFeAsO at initial temperatures of 300 and 100 K, above and below the known structural and magnetic phase-transition temperatures, respectively. With nanometer-picosecond resolution, we resolved an initial (precursor) sigmoidlike response arising from photothermal expansion and lattice reorientation that precedes the onset of propagating coherent acoustic phonons (CAPs). In the specific regions probed, the precursor response at 100 K is shorter than at 300 K (${t}_{0.5;100\phantom{\rule{0.16em}{0ex}}\mathrm{K}}=11.3\phantom{\rule{0.16em}{0ex}}\mathrm{ps}$ vs ${t}_{0.5;300\phantom{\rule{0.16em}{0ex}}\mathrm{K}}=17.8\phantom{\rule{0.16em}{0ex}}\mathrm{ps}$), and the CAP oscillation frequency is lowered with cooling (${f}_{\mathrm{CAP};100\phantom{\rule{0.16em}{0ex}}\mathrm{K}}=12\phantom{\rule{0.16em}{0ex}}\mathrm{GHz}$ vs ${f}_{\mathrm{CAP};300\phantom{\rule{0.16em}{0ex}}\mathrm{K}}=21\phantom{\rule{0.16em}{0ex}}\mathrm{GHz}$), correlated to known lattice softening due to the structural phase change. The transient CAP behaviors at 300 K are dispersive, displaying an exponentially decaying phase velocity over the first nanosecond. Further, the CAP symmetry at 300 K matches a first-order antisymmetric shear mode (${A}_{1}$), while at 100 K it is best matched by a mostly nondispersive zero-order symmetric mode (${S}_{0}$). These findings illustrate the sensitivity of UEM imaging to spatially heterogeneous dynamics in the Fe-pnictide materials and more broadly in other quantum materials.

Nonlinear vibrations of a beam with non-ideal boundary conditions and subjected to two correlated or uncorrelated broadband random excitations - experiments, modeling and simulations

This study presents experiments and numerical simulations of a beam with non-ideal boundary conditions and subjected to two broadband correlated or uncorrelated random excitations. Modeling of a beam with non-ideal boundary conditions and additional static pretension, as well as the modeling of multipoint correlated or uncorrelated random excitations, are developed. Based on numerical simulations via an extension of the Harmonic Balance Method, comparisons between experiments and numerical simulations are performed for various broadband bi-point correlated random excitations. In order to achieve such an objective, experiments are performed for four types of correlation between the two broadband random excitations: correlated excitations in phase, opposite phase and quadrature phase, and uncorrelated excitations. Experimental results demonstrate that the different configurations of broadband random excitations drastically affect, not only the amplification or attenuation of some symmetric or anti-symmetric modes of the beam in the vicinity of their primary resonance, but also the appearance of harmonics and a combination of harmonic components. Good correlations are observed between experiments and numerical simulations, thus validating the proposed modeling and the computational strategy for the prediction of the nonlinear vibrational phenomena of the beam system subjected to two correlated or uncorrelated broadband random excitations.To be noted that the experimental data set including the input and output measurements of the four configurations are provided as supplementary files on Talik et al. [1].

3D intra-cellular wave dynamics in a phononic plate with ultra-wide bandgap: attenuation, resonance and mode conversion

The intra-cellular wave dynamics of a water jetted phononic plate are experimentally investigated by means of high-resolution three-dimensional (3D) scanning laser Doppler vibrometry. The study is focused on the vibrational behavior around the ultra-wide bandgap of the plate (with a relative bandgap width of 0.89), as the critical frequency range of its phononic functionality. Broadband vibrational excitations are applied using a piezoelectric transducer and both in-plane and out-of-plane operational deflection shapes of the unit-cells are analyzed with respect to mode shapes calculated by finite element (FE) simulation. Attenuation and resonance of both symmetric and antisymmetric wave modes are validated, and it is shown that despite the absence of in-plane wave energy actuation, the symmetric modes are effectively excited in the phononic lattice, due to mode conversion from co-existing antisymmetric modes. Supported by FE modal analysis, this mode conversion observation is explained by the slight through-the-thickness asymmetry introduced during manufacturing of the phononic plate which leads to coupling of modes with different symmetry. The results confirm the potential of such detailed 3D inspection of phononic crystals (and in general acoustic metamaterials) in gaining full insight about their intracellular dynamics, which can also illuminate discrepancies with respect to idealized numerical models that might be due to manufacturing imperfections.

Van Allen Probes Observations of Symmetric Stormtime Compressional ULF Waves.

Previous spacecraft studies showed that stormtime poloidal ultralow‐frequency (ULF) waves in the ring current region have an antisymmetric (second harmonic) mode structure about the magnetic equator. This paper reports Van Allen Probes observations of symmetric ULF waves in the postnoon sector during a moderate geomagnetic storm. The mode structure is determined from the presence of purely compressional magnetic field oscillations at the equator accompanied by strong transverse electric field perturbations. Antisymmetric waves were also detected but only very late in the recovery phase. The symmetric waves were detected outside the plasmasphere at L = 3.0–5.5 and had peak power at 4–10 mHz, lower than the frequency of the local fundamental toroidal standing Alfvén wave. During the wave events, the flux of protons was enhanced at energies below ∼5 keV, which appears to be a prerequisite for the waves. The protons may provide free energies to waves through drift resonance instability or drift compressional instability, which occur in the presence of radial gradients of plasma parameters.

Crossover/Veering in V-Shaped MEMS Resonators

We investigate experimentally and theoretically mode crossing and mode veering between the first two symmetric and anti-symmetric modes of an electrothermally and electrostatically actuated in-plane V-shaped micro-beam. The micro-beam is sandwiched between four electrodes to electrostatically activate both modes of vibrations and to offer various actuation options. When tuning the electrothermal voltage, the natural frequencies of the two modes approach each other until they cross. Under an asymmetrical electrostatic actuation, it is shown experimentally that the system undergoes a transition between modes crossing to veering. In addition, an analytical study is presented based on a Galerkin-based reduced-order model of a nonlinear Euler–Bernoulli shallow arch beam equation. The analytical results are compared to experimental data showing excellent agreement. The effects of the shape profile, Cosine and Arc, and micro-beam length are theoretically investigated on the transition between the crossing and veering behavior for the first four vibration modes. [2021-0223]

Internal damages detection for structural timber members using low-frequency anti-symmetric guided wave

Structural timber is one of the commonly used construction materials. Timber can contain natural internal defect such as knot or natural decay due to its anatomical complexity. Moreover, internal damages or stiffness reductions can also be induced by environmental and biological factors such as weathering and termite attacks. This hidden internal damage increases the difficulty of damage detection using conventional non-destructive testing (NDT) methods. Ultrasonic guided wave (GW) damage detection technique is one of the promising damage detection techniques, which can be employed to achieve an effective and robust damage inspection in timber. However, limited attention has been paid to the use of GW for damage detection in timber, due to the material anisotropy and inhomogeneity. This paper assesses the capability of GW in detecting different sizes of the internal damages in a structural red oak timber using the fundamental anti-symmetric mode (A0) of GW. Measured GW signals in forward and backward scattering directions are used to calculate the reflection and transmission ratios for different sizes of internal damages. A series of comprehensive experimental and numerical parametric studies are carried out using three-dimensional (3D) finite element (FE) simulations. Good agreement is obtained between numerical and experimental results. The experimentally verified FE model is utilized to further investigate the wave reflection and transmission phenomena from different characteristics of internal damages, such as different lengths, widths, thicknesses, and through thickness locations. The outcomes of this study demonstrate the robustness of GW technique in detecting conspicuous internal damages in structural timber. It demonstrates the feasibility of quantitative assessments of internal damages in timber using A0 GW.

Flexural wave bandgap properties in metamaterial dual-beam structure

In this paper, a metamaterial dual-beam structure is firstly constructed by periodically adding pillars between two identical homogeneous beams. By simplifying the pillar as an equivalent spring-mass-spring system, another metamaterial dual-beam structure carrying periodic spring-mass-spring resonators is then established. This equivalent representation method is validated through both a vibration transmission analysis and a band structure analysis. Two bands corresponding to the antisymmetric and symmetric flexural vibration modes respectively open a flexural narrow bandgap in the low frequency range. Then, the improved structure with high-static-low-dynamic stiffness (HSLDS) resonators is proposed in the need of widening the bandgap and lowering the beginning frequency. Results show that with the decrease of the proportion of the residual stiffness, the beginning frequency of the bandgap can be reduced, while the ending frequency keeps unchanged. This results in a relative bandgap as high as 1.465, which is about 4.4 times of the original structure.

Fast PSP measurement of three-dimensional low-frequency unsteadiness in sidewall-confined shock wave/turbulent boundary layer interaction

The presence of sidewalls exerts three-dimensional (3D) effects on shock wave/turbulent boundary layer interaction (SBLI), which results in experimental challenges due to the highly unsteady flow with complex 3D patterns. The low-frequency large-scale flow patterns of a 24° sidewall-confined compression ramp interaction were investigated using fast pressure-sensitive paint. The experiment was conducted in a Mach 2.5 wind tunnel, and the unit Reynolds number was Re = 1.12 × 107/m. The sampling rate of the fast pressure-sensitive paint was 4 kHz, and the spatial resolution was 0.175 mm/pixel. In-situ calibration via several pressure transducers was used to determine the time-resolved global pressure distributions. Correlation analysis and proper orthogonal decomposition analysis were used to extract the large-scale unsteady flow patterns. The time-averaged results provide a global view of the typical flow structure, which consists of primary interaction zone and corner interaction zones. The time-resolved results show the asymmetry of the instantaneous pressure distribution induced by the sidewalls, and spanwise differences of the pressure fluctuation amplitudes in the interaction zones are observed. In addition to the common streamwise breathing mode, an interesting spanwise antisymmetric mode is revealed that provides clear evidence of the strong 3D effects in sidewall-confined SBLI.

Control of roughness-induced transition under the influence of inflow disturbance

Control of roughness-induced transition influenced by two- or three-dimensional inflow disturbances using spanwise-uniform wall blowing is investigated from the perspective of unstable modal growth with direct numerical simulations. Visualization of flow structures for two-dimensional disturbances indicates that the counter-rotating vortex pair in the wake bends the high-shear layer, which is related to both symmetric and antisymmetric unstable modes. The interaction of these structures is found to be responsible for the initial growth of the symmetric mode, which dominates the transition process. Both upstream and downstream position wall blowing are effective for transition control. Upstream position blowing works by lifting up the boundary layer and diminishing the wake structures, resulting in inhibition of the symmetric mode. Downstream position blowing has an even better effect, attributable to reconfiguration of the streamwise vorticity field in the wake and further distortion of the high-shear layer. When the inflow disturbance is a three-dimensional wave, the antisymmetric mode becomes dominant in the near-wake region, resulting in more rapid initial growth of the symmetric mode. As oblique waves have higher growth rates in a compressible boundary layer, the background disturbance is obviously stronger. Therefore, transition occurs earlier than for two-dimensional disturbances. When upstream position blowing is introduced, the spanwise shear related to the antisymmetric mode is reduced, and the background disturbance is suppressed as the disturbed boundary layer is pushed away from the roughness. Downstream position blowing can still inhibit the symmetric mode, but the antisymmetric mode is barely influenced, since this blowing is less effective in suppressing the counter-rotating vortex pair.

Surface-Enhanced Raman Scattering on Silver Sinusoidal Nanograting: Impact of Interactions of Grating-Coupled Surface Plasmon Polaritons

In this study, we reported a silver sinusoidal nanograting used in microchannels, forming H2O/Ag/NOA/SiO2 heterostructure and studied the impact of interactions of grating-coupled surface plasmon polaritons (SPPs) on surface-enhanced Raman scattering (SERS). There are two modes of \(E_{x} (z)\) in the Ag grating, namely, the anti-symmetric mode (AM) at the high SPP resonance frequency and the symmetric mode (SM) at the low-SPP resonance frequency. When the refractive indices of the upper and lower dielectrics of the thin Ag grating are close, there will be a strong coupling between the SPPs of the two interfaces, which will increase the transmittance but reduce the maximum electromagnetic (EM) of the SERS substrate. Additionally, the thinner the Ag grating, the stronger the coupling, accompanied by the frequency shift of the two coupling modes. Our experimental results are supported by FDTD calculations, which have corroborated the importance of the interactions of grating-coupled SPPs in the design of SERS substrate.

Электрозвуковые волны в PT-симметричной структуре пьезоэлектриков вблизи особой точки

The spectral properties of electrosound waves in the gap of PT-symmetric piezoelectrics of symmetry class 6 are investigated theoretically. It is found that at a certain level of loss and gain in piezoelectrics, the symmetric and antisymmetric modes are coupled. The coupling point defines an exceptional point of PT-symmetric structure. It is shown that the frequency dependence of the amplitude at the exceptional point has an extremely narrow resonance peak, which opens up the possibility of creating supersensitive sensors based on PT-symmetric physical structures.

Semi-Analytical Finite Element Method for calculating dispersion curves of a CFRP plate

Composite Materials are widely used thanks to their mechanical characteristics. Non destructive evaluation requires knowing the exact dispersion curves to determine the propagative waves and to resolve the phase velocity of symmetrical and antisymmetrical modes. The aim of this paper is to plot the dispersion curves of a Carbon Fibre Reinforced Polymer plate using Semi Analytical Finite Element Method algorithm. This method combines the analytical expression of the displacement and the finite element method procedure. The resultant advantage is both simplicity and rapidity. The obtained results showed that the method accuracy depends on the elements number of meshing. To ensure good precision and fast computation of the method, the elements number and the order of the interpolation functions must be optimized.

Ab initio study of CsBiO[formula omitted] oxide-perovskite, dynamical stability and bonding analysis

The quest for a new stable oxide perovskite is a crucial challenge for use in many fields of applications. However, the Jahn-Teller effect in conjunction with symmetry-breaking distortion in some compounds acts as a key ingredient for several intriguing quantum phenomena causing instability in the ideal cubic structure. Using density functional theory (DFT) calculations, we find that the CsBiO3 compound is rather stable in a distorted tetragonal structure than in an ideal cubic one. To analyze this stability, we investigated deeply the thermodynamic and dynamical properties of the title compound. The results suggest that the octahedral distortion is due to the anti-symmetric breathing modes moving the BiO6 octahedra. Both electronic and topological analyses of electron density showed that these distortions found their origin in the sp lone pair bonds in the Bi units.

Nonlinear liquid sloshing in an upright circular container: Modal responses and higher-order harmonics

Nonlinear sloshing in an upright circular container near the lowest natural frequency is analyzed by using a fully nonlinear overset-mesh-based harmonic polynomial cell method, two weakly nonlinear Narimanov–Moiseev-type multimodal models and a linear multimodal method. Modal responses are extracted from the fully nonlinear results based on a simple but accurate least-square procedure using the time series of free-surface wave elevations, which provides new ways to delve into the underlying modal responses and energy transfer between modes, as well as to verify the validity of ordering assumptions in the weakly nonlinear models. Wavelet analyses are also performed for the wave elevations and generalized coordinates of the modes to better understand the time-frequency information of the higher harmonics of the sloshing responses and energy transfer in a nonlinear process. Planar harmonic sloshing state, swirling harmonic sloshing state, and periodically modulated sloshing state are analyzed. It is found that the energy is more dispersed among different modes in the periodically modulated sloshing state, which means higher natural modes are consequential. In general, energies are found to transfer from lower to higher natural modes and between symmetric and antisymmetric natural modes. The results also show that the O(ε1/3) and O(ε2/3) responses are dominated by only first and second harmonics, respectively, while the O(ε) response contains non-negligible first and third harmonic contribution. At last, the influence of initial disturbance is examined, demonstrating that different initial disturbances may lead to the different rotation direction of the swirling waves and the sloshing-wave responses in the transient stage, while the main characteristics of the sloshing waves are robust and independent of initial conditions.

Nonlinear transmission line (NTL) model study of electromagnetic effects in high-frequency asymmetrically driven capacitive discharges

In this work, we generalize a nonlinear transmission line (NTL) model introduced in a previous work [Liu et al. Plasma Sources Sci. Technol. 30, 045017 (2021)] to achieve more comprehensive simulations. The improved NTL model allows a variable-sized (instead of a one grid cell) vacuum spacer between the powered and grounded electrodes and takes into account the (previously neglected) electron-neutral elastic collision frequency term in the plasma dielectric constant, and the radial variation of the plasma density. Using this model, we study the effects of the spatial and series resonances, associated with the (axially) z-antisymmetric and z-symmetric radially propagating surface wave modes, and the nonlinear harmonic excitations on the plasma uniformity. We conduct simulations in which we increase the driving frequency f from 30 to 120 MHz for an asymmetric capacitive argon discharge at a fixed pressure and electron power of pg=20 mTorr and Pe=40 W, respectively. The first antisymmetric mode resonance frequency fa1 occurs between 80 and 90 MHz, and the first symmetric mode resonance frequency fs1 occurs at about 100 MHz. The powered electrode sheath becomes smaller than the grounded electrode sheath for f≈fa1, and the source voltage reaches a minimum for f≈fs1. The radially varying electron power density shows a narrow center peak due to the finite mode wavelengths and the nonlinearly excited harmonics, which can be further enhanced by spatial or series resonances. A second peak appears above the vacuum spacer edge as f exceeds fs1 at 120 MHz because the shorter wavelengths at higher f allow secondary maxima of the surface waves to form within the reactor.

Electrosound waves in a PT-symmetric piezoelectric structure near the exceptional point

The spectral properties of gap electroacoustic waves in PT-symmetric piezoelectric structures of symmetry class 6 are investigated theoretically. It was found out that, at a certain level of loss and gain in piezoelectrics, the symmetric and antisymmetric modes intersect. The intersection point defines an exceptional point of the PT-symmetric structure. It was shown that the frequency dependence of the amplitude at the exceptional point has an extremely narrow resonance peak, which opens up the possibility of creating supersensitive sensors based on PT-symmetric physical structures. Keywords: PT-symmetry, piezoelectrics, electroacoustic waves, gap structure.

Anti-parity-time symmetry enabled on-chip chiral polarizer

Encircling an exceptional point (EP) in a parity-time (PT) symmetric system has shown great potential for chiral optical devices, such as chiral mode switching for symmetric and antisymmetric modes. However, to our best knowledge, chiral switching for polarization states has never been reported, although chiral polarization manipulation has significant applications in imaging, sensing, communication, etc. Here, inspired by the anti-PT symmetry, we demonstrate, for the first time to our best knowledge, an on-chip chiral polarizer by constructing a polarization-coupled anti-PT symmetric system. The transmission axes of the chiral polarizer are different for forward and backward propagation. A polarization extinction ratio of over 10 dB is achieved for both propagating directions. Moreover, a telecommunication experiment is performed to demonstrate the potential applications in polarization encoding signals. It provides a novel functionality for encircling-an-EP parametric evolution and offers a new approach for on-chip chiral polarization manipulation.