Antisymmetric Modes Research Articles

Effect of angle of attack on flow past a partially-burrowed, isolated freshwater mussel

Though freshwater mussels in rivers tend to be aligned with the flow, the angle of attack can increase significantly during periods when the stage and discharge vary rapidly (e.g., floods). As the near-bed flow changes, mussels can experience larger drag forces before they reorient themselves parallel to the flow and/or increase their level of burial into the substrate to avoid dislocation. The present paper used a previously validated eddy-resolving numerical model to study the changes in the structure of the near-wake flow, horseshoe vortex system, drag forces acting on the exposed part of the mussel shell, sediment entrainment mechanisms and dilution of the jet of filtered water originating in the excurrent siphon with the increase in the magnitude of the angle of attack, |θ|, for an isolated mussel placed in an open channel with incoming fully-developed, turbulent flow. Given that the mussel may modify its mean volumetric discharge through the two siphons and level of mussel burial as θ changes, simulations are performed with and without active filtering and with two heights of the exposed part of the mussel, h. The raise in |θ| increases the total drag force acting on the exposed part of the shell, the entrainment into the excurrent-siphon jet containing filtered water, the circulation of the horseshoe vortices, the strength of the anti-symmetric wake shedding mode and the circulation of the main streamwise-oriented (base) vortex forming behind the mussel. Results show that, for all angles of attack, the downflow induced by the primary base vortex generates a large region where the bed shear stresses are strongly amplified. So, particulate deposition is expected to be low and significant local scour may occur in the near wake of a partially burrowed mussel.

Dynamical characteristics of the surface plasmon-polariton wave supported by a thin metal film

We study the energy and momentum of the surface plasmon-polariton (SPP) excited in a symmetric three-layer ‘insulator–metal–insulator’ structure, which is known to support the symmetric (S) mode (which, under certain conditions, possesses the negative group velocity) as well as the antisymmetric (AS) mode with always positive energy flow. The electric and magnetic field vectors are calculated via both the phenomenological and the microscopic approach; the latter involves the hydrodynamic model accounting for the quantum statistical effects for the electron gas in metal. Explicit representation for the energy and momentum constituents in the dielectric and in the metal film are obtained, and the wavenumber dependences of the energy and momentum contributions for the whole SPP are analyzed numerically. The various energy and momentum constituents are classified with respect to their origin (‘field’ or ‘material’), and the physical nature (orbital (canonical) and spin (Belinfante) momentum contributions). The pictures characteristic for the S and AS modes are systematically compared. The results can be useful for the studies and applications of the SPP-induced thin-film effects, in particular, for the charge and spin dynamics in thin-film plasmonic systems.

Antisymmetric Mode Cancellation for High-Q Cavities in a Double-Disk

High-quality factor dielectric microcavities like whispering-gallery-mode resonators can be widely applied in fields such as laser, optical sensor, switch, and filter. We propose the whispering-gallery-mode double-disk cavity comprising dielectric disks apart along the face of the disk. The electric field of each disk with opposite phases was excited; the emitted radiation field was annihilated by destructive interference. We numerically achieved a 5.67-fold enhancement in quality factor at the double-disk cavity with a radius, thickness, and gap distance of 850 nm, 220 nm, and 220 nm, respectively, compared to a single dielectric disk with the same structural conditions.

Ab initio spatial coupled-mode theory of Fano resonances in optical responses of multilayer interference resonators

To reveal origins of resonance characteristics of multilayer interference structures, we developed an ab initio spatial coupled-mode theory using the approximations of general electromagnetic theory of wave propagation in stratified media. In contrast to the conventional coupled-mode theory, the coefficients of developed coupled-mode models, which describe the Fano resonance behavior of interference field enhancement, are given by analytical functions of structural and optical parameters of the resonance systems. The results of analytical modeling of low- and high-loss resonator systems supporting waveguide, Fabry-P\'erot, symmetric, and antisymmetric plasmonic normal modes agree very well with electromagnetic numerical simulations. We demonstrate also that the conventional spatial phenomenological coupled-mode theory is accurate only for low-loss structures.

Compact nonvolatile 2×2 photonic switch based on two-mode interference.

On-chip nonvolatile photonic switches enabled by phase change materials (PCMs) are promising building blocks for power-efficient programmable photonic integrated circuits. However, large absorption loss in conventional PCMs (such as Ge2Sb2Te5) interacting with weak evanescent waves in silicon waveguides usually leads to high insertion loss and a large device footprint. In this paper, we propose a 2×2 photonic switch based on two-mode interference in a multimode slot waveguide (MSW) with ultralow loss Sb2S3 integrated inside the slot region. The MSW supports two lowest order TE modes, i.e., symmetric TE00 and antisymmetric TE01 modes, and the phase of Sb2S3 could actively tune two-mode interference behavior. Owing to the enhanced electric field in the slot, the interaction strength between modal field and Sb2S3 could be boosted, and a photonic switch containing a ∼9.4 µm-long Sb2S3-MSW hybrid section could effectively alter the light transmission between bar and cross ports upon the phase change of Sb2S3 with a cross talk (CT) less than -13.6 dB and an insertion loss (IL) less than 0.26 dB in the telecommunication C-band. Especially at 1550 nm, the CT in the amorphous (crystalline) Sb2S3 is -36.1 dB (-31.1 dB) with a corresponding IL of 0.073 dB (0.055 dB). The proposed 2×2 photonic switch is compact in size and compatible with on-chip microheaters, which may find promising applications in reconfigurable photonic devices.

Tunable narrow-linewidth surface plasmon resonances of graphene-wrapped dielectric nanoparticles in the visible and near-infrared

We investigate the plasmon resonance in the light interaction with a graphene nanoshell which consists of graphene wrapped around a low − loss dielectric core. In this work, Polystyrene (PS) and Titanium dioxide (TiO 2 ) cores are proposed. This study is restricted to small nanoparticles with diameters significantly smaller than the wavelength of incident light. Therefore, we adopt the quasi − static approach that provides in − depth insights into the plasmon resonance of core − shell nanoparticles. The proposed graphene nanoshell can support two localized surface plasmon resonances (LSPRs). These plasmon resonance modes correspond to the symmetric and antisymmetric dipole-like modes described in the hybridization theory. The observed resonance modes are characterized by a narrow bandwidth and a broadband resonance tunability in the visible to near-infrared (NIF) range. The impacts of the optical properties of graphene, core material, and nanoshell geometry on surface plasmon features are explored. We show that the features of each plasmonic resonance meet the demands for specific nanoscale applications such as biosensing, photothermal therapy, and in vivo imaging. • We study the plasmon resonance of graphene nanoshells with low − loss dielectric core. • In our model, Polystyrene (PS) and Titanium dioxide (TiO 2 ) cores are proposed. • Graphene nanoshells exhibits two plasmon resonances, sphere and cavity plasmons. • Plasmon resonances have a very narrow bandwidth with a broadband tunability. • Features of plasmon resonances are attractive for nanosensing/medical applications.

Dual resonance modes in two-port SAW resonator sensors: an analysis through scattering matrix approach

We report dual resonance modes occurring in two-port surface acoustic wave (SAW) resonators attached with typical sensing film and investigated the significance in sensing applications. Two-port SAW resonators operating at 303 MHz frequency were modelled using the scattering matrix approach and resonance frequency characteristics of the resonators were studied for mass loading caused by an arbitrary film attached between the interdigital transducers of the resonator. Depending on the mass loading, symmetric and antisymmetric modes occur around the near resonance frequency of the SAW device. Either a single-mode or both modes are dominant depending on the phase shift caused by the mass loading of the film. Choosing arbitrary film thickness could result in complexity to track the resonance frequency shift and degrade sensing performance. The approach used for analysis can aid to determine the optimum sensing film thickness for two port SAW resonator-based sensors that can exhibit better linearity.

Single-mode lasing in an AlGaInAs/InP dual-port square microresonator.

Mode selection is crucial to achieving stable single-mode lasing in microlasers. Here, we demonstrate experimentally a dual-port square microresonator for single-mode lasing with a side-mode-suppression ratio (SMSR) exceeding 40 dB. By connecting waveguides at two opposite vertices, the quality factor for the antisymmetric mode (ASM) is much higher than that of the symmetric mode (SM), enabling single-mode lasing. Furthermore, far-field interference patterns similar to Young's two-slit interference are observed. This microlaser is capable of providing two optical sources simultaneously for optical signal processing in high-density integrated photonic circuits.

Quantum criticality driven by the cavity coupling in the Rabi-dimer model

The superradiant phase transition (SPT) controlled by the interacting strength between the two-level atom and the photons has been a hot topic in the Rabi model and the Rabi-dimer model. The latter describes two Rabi cavities coupled with an inter-cavity hopping parameter. Moreover, the SPT in the Rabi-dimer model is found to be the same universal class that in the Rabi model by investigating the correlation-length critical exponent. In this paper, we are concerned about whether the inter-cavity hopping parameter between two Rabi cavities (i.e., the Rabi-dimer model) will induce the SPT and to which the universal class of the phase transition belongs. We analytically derive the phase boundary of the SPT and investigate the ground-state properties of the system. We uncover that the inter-cavity induced SPT can be apparently understood from the ground-state energy and the ground-state photon population, as well as the ground-state expectation value of the squared anti-symmetric mode. From the scaling analysis of the fidelity susceptibility, we numerically verify that the SPT driven by the cavity coupling belongs to the same universal class as the one driven by the atom–cavity interaction. Our work enriches the studies on the SPT and its critical behaviors in the Rabi-dimer model.

Coupled Plasmon Wave Dynamics beyond Anomalous Reflection: A Phase Gradient Copper Metasurface for the Visible to Near-Infrared Spectrum

In nanoscale photonic devices, the demand for multifunctionality from 2D metasurface optics has increased rapidly. To explore the required fine-tuning in the design metrics, we reinvestigated the trapezoid-shape copper metasurface using finite-difference time-domain simulation to efficiently utilize linearly polarized light for two different functionalities. From the plasmonic band structure, we could see how the degree of asymmetry in the geometry affected the efficient resonance coupling of the traveling plasmonic modes, along with the different types of mode hybridization profiles that were related to the nanoantenna’s geometric shape. By tuning the nanoantenna’s length, we could excite the effective plasmon mode that was supported by this configuration and guide surface waves unidirectionally from the normal incidence free-space light within the visible to infrared range. The directed surface plasmon polaritons had both antisymmetric and symmetric modes that oscillated between the top and bottom surfaces of the continuous metal layer, depending on the nanoantenna’s length and wavelength. This proposed copper metasurface was optimized for a far-field application of broadband (600–900 nm) anomalous beam steering for an average of 60% efficiency with a maximum angle of 64°. This work offers more understanding of a metasurface being implemented in small plasmonic devices, waveguide mode controlling and beam steering with wavelength-dependent functionalities.

Assessment of Damage in Composite Pressure Vessels Using Guided Waves.

This paper deals with guided wave-based structural health monitoring of composite overwrapped pressure vessels adopted for space application. Indeed, they are well suited for this scope thanks to their improved performance compared with metallic tanks. However, they are characterized by a complex damage mechanics and suffer from impact induced damage, e.g., due to space debris. After reviewing the limited progress in this specific application, the paper thoroughly covers all the steps needed to design and verify guided wave structural health monitoring system, including methodology, digital modelling, reliability, and noise estimation for a correct decision-making process in a virtual environment. In particular, propagation characteristics of the fundamental anti-symmetric mode are derived experimentally on a real specimen to validate a variety of finite element models useful to investigate wave interaction with damage. Different signal processing techniques are demonstrated sensitive to defect and linearly dependent upon damage severity, showing promising reliability. Those features can be implemented in a probability-based diagnostic imaging in order to detect and localized impact induce damage. A multi-parameter approach is achieved by metrics fusion demonstrating increased capability in damage detection with promising implication in enhancing probability of detection.

Modulated flexural edge waves in a plate with its free edge structured by an array of grooves

A flexural edge wave (FEW) is a guided wave propagating along the free edge of a semi-infinite isotropic elastic thin plate. Here, we investigate the FEW in a plate with its free edge structured by an array of grooves, revealing the essence of a kind of modulated FEW. We analytically solve the dispersion relation of the modulated FEW by developing the coupled mode theory (usually used in diffraction optics and acoustics) that couples diffraction modes and high-order flexural waveguide modes, and discuss the propagation characteristics of the first- and second-order modulated FEWs (symmetric and antisymmetric modes, respectively). Based on the features of the dispersion curves corresponding to the unit cells of the plate with grooves of graded depths, we numerically and experimentally realize the rainbow trapping of the first-order modulated FEW and the mode conversion of the second-order modulated FEW to the bulk wave. Our work provides new ideas for the manipulation of flexural waves and has the potential to develop corresponding acoustic devices based on the FEW.

Ultrathin Resistive Sheets for Broadband Coherent Absorption and Symmetrization of Acoustic Waves

The manipulation of acoustic waves is experimentally and theoretically investigated based on the concept of coherent perfect absorption. It is shown that the absolute control of antisymmetric modes of an acoustic duct is possible using an ultrathin wire mesh. By varying the relative phase of two counterpropagating (normal or oblique) incident waves, the absorption due to wire mesh can be tuned from zero to unity. Experimental as well as numerical results demonstrate that this phenomenon is extremely broadband---allowing, for instance, the symmetrization of the acoustic wave with a short pulse in the time domain.

Magnetoplasmonic coupling in graphene nanodisk dimers: An extended coupled-dipole model for circularly polarized states

Plasmonic coupling is one of the most important effects in compact plasmonic systems, and it has been studied intensively. In contrast, magnetoplasmonic coupling (MPC) is rarely mentioned, even in graphene nanostructures supporting the strong magneto-optic effect. Here, we theoretically investigate MPC in graphene nanodisk dimers in the presence of either parallel (case I) or antiparallel (case II) magnetic fields. We find the hybridized modes always appear for two states with same chirality, while their excitations depend on incident polarization. Moreover, two antisymmetric modes are dark in case I, but all four modes are bright in case II. To provide better insight, an extended coupled-dipole model is presented, in which the fundamental circularly polarized magnetoplasmons are decomposed into two linear and orthogonal dipoles, with a $\ensuremath{\pi}/2$ phase difference, and then the coupling is described by two linear dipoles along the two orthogonal directions separately. The parameters for the magneto-optic effect and coupling strengths are independent and can be easily extracted from their individual simulations. The eigenvalues and wave functions obtained from the model can describe well the resonance frequency and excitation strength of each hybridized mode. We finally discuss MPC in touching graphene nanodisks, where the charge transfer plasmon is immune to the magneto-optic effect, and in case II, a circular resonance state will be replaced by a linear one with the incident electric field along the touching direction. In this paper, we provide a general framework for investigating mode coupling of two circular states and pave the way to magneto-optic and plasmonic applications.

Attenuation of the unsteady loading on a high-rise building using feedback control

The unsteady wind loading on high-rise buildings has the potential to influence strongly their structural performance in terms of serviceability, habitability and occupant comfort. This paper investigates numerically the flow structures around a canonical high-rise building immersed in an atmospheric boundary layer, using wall-resolved large eddy simulations. The switching between two vortex shedding modes is explored, and the influence of the atmospheric boundary layer on suppressing symmetric vortex shedding is identified. It is shown that the antisymmetric vortex shedding mode is prevalent in the near wake behind the building, with strong coherence between the periodic fluctuations of the building side force and the antisymmetric vortex shedding mode demonstrated. Two feedback control strategies, exploiting this idea, are designed to alleviate the aerodynamic side-force fluctuations, using pressure sensing on just a single building wall. The sensor response to synthetic jet actuation along the two ‘leading edges’ of the building is characterised using system identification. Both the designed linear controller and the least mean square adaptive controller attenuate successfully the side-force fluctuations when implemented in simulations. The linear controller exhibits a better performance, and its effect on the flow field is to delay the formation of dominant vortices and increase the extent of the recirculation region. Feedback control that requires a smaller sensing area is then explored, with a comparable control effect achieved in the attenuation of the unsteady loading. This study could motivate future attempts to understand and control the unsteady loading of a high-rise building exposed to oncoming wind variations.

2D slowness visualization of ultrasonic wave propagation for delamination detection in CFRP laminates

Visualizing the propagation of Lamb waves generated by scanning laser injection is a promising and effective method for the nondestructive inspection of carbon fiber reinforced plastic (CFRP) structures. We evaluate the use of two-dimensional time-slowness maps of the acquired data for defect detection in CFRP laminates. The slowness maps can be constructed from the differences in wave propagation characteristics between two adjacent laser injection points. Using a CFRP plate, in which an artificial delamination is located at the center, we show that the delamination can be clearly observed as a hotspot in the two-dimensional slowness map. The visible hotspot is due to the dispersion of the lowest antisymmetric (A0) mode of the Lamb wave: the A0 mode slows down at the delaminated region. We also demonstrate that our method can be applied to an L-shaped structure. Two-dimensional slowness images can be useful for developing and applying practical inspection methods based on laser ultrasonic waves.

Non-modal growth of finite-amplitude disturbances in oscillatory boundary layer

The essence of sub-critical transition of oscillatory boundary-layer flows is the non-modal growth of finite-amplitude disturbances. The current understanding of the mechanisms of the orderly and bypass transitions of oscillatory boundary-layer flows is limited. The present study adopts optimisation approaches to predict the maximum energy amplification of two- and three-dimensional perturbations in response to the optimal initial disturbance with or without external forcing. A series of direct numerical simulations are also performed to compare with the results obtained from the stability analyses. In particular, the optimal initial perturbation similar to a Tollmien–Schlichting (T–S) wave yields the largest transient growth under the combined effects of the Orr mechanism and inflectional point instability. With a considerable level of two-dimensional disturbance, the vortex tube nonlinearly develops from the T–S-like wave, and then either deforms into a $\varLambda$ -vortex in the near-wall region or rolls up to the free shear region. The further burst of turbulence can follow the first pathway as K-type transition or the second one as vortex tube breakdown due to the elliptical instability. Additionally, non-modal growth can initiate the inception of streaky structures by favourable three-dimensional initial perturbations and/or forcing. The secondary instabilities responsible for the streak breakdown are classified as the varicose (symmetric) and sinuous (anti-symmetric) modes. Under a sufficiently high level of three-dimensional disturbance, the bypass transition is predominantly characterised by the formation of the sinuous mode and turbulent spots, which leads to the suppression of inflection point instability.

Simultaneous gas and magnetic sensing using a single heated micro-resonator

In this paper, we propose a single device for simultaneous measurement of in-plane magnetic field and gas concentration. The concept is based on tracking simultaneously the resonant frequency of the first two symmetric and anti-symmetric modes of an electrothermally actuated out-of-plane buckled micro-beam. First, the Lorentz-force magnetic sensing was investigated at different electrothermal voltages in air. Powered with 3.78 mW, the magnetic sensor achieves a sensitivity of 0.0867 /T at the second mode with good linearity lower than 0.1%. On the other hand, the gas-sensing technique is based on the thermal conductivity mechanism. We simultaneously measure the frequencies shift of the first and the second modes while changing the gas concentration and the magnetic field (Bx). At Bx = 55 mT, Helium (He) and Argon (Ar) sensing yield the highest sensitivity of 4.15%/%He in the range of 2.5–10% He and 2.22%/%Ar in the range of 10–20%Ar. Additionally, the results show a response/recovery time of 200 s/180 s (He) and 580 s/320 s (Ar). Herein, for the first time, we experimentally demonstrate the potential for employing a multimode micro-resonator for magnetic field and gas sensing. The proposed multi-sensing device has the characteristics of simple design, low cost, small size, and good linearity, making it suitable for smart environmental monitoring applications.

Spectral Analysis of Flow around Single and Two Crossing Circular Cylinders Arranged at 60 and 90 Degrees

Two modal decomposition techniques, including proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD), were used to identify the wake patterns past single and two crossing cylinders in 60° and 90° arrangements with gap ratio G = 4. The flow was simulated using direct numerical simulations (DNS) for Reynolds numbers Re = 100. From modal analysis, the flow’s spatial scale decreased with increasing modal frequency. Two main modes were identified in the wake of the cylinders, namely spatially antisymmetric and symmetric modes. Antisymmetric and symmetric modes were related to the cylinders’ vortex shedding and shedding vortices’ shift motion, respectively, whose frequencies were odd and even multiples of the cylinders’ lift force frequency. In addition, a low-frequency mode concerning the shadowing effect of the downstream cylinder (DC) in 90° arrangement was found in the wake of the DC centre.

Phase Matching via Plasmonic Modal Dispersion for Third Harmonic Generation.

The overall effectiveness of nonlinear optical processes along extended nonlinear media highly depends on the fulfillment of the phase‐matching condition for pump and generated fields. This is traditionally accomplished by exploiting the birefringence of nonlinear crystals requiring long interaction lengths (cm‐scale). For nonbirefringent media and integrated photonic devices, modal phase matching can compensate the index mismatch. Here, the various interacting waves propagate in transverse modes with appropriate phase velocities, but they suffer from a low refractive index contrast and cm‐scale interaction lengths. This work harnesses modal phase matching for third‐harmonic generation (THG) in plasmonic waveguides using an organic polymer (poly[3‐hexylthiophene‐2,5‐diyl]) as the nonlinear medium. One demonstrates experimentally an effective interaction area as small as ≈ 0.11 µm2 and the phase‐matched modal dispersion results in THG efficiency as high as ≈ 10–3 W‐2 within an effective length scale of ≈ 4.3 µm. THG also shows a strong correlation with the polarization of the incident laser beam, corresponding to the excitation of the antisymmetric plasmonic modes, corroborating that plasmonic modal phase matching is achieved. This large reduction in device area of orders of magnitude is interesting for various applications where space is critical (e.g., device integration or on‐chip applications).