Plane Wave Excitation Research Articles

Active acoustic cloaking of cylindrical shells in low Mach number flow

The vibro-acoustic response of a two-dimensional cylindrical shell in low Mach number flow is herein derived. The analytical model takes into account the structural elasticity and coupling of the shell vibration with its interior and exterior acoustic fields in the presence of a moving fluid. The cylindrical shell is modelled using Donnell-Mushtari theory. Taylor transformations are employed to transfer the convected wave equation into the ordinary wave equation which was then solved using scattering theory. Three excitation cases corresponding to a plane wave, an external monopole source and a radial point force applied directly to the shell are considered. Shell circumferential resonances and interior acoustic resonances are identified. Two active control strategies are then applied to acoustically cloak the cylindrical shell at its acoustic and structural resonances. The first control approach employs acoustic control sources in the exterior fluid domain. In the second approach, control forces are applied to directly excite the elastic shell, whereby the structural response is actively modified to manipulate the scattered and radiated acoustic fields arising from plane wave excitation of the shell. Results show that the second approach is superior in terms of both reduced control effort and cloaking of the global exterior domain. For both control approaches, the performance of the active cloak is shown to deteriorate if the convected flow field is not accounted for in the control process.

Wideband sound absorption and transmission through micro-capillary plates: Modelling and experimental validation

The current work presents theoretical and experimental studies that investigate sound propagation through micro-capillary plates (MCP) under a general plane wave excitation and in the no-flow case. MCPs are characterized by micrometric channels radius with Knudsen number greater than 0.001 so that a slip-flow model has been derived for their viscous transfer impedance. It is found that the slip-flow model should be used instead of the continuum model to predict the transfer impedance of MCPs with channels radii lower than 2 μm as well as their absorption coefficient under near-grazing incident excitations. Otherwise, both approaches provide similar results, as confirmed by comparison with finite element simulations. Due to their high porosity, MCPs provide minute reactance and constant resistance that can be tailored to achieve target absorption over a broad frequency range. Plane wave impedance tube experiments have shown that a near-optimal MCP termination can provide a low frequency flat absorption spectrum that stays above 0.7 up to a Helmholtz number of 1.84. Measurements on rigidly-backed MCPs have led to ultra-wideband absorption with a half-bandwidth spanning up to 12 octaves around the absorber Helmholtz resonance. Expressions have been derived to find the optimal channels radius that maximize the MCPs dissipation under general incidence angle and assuming anechoic or rigid backing. The sensitivity of the MCPs optimal transfer resistance to their load impedance has been examined. It provides a design chart to find the MCP optimal parameters that achieve specific broadband absorption value under general incidence and practical load conditions.

Resonant Reflection From Cylindrical Grating-Waveguide Under Holistic Excitation

The phenomenon of resonant reflection from a grating-coupled waveguide mode is conceptually, technologically, and experimentally transposed from a planar corrugated waveguide structure under plane wave excitation to a circularly symmetrical waveguide at the inner wall of a tube under cylindrical wave excitation. The mode coupling element is an azimuthally periodic wall corrugation having an integer number of lines parallel to the tube axis. The grating is defined by diffractive coordinate transform of a radial grating phase-mask transverse to the tube axis under axial beam exposure onto a photoresist-coated TiO2 sol-gel wall-waveguide. The holistic excitation of a waveguide mode is achieved by transforming a broad spectrum, centered axial incident beam into a cylindrical wave by a centered, 90° apex reflective cone. The expected resonantly reflected cylindrical wave at the mode-coupling wavelength is in turn transformed back into an axial beam by the cone. TE resonant reflection is demonstrated experimentally.

Metasurface-based total internal reflection microscopy

Recent years have seen a tremendous progress in the development of dielectric metasurfaces for visible light applications. Such metasurfaces are ultra-thin optical devices that can manipulate optical wavefronts in an arbitrary manner. Here, we present a newly developed metasurface which allows for coupling light into a microscopy coverslip to achieve total internal reflection (TIR) excitation. TIR fluorescence microscopy (TIRFM) is an important bioimaging technique used specifically to image cellular membranes or surface-localized molecules with high contrast and low background. Its most commonly used modality is objective-type TIRFM where one couples a focused excitation laser beam at the edge of the back focal aperture of an oil-immersion objective with high numerical aperture (N.A.) to realize a high incident-angle plane wave excitation above the critical TIR angle in sample space. However, this requires bulky and expensive objectives with a limited field-of-view (FOV). The metasurface which we describe here represents a low cost and easy-to-use alternative for TIRFM. It consists of periodic 2D arrays of asymmetric structures fabricated in TiO2 on borosilicate glass. It couples up to 70% of the incident non-reflected light into the first diffraction order at an angle of 65° in glass, which is above the critical TIR angle for a glass-water interface. Only ∼7% of the light leaks into propagating modes traversing the glass surface, thus minimizing any spurious background fluorescence originating far outside the glass substrate. We describe in detail design and fabrication of the metasurface, and validate is applicability for TIRFM by imaging immunostained human mesenchymal stem cells over a FOV of 200 µm x 200 µm. We envision that these kinds of metasurfaces can become a valuable tool for low-cost and TIRFM, offering high contrast, low photodamage, and high surface selectivity in fluorescence excitation and detection.

Metasurfaces homogenization technique based on the computation of the average value of the contravariant tensors elements

The purpose of this paper is to present a 2D metasurfaces homogenization based on the computation of the average value of the contravariant tensors elements. A simple semi-analytical model for the comprehension of the extraordinary optical transmission (EOT) through a 2D array of periodic subwavelength metasurface is analyzed through the proposed method. In this single mode model, the mono layer of the metasurface film is considered as homogeneous medium. Therefore, the electromagnetic response of this structure to a plane wave excitation is equivalent to that of a slab with homogeneous equivalent properties. In a coordinates system where the coordinate curves xj coincide with the physical discontinuity of the material, the covariant formalism involving contravariant components of density flux and covariant components of the electromagnetic field, allows to efficiently handle the continuity properties of the electromagnetic field. By averaging all these quantities involving local fields over the periods of the unit cell, and assuming that, these macroscopic fields and densities satisfy a constitutive relationship, the electromagnetic parameters such as averaged permittivity and permeability can be defined.

Analyzing Radiated Susceptibility of Superconducting Microstrip Transmission Line Under Plane Wave Excitation

Several studies have been conducted on an externally-excited microstrip transmission line; however, no study, to the best knowledge of the researchers, has examined the superconducting microstrip transmission lines (SMTL), which are the main part of the superconducting circuits. This article aims to investigate the radiated susceptibility of SMTL under plane wave excitation. The analysis is based on modeling the superconducting film as a current sheet with specified impedance. It is revealed that the equivalent sheet impedance depends on the current distribution within the film; hence, London's second equation is employed to obtain an integral equation to acquire the current distribution and, subsequently, the equivalent sheet impedance. Then, the susceptibility of the SMTL is calculated after applying the boundary condition at the equivalent impedance surface. The procedure is performed using spectral-domain approach and verified by COMSOL software. Finally, the susceptibility of the SMTL is examined at different temperatures and for different film thicknesses. The results show that the lower temperature makes the SMTL more susceptible to an external electromagnetic field. Moreover, the same behavior is observed for increased film thickness.

Numerically stable formulation of Mie theory for an emitter close to a sphere.

Numerical implementations of Mie theory make extensive use of spherical Bessel functions. These functions are, however, known to overflow/underflow (grow too large/small for floating point precision) for orders much larger than the argument. This is not a problem in applications such as plane wave excitation, as the Mie series converge before these numerical problems arise. However, for an emitter close to the surface of a sphere, the scattered field in the vicinity of the sphere is expressed as slowly converging series, with multipoles up to order 1000 required in some cases. These series may be used to calculate experimentally relevant quantities such as the decay rate of an emitter near a sphere. In these cases, overflow/underflow prevents any calculation in double precision using Mie theory, and alternatives are either computationally intensive (e.g., arbitrary precision calculations) or not accurate enough (e.g., the electrostatics approximation). We present here a formulation of Mie theory that overcomes these limitations. Using normalized Bessel functions where the large growth/decay is extracted as a prefactor, we re-express the Mie coefficients for scattering by spheres in a normalized form. These normalized expressions are used to accurately compute the series for the electric field and decay rate of a dipole emitter near a spherical surface, in cases where the Mie coefficients would normally overflow before any degree of accuracy can be obtained.

Correlation-based Doppler-angle estimation with plane-wave excitation

Abstract Conventional Doppler imaging does not provide angle information, which makes it difficult to obtain flow information, especially for lateral flows. A correlation-based transit-time approach has been developed for estimating the Doppler angle under the framework of a focused-beam architecture, which has a fast computation capability. However, there is still a trade-off between the estimation accuracy and data acquisition time, and so this study applied plane-wave excitation with the aim of mitigating this limitation. In addition, three Doppler-angle estimation methods based on (1) the fast Fourier transform, (2) correlation, and (3) an autoregressive model were used to estimate Doppler angles. The estimation performances and computation loads of the three approaches were compared using simulations and a flow phantom. The simulation results showed that the Doppler angle can be correctly estimated with a sufficient aspect ratio, and Doppler-angle estimation using plane-wave excitation outperformed the focused-beam method. The flow-phantom experiments revealed the minimum number of ensembles required for the different approaches. The velocity-vector profiles were also demonstrated using the correlation and autoregressive methods, to show the feasibility of both lateral and axial velocity estimation using the proposed methods.

The convolutional interpretation of registration-based plane wave steered pulse-echo local sound speed estimators

Pulse-echo reconstruction of sound speed has long been considered a difficult problem within the domain of quantitative biomedical ultrasound. However, recent results (Jaeger 2015 Ultrasound Med. Biol. 41 235–50; Jaeger and Frenz 2015 Ultrasonics 62 299–304; Jaeger et al 2015 Phys. Med. Biol. 60 4497–515) have demonstrated that pulse-echo reconstructions of sound speed are achievable by exploiting correlations in post-beamformed data from steered, plane-wave excitations in the presence of diffuse scatterers. Despite these recent advances, a coherent theoretical imaging framework for describing the approach and results is lacking in the literature. In this work, the problem of sound speed reconstruction using steered, plane-wave excitations is reformulated as a truncated convolutional problem, and the theoretical implications of this reformulation are explored. Additionally, a matrix-free algorithm is proposed that leverages the computational and storage advantages of the fast Fourier transform (FFT) while simultaneously avoiding FFT wraparound artifacts. In particular, the storage constraints of the approach are reduced down from to over full matrix reconstruction, making this approach a better candidate for large reconstructions on clinical machines. This algorithm was then tested in the open source simulation package k-Wave to assess its robustness to modeling error and resolution reduction was demonstrated under full-wave propagation conditions relative to ideal straight-ray simulations. The method was also validated in a phantom experiment.

Self-Trapped Nanoparticle Binding via Waveguide Mode

In this paper, we study a stable optomechanical system based on a nanoparticle chain coupled to a waveguide mode. Under the plane wave excitation the nanoparticles form a stable self-organized periodic chain array along the direction of the waveguide through the transverse binding effect. We show that owing to the long-range interaction between the nanoparticles the trapping potential for each nanoparticle in the chain increases linearly with the system size making the formation of long chains more favourable. We propose that this effect can be observed with an optical nanofiber which is a versatile platform for achieving optical binding of atoms and nanoparticles. Our calculations show that binding energy for two nanoparticles is in the range of $9\div13~kT$ reaching the value of 100 $kT$ when the size of the chain is increased to 20 nanoparticles that makes the potential experimental observation of the effect possible. We also suggest the geometry of the two counter-propagating plane waves excitation, which will allow trapping the nanoparticles close to the optical nanofiber providing efficient interaction between the nanoparticles and the nanofiber.

The Cylindrical Wave Approach for the Electromagnetic Scattering by Targets behind a Wall

An overview of the cylindrical wave approach in the modeling of through-wall radar problems with targets hidden behind a dielectric wall is reported. The cylindrical wave approach is a technique for the solution of the two-dimensional scattering by buried circular cross-section cylinders in a semi-analytical way, through expansion of the scattered fields into cylindrical waves. In a through-wall radar application, the scattering environment is made by a dielectric layer between two semi-infinite half-spaces filled by air. For this layout, two possible implementations of the cylindrical wave approach have been developed in the case of plane-wave excitation. The first was an iterative scheme with multiple-reflection scattered fields, and the second was a fast and non-iterative solution, through suitable basis functions (i.e., reflected and transmitted cylindrical waves). Such waves take into account all the interactions of the source field with the interfaces bounding the dielectric layers and the targets. The non-iterative approach was also extended for excitation from the radiated field by a line source. A final system was derived for the computation of the scattered field by PEC or dielectric targets. Numerical results show the potentialities of the cylindrical wave approach in the modeling of through-wall radar, in particular in the evaluation of the scattered fields by human targets in a building’s interior, modeled with a two-dimensional approach.

Doppler ultrasound bandwidth imaging: A new technique for mapping unstable flow

Unstable flow plays an important role in the initiation and progression of atherosclerosis. Yet, mapping flow instability in arteries is challenging because such event is dynamic over both space and time. In this work, we present a new framework called Doppler ultrasound bandwidth imaging (DUBI) that makes use of high-frame-rate plane wave excitation and Doppler bandwidth analysis principles to identify unstable flow regions within an image view. DUBI works by performing autoregressive modeling at every pixel position to estimate the instantaneous Doppler bandwidth, which in principle is broadened by the wider range of velocities attributed to the emergence of unstable flow. DUBI’s performance in mapping unstable flow was tested with laminar and turbulent flow conditions in a nozzle-flow phantom. Results showed that DUBI was able to quantify the difference in Doppler bandwidth magnitude (increased from 2.1 to 5.2 kHz) as the flow condition changed from laminar to turbulent. Also, DUBI was found to be able to identify and map the unstable flow region with the image view, outperforming conventional Doppler variance imaging. This observation was substantiated by receiver operating characteristic analysis, in which DUBI achieved a sensitivity of 0.72 and specificity of 0.83 (vs 0.66 and 0.68, respectively, for conventional Doppler variance imaging). We anticipate that DUBI can be applied in vivo to obtain useful information for clinical diagnosis of atherosclerosis.

Silicon Nitride based Medium Contrast Gratings for Doubly Resonant Fluorescence Enhancement

We present the design, fabrication, and experimental characterization of silicon nitride based medium-index contrast gratings on glass substrate for fluorescence enhancement in the yellow to red spectral range with resonances for both incident excitation and fluorescence emission wavelengths under surface normal incidence. A comparison of the design space to realize resonant field enhancement in high-index contrast silicon and medium-index contrast silicon nitride grating structures is presented. The one-dimensional sub-wavelength grating structures studied here are designed with large duty cycle (∼80%) to account for the medium refractive index contrast (Δn ∼ 0.5) between silicon nitride and the glass substrate to ensure that the device operates in the two-mode regime. The resonant enhancement of fluorescence is experimentally verified using rhodamine-B isothiocyanate dye as the fluorophore of interest. A resonant enhancement of 10.8 times is demonstrated in this sample when compared to un-patterned film for transverse electric-transverse magnetic (TE–TM) polarization combination. We have also performed simulation study with plane wave excitation and incoherent dipole array emission to model the resonant excitation and emission processes, respectively. The simulations corroborate well with the best observed experimental results for the doubly resonant fluorescence configuration. Silicon nitride based medium contrast gratings are a promising platform to fabricate scalable structures for resonant enhancement of light–matter interaction with potential applications in high-sensitivity biological fluorescence assays and as a platform for polarization selective interrogation of light emission from nanoscale emitters attached to the grating.

Efficient Transmit Delay Calculation in Ultrasound Coherent Plane-Wave Compound Imaging for Curved Array Transducers

The recently introduced plane-wave compounding method based on multiple plane-wave excitation has enabled several new applications due to its high frame rate (>1000 Hz). In this paper, a new efficient transmit delay calculation method in plane-wave compound imaging for a curved array transducer is presented. In the proposed method, the transmit delay is only calculated for a steering angle of 0° and is shifted along the element of the transducer to obtain other transmit delays for different steering angles. To evaluate the performance of the proposed method, the computational complexity was measured for various transmission conditions. For the number of elements and plane-wave excitations of 128 and 65, respectively, the number of operations was substantially decreased in the proposed method compared with the conventional method (256 vs. 8320). The benefits of the proposed method were demonstrated with phantom and in vivo experiments, where coherent plane-wave compounding with 65 excitations provided larger CR and CNR values compared to nine excitations (−22.5 dB and 2.7 vs. −11.3 dB and 1.9, respectively). These results indicate the proposed method can effectively reduce the computational complexity for plane-wave compound imaging in curved array transducers.

3D-printed phononic crystal lens for elastic wave focusing and energy harvesting

We explore elastic wave focusing and enhanced energy harvesting by means of a 3D-printed Gradient-Index Phononic Crystal Lens (GRIN-PCL) bonded on a metallic host structure. The lens layer is fabricated by 3D printing a rectangular array of cylindrical nylon stubs with varying heights. The stub heights are designed to obtain a hyperbolic secant distribution of the refractive index to achieve the required phase velocity variation in space, hence the gradient-index lens behavior. Finite element simulations are performed on composite unit cells with various stub heights to obtain the lowest antisymmetric mode Lamb wave band diagrams, yielding a correlation between the stub height and refractive index. The elastic wave focusing performance of lenses with different design parameters (gradient coefficient and aperture size) is simulated numerically under plane wave excitation. It is observed that the focal points of the wider aperture lens designs have better consistency with the analytical beam trajectory results. Experiments are conducted using a PA2200 nylon lens bonded to an aluminum plate to demonstrate wave focusing and enhanced energy harvesting within the 3D-printed GRIN-PCL domain. The piezoelectric energy harvester at the focal region of the GRIN-PCL produces 3 times more power output than the baseline harvester at the same distance in the flat plate region. The results show that 3D printing can provide a simple and practical method for implementing phononic crystal concepts with minimal modification of the host structure.

Influence of 2D heterogeneous elastic soil properties on surface ground motion spatial variability

Spatial variability of earthquake ground motion (SVEGM) refers to the differences in amplitude and phase between recordings of the same earthquake at different locations. In the near-surface, geological processes (sedimentation, erosion) and anthropogenic activities can lead to small scale spatial heterogeneities of soil mechanical properties, which may affect SVEGM. In this paper, the effect of shallow 2D spatial variability of the shear wave velocity (Vs) on the surface ground motion is assessed through a set of numerical experiments, using a simple 2D velocity structure (a sedimentary layer over a half-space). Non-linearity or damping are not considered in the wave propagation calculation in order to solely focus on the effects of soil elastic property variability. Vs is modeled as a random field using the EOLE method (Expansion Optimal Linear Estimation) and considering three statistical parameters: the coefficient of variation, and the horizontal and vertical autocorrelation distances. Seismic ground motions are numerically simulated for a plane wave excitation with SV polarization. Modeling results clearly highlight the scattering of surface waves by ground heterogeneities, leading to large spatial variations in surface ground motion. We computed surface ground motion indicators (resonance frequency, spectral amplification, Arias intensity and duration) and we showed that their spatial variability is mainly controlled by the Vs coefficient of variation. A comparison between 2D and 1D ground motion probabilistic modeling shows that the 1D probabilistic approach may correctly reproduce average fundamental resonance frequencies and corresponding amplification. However, the 1D approach significantly under-predicts both ground motion amplification at higher frequencies and related variabilities, as well as Arias intensities and inferred durations, which are all controlled by the generation of locally diffracted surface waves.

Design of perovskite photonic crystals for emission control

Here we design photonic nanostructures for perovskite thin film of CH3NH3PbI3 (MAPbI3). Optical properties of perovskite thin film are reported with the emission spectra peaked at 765 nm with FWHM of 50 nm. Based on the experimentally obtained optical constants, two-dimensional perovskite photonic crystals are designed through plane wave expansion and finite element methods. The calculated band structure of 2D photonic crystal slab shows the photonic band gap at 750-800 nm, originated from the x-y plane wave excitation. Incorporating one-hole and three-hole missing cavities in the photonic crystal slabs generates one and two modes in electric field spectra, respectively, indicating that the one hole missing cavity is more efficient for low power consumption of light source devices.

Impact of NID dielectric–magnetic medium on reflection and transmission characteristics of a Lorentz–Drude slab

Impact of non-integer dimensional (NID) dielectric–magnetic medium on the behavior of power transmitted and reflected from a planar NID Lorentz–Drude slab is analyzed for uniform electromagnetic plane wave excitation. The dispersive slab is terminated by NID dielectric–magnetic medium and dispersion in the slab has been indulge through the Lorentz–Drude model. It is assumed that medium inside the Lorentz–Drude slab and medium backing the slab are NID in direction perpendicular to surface of the slab but their dimensions are described by different NID parameters. According to the characteristics of the model, while the operating frequency increases the NID Lorentz–Drude slab operates as epsilon negative (ENG), double negative (DNG), mu negative (MNG), double positive (DPS) metamaterial respectively. Effect of NID parameter of the terminating medium, dielectric magnetic properties of the terminating medium and impact of their combination on the powers is noted one by one. It is noticed that parameters describing the NID dielectric–magnetic medium significantly change the behavior of powers. These parameters can be effectively used to control the power according to the requirements.

Second-harmonic generation in Mie-resonant dielectric nanoparticles made of noncentrosymmetric materials

We develop a multipolar theory of second-harmonic generation (SHG) by dielectric nanoparticles made of noncentrosymmetric materials with bulk quadratic nonlinearity. We specifically analyze two regimes of optical excitation: illumination by a plane wave and single-mode excitation, when the laser pump drives the magnetic dipole mode only. Considering two classes of nonlinear crystalline solids (dielectric perovskite material and III-V semiconductor), we apply a symmetry approach to derive selection rules for the multipolar composition of the nonlinear radiation. The developed description can be used for design of efficient nonlinear optical nanoantennas with reconfigurable radiation characteristics.

Fano interferences of electromagnetic modes in dielectric nanoblock dimers

The optical properties of a dielectric dimer consisting of two parallel identical nanoblocks are theoretically investigated. Each dielectric nanoblock has a magnetic dipole mode and a broad electric dipole mode, when the polarization of the plane wave excitation is along the long axis of the nanoblock. The two modes are spectrally overlapped. In such a dimer, the magnetic fields associated with the magnetic dipole modes on the two nanoblocks will rotate and form a subradiant mode. Multipole decomposition calculations show that the subradiant mode is induced by the spatial combination of the magnetic fields of a magnetic quadrupole mode and a toroidal dipole mode. The electric field associated with the subradiant mode undergoes destructive Fano interference with that of a broad electric dipole mode and results in a pronounced Fano dip on the scattering spectrum of the dimer. The Fano resonance varies with the distance between the two nanoblocks and the size of each nanoblock. The case with the polarization along the short axis of each nanoblock is also considered. Fano interference between a subradiant mode and a radiant electric mode can also appear in this configuration. But here the subradiant mode corresponds to the combined magnetic quadrupole and toroidal dipole modes on each nanoblock. Our study reveals complex electromagnetic interactions in simple dielectric nanostructures and could find applications in dielectric photonic devices.