Transmission Of Plane Waves Research Articles

Mechanochromics of stretchable nano metasurfaces: non-close-packed hexagonal lattices and tunable structural coloration

Under the bio-mechanism of a chameleon’s color change, in this letter, a 2D nano metasurface, mechanical-strain-actuated photonic crystal, is proposed. The lattice, a nano hexagon with non-close-packed cylinder holes, is analyzed via a mechanochromic coupling model and plane wave transmission to verify the optical structural color in strain. The metasurface material was fabricated by soft lithography on a stretchable substrate. The strain versus reflectivity spectra were measured, showing a full visible range color change, at a strain of 29%. The RGB colors in the strained state were maintained after 2000 repeated loading/unloading cycles, illustrating fatigue insensitivity.

Acoustic wave propagation in effective graded fully anisotropic fluid layers.

This work deals with the sound wave propagation modeling in anisotropic and heterogeneous media. The considered scattering problem involves an infinite layer of finite thickness containing an anisotropic fluid whose properties can vary along the layer depth. The specular transmission and reflection of an acoustic plane wave by such a layer is modeled through the state vector formalism for the acoustic fields. This is solved using three different numerical techniques, namely, the transfer matrix method, Peano series, and transfer Green's function. These three methods are compared to demonstrate the convergence of the numerical solutions. Moreover, the implemented numerical procedures allow the authors to retrieve the internal acoustic fields and show their dependency along with the fluid anisotropic properties. Results are presented to illustrate the changes in absorption that can be achieved by tuning the fluid anisotropy as well as the variation of these properties across the depth of the layer. The results presented are in very good agreement across the different methods. Given that many porous materials can be modeled as equivalent fluids, the results presented show the potential offered by such numerical techniques, and can further give more insight into inhomogeneous anisotropic porous materials.

Active control of plane waves: Transmission, reflection, absorption and steering

Active control of an incident acoustic wave using an array of secondary source has been extensively investigated. In general, the objective is to attenuate the propagating wave so as to generate a space with a reduced sound pressure level. However, there are a variety of applications where more flexible manipulation of the acoustic wave propagation is required. For example, in architectural acoustics it may be desirable to achieve a particular level of absorption rather than total cancellation of the acoustic wave. This paper presents an investigation into the use of an array of secondary sources to control the transmission, reflection, absorption and steering of an incident plane wave. A formulation is presented that enables the secondary sources to be driven to achieve the desired levels of transmission, reflection and absorption, as well as control over the angle of wave propagation. The requirements in terms of the secondary sources, error sensors and reference sensors are discussed and the physical limits on the control performance are evaluated.

Acoustic excitation and detection of guided waves in a submerged aluminum plate using an angular spectrum approach

An angular spectrum approach can be used to obtain high-resolution measurements of the acoustic plane wave reflection and transmission coefficients corresponding to a given planar sample of material. Such an approach involves exciting the sample with a compact broadband source located in the near field and then scanning the resulting reflected or transmitted field on a plane parallel to the sample surface. Fourier transforms in both space and time then yield a description of the acoustic field in terms of its angular spectrum, or expansion into time-harmonic plane waves propagating throughout a range of frequencies and angles. To demonstrate this method, measurements made on a thick aluminum plate submerged in water are shown. In this case the reflection and transmission coefficients are intricately structured, with individual features corresponding to different symmetric and antisymmetric guided wave modes propagating within the plate. The coincidence effect, by which the plate is nearly transparent acoustically at just those frequency-angle pairs that excite guided waves, is clearly seen in the measurements. Measurements made on other materials, as well as possible extensions to this method, will also be discussed. [Research supported by ONR.]

Transient Plane-Wave Transmission Through an N-Layer Structure by the Method of Subregions

The temporal transmission properties of an N -layer structure under planar wave incidence are investigated by the method of subregions via the wave matrix approach. The transmission coefficient expression is first expressed in the frequency-domain by reduced transmission coefficients and then transformed into the time domain, which includes unit step functions with different effective time periods and weightings. The analysis is validated by numerical computations and three-dimensional electromagnetic simulations at gigahertz frequencies.

Microbubble Radiation Force-Induced Translation in Plane-Wave Versus Focused Transmission Modes.

Due to the primary radiation force, microbubble displacement has been observed previously in the focal region of single-element and array ultrasound probes. This effect has been harnessed to increase the contact between the microbubbles and targeted endothelium for drug delivery and ultrasound molecular imaging. In this study, microbubble displacements associated with plane-wave (PW) transmission are thoroughly investigated and compared to those obtained in focused-wave (FW) transmission over a range of pulse repetition frequencies, burst lengths (BLs), peak negative pressures, and transmission frequencies. In PW mode, the displacements, depending upon the experimental conditions, are in some cases consistently higher (e.g., by 28%, when the longest BL was used at PRF = 4 kHz), and the axial displacements are spatially more uniform compared to FW mode. Statistical analysis on the measured displacements reveals a slightly different frequency dependence of statistical quantities compared to transient peak microbubble displacements, which may suggest the need to consider the size range within the tested microbubble population.

Plane-Wave Transmission Through an Array of Dielectric-Loaded Slots in a Thick Metallic Screen: Normal Incidence

In this paper, we derive an analytical model for plane-wave transmission through a periodic array of dielectric-loaded slots in an infinite metallic screen of finite thickness (a metagrating). We show the existence of (Fano) resonances associated with the presence of higher order modes in the slots, in addition to the well-known (Fabry-Perot) resonances associated with the dominant mode in the slots. The Fano resonances arise from a physical mechanism distinct from that which is responsible for so-called extraordinary transmission through a grating, which is strongly dependent on the coupling among the slots. Comparison of the analytical model with the results of full-wave simulations shows excellent agreement when the characteristic dimensions of the grating are small compared with a free-space wavelength.

Controllability of surface plasmon polariton far-field radiation using a metasurface

In this study, a point-scattering approach to the plane-wave optical transmission of subwavelength metal nanoslit arrays with varying angles of rotation and that of subwavelength metal supercell arrays consisting of nanoslits capable of various angles of rotation is developed. It is demonstrated that the suggested theories show good agreement with the simulations and experiments. The results show that constructive and destructive interference at each nanoslit can respectively enhance and suppress the surface plasmon polariton (SPP) far-field radiation of a metasurface. The proposed theory can predict the quantity and resonant wavelength of SPPs and provide a design scheme for an SPP device.

Wireless, Real-Time Plane-Wave Coherent Compounding on an iPhone: A Feasibility Study.

The processing power in commercially available hand-held devices has improved dramatically in recent years. In parallel, techniques used in high-frame-rate medical ultrasound imaging, especially plane-wave (PW) imaging, have reduced the number of ultrasound transmissions and amount of data necessary to reconstruct an ultrasound image. In combination, the processing power and data reduction allow all of the processing steps in ultrasound image formation, from raw ultrasound channel data to final rendering, to be performed on a hand-held device. In this study, we send the raw ultrasound channel data from a research scanner wirelessly to an off-the-shelf hand-held device. The hand-held unit's graphical processing unit is processing the raw ultrasound data into the final image, achieving real-time frame rates on the order of 60-90 frames per second (FPS) for a single-angle PW transmission. Higher quality images are achieved by trading off frame rate by coherently compounding multiple PW images, resulting in frame rates on the order of, e.g., 13 FPS when coherently compounding 7 PW transmissions. The presented setup has the potential of providing image quality which could be valuable for simple medical ultrasound diagnostic scans of, e.g., the carotid artery or thyroid. Also, since the computationally expensive beamforming can be done in off-the-shelf devices, this could reduce the price of hand-held ultrasound systems in the future.

Response of fractional order on energy ratios at the boundary surface of fluid-piezothermoelastic media

In the present investigation, the phenomenon of reflection and transmission of plane waves from the interface between fluid half space and transversely isotropic piezothermoelastic solid half space with fractional order derivative is discussed. Energy ratios are computed with the use of amplitude ratios. The law of conservation of energy across the interface is verified. It is found that the energy ratios are the functions of angle of incidence, frequency of independent wave and depend on the different piezothermoelastic material. Cadmium selenide (CdSe) piezothermoelastic material has been considered which is in welded contact with water. Variations of energy ratios corresponding to the reflected and transmitted waves with respect to different values of fractional order derivative are shown graphically. A particular reduced case is also discussed.

Reflection and transmission of elastic waves at an interface between two micropolar piezoelectric half-spaces

In the present paper, we consider a problem of reflection and transmission of plane waves at an interface between two different transversely isotropic micropolar piezoelectric half-spaces. The plane wave solution of governing equations for micropolar piezoelectric medium indicates the propagation of three coupled plane waves. An incident plane wave at an interface between two dissimilar half-spaces generates three reflected and three transmitted waves. The relations between amplitude ratios of reflected and transmitted waves are derived which, in general, depend on material parameters and angle of incidence. The explicit expressions for energy ratios of reflected and transmitted waves are also obtained. For relevant material parameters, the results are illustrated graphically to show the micropolar piezoelectric effects on variations of amplitude ratios and the square root of energy ratios against the angle of incidence.

Teaching and Learning Electromagnetic Plane Wave Reflection and Transmission Using 3D TV [Education Corner

In this article, the notion of teaching and learning electromagnetic (EM) plane wave reflection and transmission using 3D TV is presented. The necessary equipment setup for 3D TV demonstration in a classroom setting is described. The contents of 3D TV, comprising left and right images, are generated in real time by a program on a notebook PC. Projected onto the 3D TV screen, these images combine appropriately to enable stereoscopic view through the use of 3D glasses.

Dynamic coherence factor based on the standard deviation for coherent plane-wave compounding

The ultrafast imaging technique based on plane wave transmission has been a commonly investigated imaging mode in medical ultrasound imaging. Coherent plane-wave compounding (CPWC) was proposed to improve the quality of plane-wave imaging (PWI), which obtained a high-quality image by summing the low-quality images formed by transmitting plane waves at different steering angles. Coherence factor (CF) weighting algorithms can effectively improve image contrast with low computational complexity. However, this usually introduces black artifacts and degraded speckle quality. In this paper, we propose a dynamic coherence factor (DCF) that is based on the angular difference of CPWC to adaptively determine the number of plane waves and utilizes several plane waves with small angular difference to evaluate coherence. To improve resolution and contrast-to-noise ratio (CNR), adjusted DCF (ADCF) introduces a parameter related to the standard deviation to adjust DCF. Furthermore, a square neighborhood ADCF (SN-ADCF) using a 2-dimensional average filter is designed to obtain higher contrast and speckle quality. The simulated, experimental and in-vivo datasets are used to evaluate the proposed methods. Results show that, in comparison with CF, DCF can achieve improved contrast ratio (CR), CNR and speckle signal-to-noise ratio (sSNR). ADCF achieves a maximal lateral full width at half maximum (FWHM) improvement by 40% and better speckle quality than CF. SN-ADCF causes about 20% improvements upon CF in CNR and sSNR, while maintaining a similar resolution performance. SN-ADCF also provides higher lateral resolution than the generalized coherence factor (GCF) and scaled coherence factor (scCF), meanwhile obtaining a comparable contrast and speckle quality. Therefore, SN-ADCF has a satisfying comprehensive performance, which can achieve a reasonable balance among resolution, contrast and speckle quality.

Fast Acoustic Wave Sparsely Activated Localization Microscopy (fast-AWSALM): Ultrasound Super-Resolution using Plane-Wave Activation of Nanodroplets.

Localization-based ultrasound super-resolution imaging using microbubble contrast agents and phase-change nano-droplets has been developed to visualize microvascular structures beyond the diffraction limit. However, the long data acquisition time makes the clinical translation more challenging. In this study, fast acoustic wave sparsely activated localization microscopy (fast-AWSALM) was developed to achieve super-resolved frames with sub-second temporal resolution, by using low-boiling-point octafluoropropane nanodroplets and high frame rate plane waves for activation, destruction, as well as imaging. Fast-AWSALM was demonstrated on an in vitro microvascular phantom to super-resolve structures that could not be resolved by conventional B-mode imaging. The effects of the temperature and mechanical index on fast-AWSALM was investigated. Experimental results show that sub-wavelength micro-structures as small as 190 lm were resolvable in 200 ms with plane-wave transmission at a center frequency of 3.5 MHz and a pulse repetition frequency of 5000 Hz. This is about a 3.5 fold reduction in point spread function full-width-half-maximum compared to that measured in conventional B-mode, and two orders of magnitude faster than the recently reported AWSALM under a non-flow/very slow flow situations and other localization based methods. Just as in AWSALM, fast-AWSALM does not require flow, as is required by current microbubble based ultrasound super resolution techniques. In conclusion, this study shows the promise of fast-AWSALM, a super-resolution ultrasound technique using nanodroplets, which can generate super-resolution images in milli-seconds and does not require flow.

Reflection by and transmission through an ENZ interface

We have studied the reflection and transmission of traveling and evanescent plane waves, incident upon an ENZ material. The Fresnel reflection and transmission coefficients were obtained in the ENZ limit. For a p polarized incident wave, the transmission coefficient vanishes, except very close to normal incidence. The reflection coefficient is -1 for both traveling and evanescent waves. It is shown, however, that there is a finite electric field in the ENZ material, even though the transmission coefficient is zero. This field is either linearly polarized or circularly polarized. The magnetic field in the medium for p polarized illumination is zero, and therefore there can be no energy flow through the material. For s polarization, the magnetic field in the medium is circularly polarized, and energy can flow through the material, parallel to the interface.

Design and optimisation of a phased array transducer for ultrasonic inspection of large forged steel ingots

Large steel forgings are used in numerous industries. This paper investigates the possibility of adapting ultrasonic phased array testing to the inspection of forged steel blocks with dimensions of up to 1000 mm in every direction. The paper looks at two objectives in order to provide the best inspection performance: (1) the ultrasonic phased array probe optimisation and (2) the ultrasonic wave transmission sequence in a total focusing method imaging scenario. The CIVA software suite was used to optimise the phased array probe element count and width in a full matrix capture configuration. Based on the simulation results, a 32-element transducer was then built and tested in a 777 mm forged steel block using full matrix capture, plane wave and Hadamard transmission sequences. It was observed that plane wave and Hadamard sequences transmit significantly more energy inside the test sample because the elements are emitting simultaneously, therefore leading to an improved signal-to-noise ratio. However, the horizontal resolution was a strong limitation for every transmission sequence, especially for plane waves, because of the limited range of angles available in a block of large dimensions. The Hadamard transmission sequence was shown to represent the best compromise in terms of defect reflected amplitude, signal-to-noise ratio and resolution. The experimental results were compared with simulations and were found to be in very good agreement.

Microbubble translation in plane-wave ultrasound transmission

Primary radiation force is capable of translating microbubbles in the focal region of single-element and array ultrasound probes. This effect can be harnessed to enhance the contact between ligand-bearing microbubbles and targeted endothelium for applications in targeted drug delivery and ultrasound molecular imaging. In this study, displacements of lipid-coated microbubbles associated with plane-wave transmission are investigated using the multi-gate Doppler approach, and compared with focused-wave transmission at equivalent peak negative pressures. In plane wave transmission, the radiation force is nearly uniform over the field of view and therefore allows for a more uniform translation of microbubbles compared to focused wave. Statistically determined median displacements are in good agreement with the axial and lateral ultrasound beamplots both in plane-wave and focused-wave transmissions, while peak microbubble displacements reveal a number of discrepancies. Distinct size-isolated microbubble populations (diameters 1–2 μm, 3–4 μm, 4–5 μm, 5–8 μm, and polydisperse) were tested, showing important differences in their displacements and a strong driving frequency dependence thereof. These findings help tune the ultrasound transmission parameters for uniform and size-selective microbubble translations.

Investigation of ultrasonic backscatter using three-dimensional finite element simulations.

Theoretical models are commonly used to describe ultrasonic backscattering in polycrystalline materials. However, although a full multiple scattering formalism has been derived, due to the difficulty in evaluation, currently only the single and double scattering effects have been evaluated. Three-dimensional finite element (3D FE) models have recently been demonstrated to be capable of predicting ultrasonic attenuation in polycrystalline materials and thereby show great potential in overcoming this limitation. In this paper, the application of 3D FE models is extended to the backscatter problem. First, longitudinal-to-longitudinal backscattering amplitudes from single grains are predicted, where the setup and configuration of the finite element (FE) model are verified with an isotropic spherical inclusion for which an exact solution is available. Subsequently, backscatter in terms of the root-mean-square noise levels in two different pulse-echo scenarios is investigated; the first is an idealised configuration with plane wave transmission and point reception; the second represents a more realistic finite-size transducer acting with the same apodization in both transmission and reception. Comparisons of FE predictions and approximate theoretical solutions within a range of validity show good agreement; however, the results demonstrate that 3D FE is useful where the simple Independent Scatterer models break down. As computing power increases, 3D FE is an increasingly viable tool to further the understanding of wave propagation in polycrystalline materials.

Using reflection and transmission coefficients to retrieve surface parameters for an anisotropic metascreen: With a discussion on conversion between TE and TM polarizations

In recent work, we derived generalized sheet transition conditions (GSTCs) for electromagnetic fields at the surface of a metascreen (a metasurface with a “fishnet” structure, i.e., a periodic array of arbitrary spaced apertures in a relatively impenetrable surface). The effective electric and magnetic surface susceptibilities and surface porosities that appear explicitly in the GSTCs are uniquely defined and as such serve as the physical quantities that most appropriately characterize the metascreen. These surface parameters are related to the geometry of the apertures that constitute the metascreen and can exhibit anisotropic properties if this geometry is sufficiently asymmetric. These anisotropic properties can result in mode conversion between transverse electric (TE) and transverse magnetic (TM) polarizations when a plane-wave is incident onto a metascreen. Here, we use these GSTCs to derive the plane-wave reflection (R) and transmission (T) coefficients of an anisotropic metascreen, in which the coupling between TE and TM polarizations is illustrated. These expressions for R and T are used to develop a retrieval approach for determining the uniquely defined effective surface susceptibilities and surface porosities that characterize the metascreen from measured or simulated data. We present an example of a metascreen whose apertures are filled with a high-contrast dielectric, which shows interesting resonances at frequencies where no resonances exist when the apertures are not filled.

Echo-mode aberration tomography: Sound speed imaging with a single linear array

Tomographic sound speed imaging has previously demonstrated the capability of producing images of comparable quality to that of X-ray CT and MRI. Traditionally, such reconstructions have only been achievable in transmission mode, either using diametrically opposed linear arrays or ring arrays. This is due to the conventional wisdom that forward scatter data are necessary for reconstruction in the general case, and consequentially, such setups are typically limited to easily externalized, soft tissues such as the female breast and thus are impractical for clinical usage. Recently, it has been demonstrated that in the presence of diffuse scatterers (Jaeger, 2015), pulse-echo reconstructions of slowness (inverse sound speed, proportional to refractive index) is feasible with a conventional single conventional linear array. By correlating data acquired with steered plane wave transmissions, depth dependent maps of phase lags can be generated and subsequently used to solve a multilinear inverse problem. The resulting images allow for baseband, speckle-free characterization of the underlying medium, which is complementary to the data acquired in traditional B-mode ultrasound. In this presentation, the fundamentals of echo-mode aberration tomography will be reviewed, completely with algorithmic formulation, beamformation considerations, and current challenges in practical reconstruction.