arXiv subjects

David A. Powell

Publications and source records attributed to David A. Powell.

At least 19 recordsLinked to original sources

arXiv2025

Metagrating-based Single-pixel Acoustic Direction Finding

Acoustic metamaterials provide new opportunities for compact and efficient wavefront manipulation, extending beyond conventional bulky and power-intensive phased-arrays. In this work, we exploit the spatial encoding properties of the acoustic metagrating aperture to transform incident acoustic fields into compressed measurements for single-pixel acoustic source localisation. The proposed method enables accurate direction finding of acoustic sources over both 180 and 360 degrees angular ranges. Numerical simulations confirm the robustness of the metagrating-based compressive sensing approach against noise and limited sampling. Experimental validation is conducted to verify its feasibility with practical metagrating prototyes. Compared wiith tranditional array-based localisation techniques, the single-pixel metagrating system significantly reduces hardward complexity while maintaining high localisation accuracy. These findings demonstrate the potential of integrating compressive sensing with acoustic metagratings for compact, low-cost, scalable source detection systems, with prospective applications in industrial monitoring, target tracking and non-destructive health monitoring.

physics.app-ph↗

arXiv2024

Reconfigurable Manipulation of Sound with a Multi-material 3D-Printed Origami Metasurface

The challenge in reconfigurable manipulation of sound waves using metasurfaces lies in achieving precise control over acoustic behavior while developing efficient and practical tuning methods for structural configurations. However, most studies on reconfigurable acoustic metasurfaces rely on cumbersome and time-consuming control systems. These approaches often struggle with fabrication techniques, as conventional methods face limitations such as restricted material choices, challenges in achieving complex geometries, and difficulties in incorporating flexible components. This paper proposes a novel approach for developing a reconfigurable metasurface inspired by the Kresling origami pattern, designed for programmable manipulation of acoustic waves at an operating frequency of 2000 Hz. The origami unit cell is fabricated using multi-material 3D printing technology, allowing for the simultaneous printing of two materials with different mechanical properties, thus creating a bistable origami-based structure. Through optimization, two equilibrium states achieve a reflection phase difference of π through the application of small axial force, F, or torque, T. Various configurations of the metasurface, generated from different combinations of these two equilibria, enable distinct reflective behaviors with switchable and programmable functionalities. The principle of this work simplifies the shaping of acoustic waves through a straightforward mechanical mechanism, eliminating the need for complex control systems and time-consuming adjustments. This innovative approach paves a novel and effective perspective for developing on-demand switchable and tunable devices across diverse fields, including electromagnetics, mechanics, and elastics, leveraging multi-material printing technology.

physics.app-ph↗

arXiv2024

Reconfigurable Acoustic Metalens with Tailored Structural Equilibria

The ability to concentrate sound energy with a tunable focal point is essential for a wide range of acoustic applications, offering precise control over the location and intensity of sound pressure maxima. However, conventional acoustic metalenses are typically passive, with fixed focal positions, limiting their versatility. A significant obstacle in achieving tunable sound wave focusing lies in the complexity of precise and programmable adjustments, which often require intricate mechanical or electronic systems. In this study, we present a theoretical and experimental investigation of a reconfigurable acoustic metalens based on a bistable origami design. The metalens comprises eight flexible origami units, each capable of switching between two stable equilibrium states, enabling local modulation of sound waves through two distinct reflection phases. The metalens can be locked into specific symmetric or asymmetric configurations by manually tailoring the origami units to settle either of the two states. Each configuration generates a unique phase profile, focusing sound energy at a specific point. This concept allows the focal spot to be dynamically reconfigured both on and off-axis. Furthermore, the approach introduces a simple yet effective mechanism for tuning sound energy concentration, offering a solution for flexible acoustic manipulation.

physics.app-ph↗

arXiv2023

Ultra-broadband Noise-Insulating Periodic Structures Made of Coupled Helmholtz Resonators

Acoustic metamaterials and phononic crystals represent a promising platform for the development of noise-insulating systems characterized by a low weight and small thickness. Nevertheless, the operational spectral range of these structures is usually quite narrow, limiting their application as substitutions of conventional noise-insulating systems. In this work, the problem is tackled by demonstration of several ways for the improvement of noise-insulating properties of the periodic structures based on coupled Helmholtz resonators. It is shown that tuning of local coupling between the resonators leads to the formation of ultra-broad stop-bands in the transmission spectra. This property is linked to band structures of the equivalent infinitely periodic systems and is discussed in terms of band-gap engineering. The local coupling strength is varied via several means, including introduction of the so-called chirped structures and lossy resonators with porous inserts. The stop-band engineering procedure is supported by genetic algorithm optimization and the numerical calculations are verified by experimental measurements.

physics.app-ph↗

arXiv2022

Planar Metasurface Antenna with Tunable via Boundaries for Computational Imaging

The fusion of metasurface antennas and computational imaging facilitates the design of microwave imaging systems which require no lenses, phase shifters or moving parts. The technique involves the generation of appropriately designed diverse measurement modes to encode the scene information into a small number of measurements. We propose a novel boundary-tunable parallel plate waveguide-based metasurface antenna for computational microwave imaging. The proposed antenna leverages a switchable boundary of two layers of vias, to efficiently change the waveguide modes supported by the antenna cavity, leading to diverse measurement modes in the scene plane. The superiority of the boundary tuning approach over the frequency diversity approach for the same antenna is confirmed using the singular value decomposition. Synthetic imaging is performed using a coupled dipole model, which quantitatively proves the efficacy of the proposed antenna along with the robustness against noise down to 15 dB SNR.

physics.app-ph↗

arXiv2022

Willis coupling-induced acoustic radiation force and torque reversal

Acoustic meta-atoms serve as the building blocks of metamaterials, with linear properties designed to achieve functions such as beam steering, cloaking and focusing. They have also been used to shape the characteristics of incident acoustic fields, which led to the manipulation of acoustic radiation force and torque for development of acoustic tweezers with improved spatial resolution. However, acoustic radiation force and torque also depend on the shape of the object, which strongly affects its scattering properties. We show that by designing linear properties of an object using metamaterial concepts, the nonlinear acoustic effects of radiation force and torque can be controlled. Trapped objects are typically small compared to the wavelength, and are described as particles, inducing monopole and dipole scattering. We extend such models to a polarizability tensor including Willis coupling terms, as a measure of asymmetry, capturing the significance of geometrical features. We apply our model to a three-dimensional, sub-wavelength meta-atom with maximal Willis coupling, demonstrating that the force and the torque can be reversed relative to an equivalent symmetrical particle. By considering shape asymmetry in the acoustic radiation force and torque, Gorkov's fundamental theory of acoustophoresis is thereby extended. Asymmetrical shapes influence the acoustic fields by shifting the stable trapping location, highlighting a potential for tunable, shape-dependent particle sorting.

physics.app-ph↗

arXiv2022

Microacoustic metagratings at ultra-high frequencies fabricated by two-photon lithography

The recently proposed bianisotropic acoustic metagratings offer promising opportunities for passive acoustic wavefront manipulation, which is of particular interest in flat acoustic lenses and ultrasound imaging at ultra-high frequency ultrasound. Despite this fact, acoustic metagratings have never been scaled to MHz frequencies that are common in ultrasound imaging. One of the greatest challenges is the production of complex structures of microscopic size. Owing to two-photon polymerization, a novel fabrication technique from the view of acoustic metamaterials, it is now possible to precisely manufacture sub-wavelength structures in this frequency range. However, shrinking in size poses another challenge; the increasing thermoviscous effects lead to considerable losses, which must be taken into account in the design. In this work we propose three microacoustic metagrating designs refracting a normally incident wave towards -35$^{\circ}$ at 2 MHz. In order to develop metaatoms insensitive to thermoviscous effects we use shape optimization techniques incorporating the linearized Navier-Stokes equations discretized with finite element method. We report for the first time microscopic acoustic metamaterials manufactured using two-photon polymerization and, subsequently, experimentally verify their effectively using a capacitive micromachined ultrasonic transducer as source and an optical microphone as a detector in a range from 1.8 MHz to 2.2 MHz. We demonstrate not just that a microacoustic metagrating can effectively redirect the normally incident wave despite the thermoviscous losses, but also that it being only 0.29$λ$ thick can allocate 90% of the transmitted energy in the $-$1st diffraction order.

physics.app-ph↗

arXiv2021

Printed tapered leaky-wave antennas for W-band frequencies

Despite their great potential in communication and sensing applications, printed leaky-wave antennas have rarely been reported at mm-wave frequencies. In this paper, tapered leaky-wave antennas operating at 80 GHz are designed, fabricated and experimentally characterized. While most continuous leaky-wave antennas use subwavelength strips or other comparably small elements, in this work, the surface impedance is discretized very coarsely using only three square patches per period. With this architecture, a wide range of surface reactance can be achieved while maintaining a minimum feature size of the metallic pattern that is feasible for printed circuit fabrication. As the analytical solution for the bandstructure of sinusoidally modulated reactance surfaces is inaccurate for coarse discretization, we find it using full-wave simulation. In order to control side lobes effectively, we use a tapered aperture illumination according to the Taylor one-parameter distribution. A comprehensive experimental demonstration is presented, including near-field and far-field measurements. Therewith, we verify the designed aperture illumination and we reveal the origin of spurious far-field features. Side lobes are effectively suppressed and spurious radiation is reduced to -18 dB compared to the main lobe.

physics.app-ph↗

arXiv2020

Characterization of Broadband Focusing Microwave Metasurfaces at Oblique Incidence

We report the characterization of an achromatic focusing metasurface at oblique incident angles. We show that in addition to the inherent off-axis aberrations that occurs due to the hyperbolic phase profile of the metasurface, the focusing performance is significantly degraded due to the meta-atoms' angular dispersion. To obtain insights into how the angular and spectral bandwidth of meta-atoms relate to the metasurface focusing performance, point-dipole models are used which incorporate different aspect's of the meta-atoms' angular response. It is emphasized that despite the meta-atoms being designed under the assumption that they support a single dipolar resonance, other resonances exist within the meta-atom geometry and become stronger at oblique incidence. These resonances disturb the designed phase and amplitude responses, resulting in lower focusing efficiency at higher incident angles. The modelling of higher order modes leads to good agreement with the experimental measurements, confirming that angular dispersion of the meta-atoms is the dominant mechanism in determining off-axis aberrations.

physics.app-ph↗

arXiv2020

Bandwidth Limit and Synthesis Approach for Single Resonance Ultrathin Metasurfaces

Metasurfaces have emerged as a promising technology for the manipulation of electromagnetic waves within a thin layer. In planar ultrathin metasurfaces, there exist rigorous narrowband design methods, based on the equivalent surface impedance of patterned metallic layers on dielectric substrates. In this work, we derive a limit on bandwidth achievable in these metasurfaces, based on constraints that their meta-atoms should be passive, causal and lossless, and that they should obey the time-bandwidth product rules of a single resonance structure. The results show that in addition to elementary design parameters involving variation of the surface impedance, the bandwidth is critically limited by the dielectric substrate thickness and permittivity. We then propose a synthesis method for broadband ultrathin metasurfaces, based on an LC resonance fit of the required surface impedance, and experimentally verify a broadband dispersive structure at millimeter-wave frequencies. This results in a bandwidth enhancement of over 90%, relative to a reference metasurface created with the narrowband design process.

physics.app-ph↗

arXiv2020

High-efficiency refracting millimeter-wave metasurfaces

Printed circuit metasurfaces have attracted significant attention in the microwave community for their versatile wavefront manipulation capability. Despite their promising potential in telecommunications and radar applications, few transmissive metasurfaces have been reported operating at millimeter-wave frequencies. Several secondary effects including fabrication tolerances, interlayer near-field coupling and the roughness of conductors are more severe at such high frequencies and can cause significant performance degradation. Additionally, very accurate experimental techniques are required in order to characterise these effects. In this work, we present highly efficient refracting metasurfaces operating at 83 GHz. We use a synthesis technique that minimizes performance degradation due to effects such as interlayer near-field coupling and conductor roughness. Our experimental characterization includes an accurate determination of the intensity of all forward propagating Floquet harmonics in a broad frequency range. The experimental data shows very good agreement with full-wave simulation and verifies our synthesis method.

physics.app-ph↗

arXiv2019

Accurate metasurface synthesis incorporating near-field coupling effects

One of the most promising metasurface architectures for the microwave and terahertz frequency ranges consists of three patterned metallic layers separated by dielectrics. Such metasurfaces are well suited to planar fabrication techniques and their synthesis is facilitated by modelling them as impedance sheets separated by transmission lines. We show that this model can be significantly inaccurate in some cases, due to near-field coupling between metallic layers. This problem is particularly severe for higher frequency designs, where fabrication tolerances prevent the patterns from being highly-subwavelength in size. Since the near-field coupling is difficult to describe analytically, correcting for it in a design typically requires numerical optimization. We propose an extension of the widely used equivalent-circuit model to incorporate near-field coupling and show that the extended model can predict the scattering parameters of a metasurface accurately. Based on our extended model, we introduce an improved metasurface synthesis algorithm that gives physical insight to the problem and efficiently compensates for the perturbations induced by near-field coupling. Using the proposed algorithm, a Huygens metasurface for beam refraction is synthesized showing a performance close to the theoretical efficiency limit despite the presence of strong near-field coupling.

physics.optics↗

arXiv2018

Refraction efficiency of Huygens' and bianisotropic terahertz metasurfaces

Metasurfaces are an enabling technology for complex wave manipulation functions, including in the terahertz frequency range, where they are expected to advance security, imaging, sensing, and communications technology. For operation in transmission, Huygens' metasurfaces are commonly used, since their good impedance match to the surrounding media minimizes reflections and maximizes transmission. Recent theoretical work has shown that Huygens' metasurfaces are non-optimal, particularly for large angles of refraction, and that to eliminate reflections and spurious diffracted beams it is necessary to use a bianisotropic metasurface. However, it remains to be demonstrated how significant the efficiency improvement is when using bianisotropic metasurfaces, considering all the non-ideal features that arise when implementing the metasurface design with real meta-atoms. Here we compare concrete terahertz metasurface designs based on the Huygens' and Omega-type bianisotropic approaches, demonstrating anomalous refraction angles for 55 degrees, and 70 degrees. We show that for the lower angle of 55 degrees, there is no significant improvement when using the bianisotropic design, whereas for refraction at 70 degrees the bianisotropic design shows much higher efficiency and fidelity of refraction into the designed direction. We also demonstrate the strong perturbations caused by near-field interaction, both between and within cells, which we compensate using numerical optimization.

physics.app-ph↗

arXiv2018

Measuring monopole and dipole polarizability of acoustic meta-atoms

We present a method to extract monopole and dipole polarizability from experimental measurements of two-dimensional acoustic meta-atoms. In contrast to extraction from numerical results, this enables all second-order effects and uncertainties in material properties to be accounted for. We apply the technique to 3D-printed labyrinthine meta-atoms of a variety of geometries. We show that the polarizability of structures with shorter acoustic path length agrees well with numerical results. However, those with longer path lengths suffer strong additional damping, which we attribute to the strong viscous and thermal losses in narrow channels.

physics.app-ph↗

arXiv2018

Time-varying Huygens' meta-devices for parametric waves

Huygens' metasurfaces have demonstrated almost arbitrary control over the shape of a scattered beam, however, its spatial profile is typically fixed at fabrication time. Dynamic reconfiguration of this beam profile with tunable elements remains challenging, due to the need to maintain the Huygens' condition across the tuning range. In this work, we experimentally demonstrate that a time-varying metadevice which performs frequency conversion can steer transmitted or reflected beams in an almost arbitrary manner, with fully dynamic control. Our time-varying Huygens' metadevice is made of both electric and magnetic meta-atoms with independently controlled modulation, and the phase of this modulation is imprinted on the scattered parametric waves, controlling their shapes and directions. We develop a theory which shows how the scattering directionality, phase and conversion efficiency of sidebands can be manipulated almost arbitrarily. We demonstrate novel effects including all-angle beam steering and frequency-multiplexed functionalities at microwave frequencies around 4 GHz, using varactor diodes as tunable elements. We believe that the concept can be extended to other frequency bands, enabling metasurfaces with arbitrary phase pattern that can be dynamically tuned over the complete 2πrange.

physics.app-ph↗

arXiv2017

Interference between the modes of an all-dielectric meta-atom

The modes of silicon meta-atoms are investigated, motivated by their use as building blocks of Huygens' metasurfaces. A model based on these modes is presented, giving a clear physical explanation of all features in the extinction spectrum. Counter-intuitively, this can show negative contributions to extinction, which are shown to arise from the interference between non-orthogonal modes. The direct and interference contributions to extinction are determined, showing that conservation of energy is preserved. The Huygens' condition of matched electric and magnetic dipole moments leads to strong forward scattering and suppressed back scattering. It is shown that higher order modes with appropriate symmetry generalise this condition, leading to multiple bands of directional scattering. The presented results are obtained using a robust approach to find the modes of nano-photonic scatterers, commonly referred to as quasi-normal modes. By utilising an integral formulation of Maxwell's equations, this work avoids the problem of normalising diverging far-fields, which other approaches require. The model and presented results are implemented in open-source code.

physics.optics↗

arXiv2016

Tunable Focusing by a Flexible Metasurface

An efficient reflective elastic metasurface with tunable focusing point is proposed. The metasurface is based on electric resonators embedded in a stretchable elastic substrate. The focal length is controlled by mean of the stretching applied applied to the sample. The results predicted by theory and numerical simulations are experimentally verified. Our proposal shows that smart engineering elastic metamaterials are an effective platform for new functional devices based on metamaterials.

physics.optics↗

arXiv2015

Purcell effect in Hyperbolic Metamaterial Resonators

The radiation dynamics of optical emitters can be manipulated by properly designed material structures providing high local density of photonic states, a phenomenon often referred to as the Purcell effect. Plasmonic nanorod metamaterials with hyperbolic dispersion of electromagnetic modes are believed to deliver a significant Purcell enhancement with both broadband and non-resonant nature. Here, we have investigated finite-size cavities formed by nanorod metamaterials and shown that the main mechanism of the Purcell effect in these hyperbolic resonators originates from the cavity hyperbolic modes, which in a microscopic description stem from the interacting cylindrical surface plasmon modes of the finite number of nanorods forming the cavity. It is found that emitters polarized perpendicular to the nanorods exhibit strong decay rate enhancement, which is predominantly influenced by the rod length. We demonstrate that this enhancement originates from Fabry-Perot modes of the metamaterial cavity. The Purcell factors, delivered by those cavity modes, reach several hundred, which is 4-5 times larger than those emerging at the epsilon near zero transition frequencies. The effect of enhancement is less pronounced for dipoles, polarized along the rods. Furthermore, it was shown that the Purcell factor delivered by Fabry-Perot modes follows the dimension parameters of the array, while the decay rate in the epsilon near-zero regime is almost insensitive to geometry. The presented analysis shows a possibility to engineer emitter properties in the structured metamaterials, addressing their microscopic structure.

physics.optics↗