Acoustic Focusing Research Articles

Acoustic metasurface development for transmitted wavefront manipulation using a level set-based topology optimization approach

A customized transmission-type metasurface design is developed for high-efficiency wavefront modulation by employing a level set-based topology optimization approach. The metasurface consists of periodically arranged supercells made of discrete unit cells aiming to form a precisely linear phase gradient of the transmitted wave ranging from 0 to 2π with high transmittances. The configuration of each unit is optimized to generate a desired phase and amplitude response of the transmitted wave at the interface. The transmitted wavefront steering realized by the phase-gradient metasurface is numerically and experimentally verified according to the diffraction theory, demonstrating that the level set-based topology optimization approach is a viable and effective technique for developing a transmitted phase-gradient metasurface. Afterwards, the designed optimal unit cells are elaborately arranged to form desired phase shift distributions for realizing fascinating acoustic functionalities, including acoustic energy focusing and acoustic Airy-like beam generation, further demonstrating the ability of the optimal units to achieve unprecedented transmitted wavefront control. This study presents a promising approach for customized transmission-type metasurface design based on the level set-based topology optimization, which is expected to be significantly valuable for acoustic device development.

Piezoelectric metasurface for high-frequency ultrasonic transducer application around 50 MHz

High-frequency ultrasound plays a vital role in ultrasonic imaging and particle manipulation. However, the control of high-frequency ultrasound fields with traditional acoustic lens presents challenges in impedance matching, precise control, material selection. Therefore, piezoelectric ultrasonic transducers prepared based on traditional methods are difficult to achieve high frequency and efficient sound beam control. To address these challenges, we have developed a piezoelectric metasurface-based ultrasonic transducer, using the encode binary information method to modulate acoustic waves. Simulation results reveal that at a frequency of 50 MHz, the system exhibits efficient acoustic focusing capabilities, which have been successfully applied in the experimental trapping of microspheres with acoustic tweezers. The design and development of high-frequency ultrasound transducers have emerged as a burgeoning research area at the confluence of acoustics and numerous interdisciplinary fields.

Fast and accurate transcranial ultrasound simulation using the asymptotic model of the Civa Healthcare platform

To ensure its efficacy and safety, transcranial ultrasound therapy treatment planning requires accurate pressure field simulations and phase law corrections. Despite their long computation time and high memory usage, full numerical methods are often used since they are considered more accurate than semi-analytical methods. This work will present the so-called “pencil method“, a fast asymptotic model embedded in the CIVA HealthCare simulation platform. It allows computation in harmonic and impulse mode and the consideration of complex configurations, including solid obstacles, considering, at each interface, refractions and reflections with or without mode conversion of the acoustic field. This model was successfully compared to a recent collaborative work by Aubry et al. that presented a set of numerical benchmarks for transcranial propagation, to allow comparisons between various modeling tools. It was used to investigate the influence of parametric variation of skull material properties on the quality of acoustic focusing through the human skull. Its ability to predict the thermal rise at the intracranial target was validated against experimental data obtained ex-vivo through human skulls. Finally, works in progress will be shared about its connection to the open-source Kranion software developed at the FUS Foundation to facilitate comparison between clinical and simulated data.

A Microflow Cytometer Enabled by Monolithic Integration of a Microreflector with an Acoustic Resonator.

Current microflow cytometers suffer from complicated fluidic integration and low fluorescence collection efficiency, resulting in reduced portability and sensitivity. Herein, we demonstrated a new flow cell design based on an on-chip monolithically integrated microreflector with a bulk acoustic wave resonator (MBAW). It enables simultaneous 3D particle focusing and fluorescence enhancement without using shear flow. Benefited by the on-chip microreflector, the captured fluorescence intensity was 1.8-fold greater than that of the Si substrate and 8.3-fold greater than that of the SiO2 substrate, greatly improving the detection sensitivity. Combined with the contactless acoustic streaming-based focusing, particle sensing with a coefficient of variation as low as 6.1% was achieved. We also demonstrated the difference between live and dead cells and performed a cell cycle assay using the as-developed microflow cytometry. This monolithic integrated MBAW provides a new type of opto-acoustofluidic system and has the potential to be a highly integrated, highly sensitive flow cytometer for applications such as in vitro diagnostics and point of care.

Multilateral Sound Manipulation Using Magnetically Tunable Apertures

In recent years, planar aperture‐based acoustic devices have been investigated due to their thin form factor, high effective transmission, and sound modulation capability. Moreover, reconfigurable and tunable devices are being continuously researched to enable multiple degrees of freedom in real time. Herein, an aperture‐based acoustic device is presented, which enables tunable multilateral ultrasonic operation. Sound waves are manipulated in multiple directions by continuously tuning aperture width or length, using a magnetically controlled rod. The position, orientation, and geometrical properties of the rod (i.e., length, topographical curvature) are exploited to create a varied interface for the impinging sound. Through experiments and simulations, acoustic wave steering in different orthogonal planes and acoustic switching using a single tunable device unit are demonstrated. Furthermore, different four‐unit device configurations are implemented for tunable beam‐formation and tunable acoustic focusing applications by utilizing the acoustic anisotropy of the system. The tunable dynamic acoustic device is scalable to audible frequencies and enables tilted operation. This straightforward and accessible proof‐of‐concept opens a paradigm for exploring multifunctional aperture‐based designs with greater degrees of freedom, bringing us a step closer toward practical applications such as acoustic device integration with walls, window panels, and other commercial products for tunable sound transmission.

Programmable inverse-designed acoustic coding metasurfaces for multifunctional broadband reflection

In this paper, we develop tunable reflection-type broadband acoustic coding metasurfaces (BACMs) using a bottom-up topology optimization method. These metasurfaces enable flexible configuration switching between the antiphase coding units of “0” and “1” across a wide frequency range through electromagnetic regulation. By introducing a customized dispersion design, the optimized structure exhibits broadband properties. The optimized coding units consist of irregular air channels and connected air cavities. The coding bits of “0” and “1” show a constant opposite phase difference over a wide frequency range through the coupling resonances. By adjusting the electromagnets, arbitrary coding sequences in the BACMs can be rapidly reconstructed. We numerically and experimentally demonstrate the acoustic focusing and splitting properties within the frequency range of 2.2 kHz–4.4 kHz.

Cavitation cloud formation and surface damage of a model stone in a high-intensity focused ultrasound field

This work investigates the fundamental role of cavitation bubble clouds in stone comminution by focused ultrasound. The fragmentation of stones by ultrasound has applications in medical lithotripsy for the comminution of kidney stones or gall stones, where their fragmentation is believed to result from the high acoustic wave energy as well as the formation of cavitation. Cavitation is known to contribute to erosion and to cause damage away from the target, yet the exact contribution and mechanisms of cavitation remain currently unclear. Based on in situ experimental observations, post-exposure microtomography and acoustic simulations, the present work sheds light on the fundamental role of cavitation bubbles in the stone surface fragmentation by correlating the detected damage to the observed bubble activity. Our results show that not all clouds erode the stone, but only those located in preferential nucleation sites whose locations are herein examined. Furthermore, quantitative characterizations of the bubble clouds and their trajectories within the ultrasonic field are discussed. These include experiments with and without the presence of a model stone in the acoustic path length. Finally, the optimal stone-to-source distance maximizing the cavitation-induced surface damage area has been determined. Assuming the pressure magnitude within the focal region to exceed the cavitation pressure threshold, this location does not correspond to the acoustic focus, where the pressure is maximal, but rather to the region where the acoustic beam and thereby the acoustic cavitation activity near the stone surface is the widest.

Graphene-based phononic crystal lenses: Machine learning-assisted analysis and design

The advance of artificial intelligence and graphene-based composites brings new vitality into the conventional design of acoustic lenses which suffers from high computation cost and difficulties in achieving precise desired refractive indices. This paper presents an efficient and accurate design methodology for graphene-based gradient-index phononic crystal (GGPC) lenses by combing theoretical formulations and machine learning methods. The GGPC lenses consist of two-dimensional phononic crystals possessing square unit cells with graphene-based composite inclusions. The plane wave expansion method is exploited to obtain the dispersion relations of elastic waves in the structures and then establish the data sets of the effective refractive indices in structures with different volume fractions of graphene fillers in composite materials and filling fractions of inclusions. Based on the database established by the theoretical formulation, genetic programming, a superior machine learning algorithm, is introduced to generate explicit mathematical expressions to predict the effective refractive indices under different structural information. The design of GGPC lenses is conducted with the assistance of the machine learning prediction model, and it will be illustrated by several typical design examples. The proposed design method offers high efficiency, accuracy as well as the ability to achieve inverse design of GGPC lenses, thus significantly facilitating the development of novel phononic crystal lenses and acoustic energy focusing.

Multifunctional comb-like acoustic metasurface for transmissive wavefront manipulation

Precise and highly efficient wavefront modulation is greatly appreciated in acoustic engineering. Here, a comb-like acoustic metasurface is developed to efficiently manipulate the transmitted wavefront. The comb-like unit consists of an array of rigid rectangular partitions supported by a base. By adjusting the height of these rectangular elements, the entire 2π transmission phase shift can be achieved. The controllability of the phase shifts is demonstrated through theoretical and numerical analysis. A particle swarm optimization algorithm is employed to determine the height of the rectangular elements to achieve accurate target phase shift while maintaining high transmission efficiency. As a case study, highly efficient acoustic focusing and waveform conversion are achieved by comb-like metasurfaces, and the effect of unit deficiency on focusing performance is discussed. Furthermore, the application of comb-like metasurfaces to generate a self-bending beam and Airy beam is verified, and the acoustic beam of self-repair performance and robustness are demonstrated. In addition, acoustic focusing is realized using two symmetric Airy beams generated from the comb-like metasurface. Experiments are conducted to verify the effectiveness of the designed metasurfaces. This work presents an effective and feasible design solution for flexible wavefront modulation.

Reconfigurable and Phase‐Engineered Acoustic Metasurfaces for Broadband Wavefront Manipulation

AbstractA novel type of phase‐engineered acoustic metasurfaces with reconfigurable properties is reported, enabling the flexible broadband manipulation of reflected wavefronts. The participants of the metasurface are elements conceived to possess even‐distributed reflected phases covering 2π span with linearity and small acoustic energy loss. The reconfigurable property of the metasurface is implemented by rearranging the fixed meta‐elements based on the phase profile, which is related to the characteristic of a specific wavefront shape. The metasurface's capability is successfully demonstrated to achieve acoustic focusing and bending within the frequency range of 2300–2800 Hz, showcasing its feasibility and adaptability. To enhance its practical applications, porous materials are incorporated, leveraging the robustness of phase differences among the meta‐elements to achieve high acoustic energy cancellation. Effective sound attenuation occurs within the frequency range of 1300–3100 Hz, even under wide‐angle incidences ranging from −80° to 80°. The work paves the way for further research on reconfigurable acoustic metasurfaces in broad frequency regions and exerts favorable implications for generally applicable structures applied in multi‐fields including biomedical acoustics, noise control, and so on.

Acoustic metascreen for broadband wavefront manipulation with stationary phase gradients

The development of an efficient acoustic metascreen utilizing artificial micro-structured metamaterials holds paramount importance for the manipulation of acoustic wavefronts. However, the expansive broadband capability of such metascreens is often compromised due to the variations in amplitude and phase profiles of the conceived structures at different frequencies. To address this issue, we introduce a broadband metascreen composed of several interconnected acoustic cavities. The concept is substantiated by the theoretical explication of the abrupt cross-section tube model. The resultant metascreen demonstrates high transmission efficiency and maintains a consistent phase profile over a wide frequency band, which implies that the relative phase disparities amongst neighboring pixels are retained at uniform intervals across the entire frequency spectrum, enabling the distinct broadband functionality of the metascreens developed. Through both simulations and experimental measurements, we exhibit the ability of the metascreen to manipulate the wavefront of transmitted waves, accomplishing acoustic focusing and self-bending across multiple frequencies. Our research paves the way for a novel and multifunctional method to manipulate broadband wavefronts through transmissive grooved acoustic metamaterial, offering a practical and promising methodology to develop advanced acoustic apparatuses.

Acoustic wave focusing using holographic acoustic metalens and its optimization using gaussian process

In this study, we present experimental and numerical results of an efficient acoustic focusing apparatus using a series of impedance-varying acoustic metasurfaces. The impedance-varying surfaces are formed by changing the cylindrical intrusions on the metasurface, comprising a pattern that directs the acoustic beam to a desired angle. We show design parameters that play an important role in the focus points, focal area, and focus efficiency and verify this through experiment and multi-physics simulation approach. In addition, we show the prediction and optimization process of the acoustic metalens using the Gaussian process.

Anomalous refraction of acoustic waves using double layered acoustic grating

The paper proposes a double layered acoustic grating for fulfilling acoustic focusing in an anomalous direction. The acoustic grating consists of two layers of rigid panels with periodically perforated slits. By optimizing the positions of the slits on the two layers, both positive and negative refractive indices can be achieved with the phase shift tailored within [-π/2, π/2]. This allows acoustic energy of an obliquely incident plane wave to converge in a predefined focusing region in any direction. The paper predicts the wave propagation manipulated by the acoustic grating based on the surface coupling approach. Then, it discusses how to optimize the slits' positions to collimate the acoustic energy of an obliquely incident plane wave in a specific direction. Such acoustic grating has various potential applications, such as deflecting outdoor noise away from sensitive areas in building acoustics, enhancing acoustic energy in a target audience area in auditorium design, collimating acoustic surface waves, etc.

Ultrasound focusing enhancement through the stiff plate with inversely optimized metamaterial lens

Conventional ultrasound focusing implementations generally rely on steering the transmitted ultrasound wave through the openings in the designed lenses, which are ineffective for ultrasound focusing through a plate without openings. Here, we have proposed an inverse optimization scheme to fabricate a metamaterial lens that can enhance ultrasound focusing through a stiff plate immersed in water without any openings. This metamaterial lens can enhance the power transmission, and at the same time focus the transmitted power well. The optimized acoustic focusing enhancement lens shows great potential in medical imaging and disease treatment.

A Manufacturability‐Driven Method for Designing Metamaterial Focusing Acoustical Lenses

Herein, a method for designing and fabricating acoustic metamaterial focusing lenses for imaging applications such as nondestructive testing is proposed. First, a refractive index distribution is designed using the time delay method for a target focal length. Second, unit cell structures that can achieve the required refractive indices are found using the effective property retrieval method. At this point, factors such as fabrication cost and manufacturing limitations are considered. Finally, the pressure field from the designed lens is calculated using finite element (FE) modelling and iterated until the required focusing performance is achieved. To increase manufacturability, the unit cells necessarily become larger than required for the homogeneous regime. As an example, a lens of focal length 200 ± 10 mm at a frequency of 40 kHz is designed using cross‐shaped unit cells and fabricated using three‐dimensional printing. The design methodology leads to 2.4 mm (wavelength/3.6) unit cells which operate near the cut‐off frequency of the first mode. The lens is experimentally tested, achieving a focal length of 208 mm and a focal width of 12 mm (1.4 × wavelength), and showing good agreement with an FE model. The focusing ability of the lens is demonstrated by measuring the size of various holes in thin plates.

Acoustic standing wave with a frequency sweeping in a microfluidic system of parallel channels

In acoustofluidics, particles driven by fluid flows in microchannels are subject to acoustic waves, which are transmitted through the channel walls. Increasing the flow rate, one of the major challenges in microfluidic operation, can be achieved by parallelization of channels. In the present paper, inducing acoustic waves in devices with 4 and 16 parallel channels in silicon-based chip is studied. The resonance quality was evaluated by acoustic focusing of particles in the channels. First, the quality of resonance was measured in every channel separately as a function of the wave frequency. Second, a methodology with a sweeping of the frequency with time was explored to compensate for the variation of the optimal frequency between the channels.As a result, for a chip with a single channel, the sweeping is less efficient because of the lower energy transfer due to a non-optimal acoustic wave frequency. However, this developed method allowed simultaneous resonance in all the channels, which is not possible with application of a single frequency.

Response of sound propagation characteristics to Luzon cold eddy coupled with tide in the Northern South China Sea

The northern part of the South China Sea is a complex ocean environment with a large range of tidal waves and the stable Luzon cold eddy, which significantly influence the sound propagation characteristics through their impact on sound speed. This study uses a 3D ocean-acoustic framework consisting of MITgcm and BELLHOP ray model to investigate the coupled effects of the Luzon cold eddy and tidal waves. Firstly, the model incorporates tidal assimilation to reconstruct the hydrographic field and compute the sound speed field. Subsequently, the sound propagation in a representative region in the northern South China Sea is simulated to compare the response of sound propagation characteristics to the Luzon cold eddy only, tide only, and the coupled effect of both. The results demonstrate that the cold eddy shifts the location of the convergence zone forward by over 5 km at most. Further, it also makes the acoustic energy focus on the first few arrivals and delays the arrival time of rays by about 0.1 s. The tidal waves intensify these effects, resulting in a further increase of the forward distance of the convergence zone by 2-5 km and a delay in arrival time by 0.02 s. Sound propagation in the coupled influence of these two dynamic processes is exposed to steady perturbations from the cold eddy and spatial-temporal perturbations from tidal waves. The model in this study provides valuable insight for underwater detection and positioning in the realistic ocean environment.

MXene-based photoacoustic transducer with a high-energy conversion efficiency.

The applications of two-dimensional transition metal carbide/nitride (MXene) in the fields of optoelectronics, sustainable energy, and sensors, among others, have been broadly investigated due to their special electrical, optical, and structural properties. In this Letter, MXene (Ti3C2Tx) has been firstly, to the best of our knowledge, adopted for the application of a photoacoustic transducer by taking advantage of the photothermal property. The efficiency of the photoacoustic transducer based on a sandwich structure of glass/MXene/polydimethylsiloxane (PDMS) has been experimentally demonstrated to be 1.25 × 10-2 by converting laser pulses into ultrasonic waves, generating a high acoustic pressure of 15.7 MPa without additional acoustic focusing. That can be explained by the great light absorption and photothermal conversion of the Ti3C2Tx layer.

Real-Time Acoustic Simulation Framework for tFUS: A Feasibility Study Using Navigation System

Transcranial focused ultrasound (tFUS), in which acoustic energy is focused on a small region in the brain through the skull, is a non-invasive therapeutic method with high spatial resolution and depth penetration. Image-guided navigation has been widely utilized to visualize the location of acoustic focus in the cranial cavity. However, this system is often inaccurate because of the significant aberrations caused by the skull. Therefore, acoustic simulations using a numerical solver have been widely adopted to compensate for this inaccuracy. Although the simulation can predict the intracranial acoustic pressure field, real-time application during tFUS treatment is almost impossible due to the high computational cost. In this study, we propose a neural network-based real-time acoustic simulation framework and test its feasibility by implementing a simulation-guided navigation (SGN) system. Real-time acoustic simulation is performed using a 3D conditional generative adversarial network (3D-cGAN) model featuring residual blocks and multiple loss functions. This network was trained by the conventional numerical acoustic simulation program (i.e., k-Wave). The SGN system is then implemented by integrating real-time acoustic simulation with a conventional image-guided navigation system. The proposed system can provide simulation results with a frame rate of 5 Hz (i.e., about 0.2 s), including all processing times. In numerical validation (3D-cGAN vs. k-Wave), the average peak intracranial pressure error was 6.8 ± 5.5%, and the average acoustic focus position error was 5.3 ± 7.7 mm. In experimental validation using a skull phantom (3D-cGAN vs. actual measurement), the average peak intracranial pressure error was 4.5%, and the average acoustic focus position error was 6.6 mm. These results demonstrate that the SGN system can predict the intracranial acoustic field according to transducer placement in real-time.

Time reversal acoustic focusing in a reverberant room: Elongating a focus, and the effect of head-scattering

Time reversal acoustics (TRA) has tremendous potential for focusing, or sound concentration, in reverberant architectural environments. While TRA provides an inherently simple means of achieving a high degree of spatiotemporal focusing, its appropriateness for listening applications in the built environment is challenged by the need to accommodate a person within a sufficiently large focal area. At its simplest, a reciprocal time reversal focusing system for audible range sound will produce a focus with a spatial spread corresponding to one-eighth of the wavelength, which is inconveniently small for speech frequencies. However, an elongated focus is introduced by deploying a directional impulse response, which can be created with a directional microphone, or synthesized by time-delay-mixing of multiple omnidirectional impulse responses. This paper reports on results of physical experiments conducted in a reverberant room, complemented by finite-different time-domain simulations in which the effect of introducing a head-sized scatterer was investigated.