Acoustic Vortex Beams Research Articles

Enhanced Broadband Manipulation of Acoustic Vortex Beams Using 3‐bit Coding Metasurfaces through Topological Optimization (Small 19/2024)

Enhanced Broadband Manipulation of Acoustic Vortex Beams Using 3‐bit Coding Metasurfaces through Topological Optimization (Small 19/2024)

Paraxial and ray approximations of acoustic vortex beams.

A compact analytical solution obtained in the paraxial approximation is used to investigate focused and unfocused vortex beams radiated by a source with a Gaussian amplitude distribution. Comparisons with solutions of the Helmholtz equation are conducted to determine bounds on the parameter space in which the paraxial approximation is accurate. A linear relation is obtained for the dependence of the vortex ring radius on the topological charge, characterized by its orbital number, in the far field of an unfocused beam and in the focal plane of a focused beam. For a focused beam, it is shown that as the orbital number increases, the vortex ring not only increases in radius but also moves out of the focal plane in the direction of the source. For certain parameters, it is demonstrated that with increasing orbital number, the maximum amplitude in a focused beam becomes localized along a spheroidal surface enclosing a shadow zone in the prefocal region. This field structure is described analytically by ray theory developed in the present work, showing that the spheroidal surface in the prefocal region coincides with a simple expression for the coordinates of the caustic surface formed in a focused vortex beam.

Transdermal drug delivery mediated by acoustic vortex beam

Ultrasound-mediated transdermal drug delivery exhibits various advantages such as biocompatibility, controllability and safety, which attracts plenty of interests within biomedical field. Current researches mostly emphasizes the acoustic cavitation generated by planar or focused waves while neglecting other physics that occur during transportation. Our experimental study illustrates the presence of an acoustic vortex (AV) beam that exhibits a lower acoustic intensity and typically means a lower dose of inertial cavitation, yet achieves a more efficient delivery. Such a result calls for the fundamental mechanism of ultrasound-mediated transdermal transfer using the AV beam. In this work, according to our knowledge, the AV beam is firstly introduced to ultrasound-mediated transdermal medication delivery. The transversal acoustic radiation force (T-ARF), which is the primary characteristic carried by the acoustic vortex beam, and its contribution to the transport enhancement are investigated. It is shown that a focused AV (FAV) beam with a maximal acoustic pressure of 200 kPa induces a pN-level T-ARF, which promotes the enlargement of pores on the stratum corneum and thereby enhances the permeability, as compared with a zero-order (non-vortex) counterpart. This contribution of the T-ARF is validated by the experimental transport on the cellulose membrane, which exhibits a significantly increased membrane porosity and delivery efficiency. The favorable results introduce the new degree of freedom into the ultrasound-mediated transdermal drug transport based on AV beam, and thereby promotes the development of a combined control strategy for more precise and efficient transdermal drug delivery in conjunction with the administration of acoustic cavitation.

Underwater double vortex generation using 3D printed acoustic lens and field multiplexing

The generation of acoustic vortex beams has attracted an increasing amount of research attention in recent years, offering a range of functions, including acoustic communication, particle manipulation, and biomedical ultrasound. However, incorporating more vortices and broadening the capacity of these beams and associated devices in three dimensions pose challenges. Traditional methods often necessitate complex transducer arrays and are constrained by conditions such as system complexity and the medium in which they operate. In this paper, a 3D printed acoustic lens capable of generating a double vortex pattern with an optional focusing profile in water was demonstrated. The performance of the proposed lens was evaluated through computational simulations using finite element analysis and experimental tests based on underwater measurements. The results indicate that by altering the positioning of the vortices’ axes, it is possible to control both the intensity and the location of the pressurized zone. The proposed approach shows promise for enhancing the effectiveness and versatility of various applications by generating a larger number of vortices and freely tailoring the focal profile with a single lens, thereby expanding the practical uses of acoustic vortex technology.

Effects of increasing orbital number on the field transformation in focused vortex beams

Acoustic vortex beams possess helical wavefronts characterized by an orbital number n. In the presence of focusing, the toroidal vortex ring for n = 1 moves out of the focal plane z = d and toward the source with increasing n, accompanied by a departure from its toroidal shape. In this talk, ray theory is developed to explain this field transformation. An expression is derived for the radius and cross-sectional area of the annular channel formed by the family of rays emanating from a thin circular ring centered at the origin in the source plane z = 0. This expression leads to explicit results for the field amplitude and caustic coordinates due to a focused vortex source with an arbitrary axisymmetric amplitude distribution. Comparison with field calculations in which diffraction is included demonstrate that the caustics describe the redistribution of the global maximum and its shift toward the source plane with increasing n. For moderate focusing gains (of order 10 or 20), a toroidal vortex ring for n = 1 transforms with increasing n into a spheroidal surface in the prefocal region 0 < z < d having volume nλd 2/6, where λ is the wavelength, inside of which exists a shadow zone. [C.A.G. was supported by the ARL:UT McKinney Fellowship in Acoustics.]

Enhanced Broadband Manipulation of Acoustic Vortex Beams Using 3-bit Coding Metasurfaces through Topological Optimization.

The acoustic coding metasurfaces (ACMs) have the ability to manipulate complex acoustic behavior by reconstructing the coding sequence. In particular, the design of broadband coding enhances the versatility of ACMs. ACMs offer significant advantages over traditional metasurfaces, including a limited number of units and flexible wave control performance. The unit quantity is determined by 2n, with 1-bit utilizing 2 units, 2-bit using 4 units, and 3-bit employing 8 units. Utilizing multiple bits allows for precise control over the phase of sound waves and enables the realization of more intricate acoustic functions. To address the requirements of broadband multi-bit applications, this paper presents the development of novel 3-bit broadband reflected acoustic coding metasurfaces (BACMs) with eight coding units. These metasurfaces are systematically designed using the bottom-up topology optimization method. A constant phase difference of 45° can be achieved across all eight coding units within a broad frequency range. Additionally, the spiral distribution of phase differences enables the construction of an acoustic vortex metasurface. Moreover, by combining the convolution method, the strategies are outlined for constructing vortex-focusing metasurfaces and vortex beam manipulation metasurfaces. These 3-bit coding metasurfaces possess significant potential in the fields of acoustic particle suspension and acoustic communication.

Acoustic vortex filter based on tunable metasurfaces

In this paper, we present an acoustic vortex filter (AVF) based on tunable metasurfaces, which can selectively filter the incident multiplexed vortices that carry different orbital angular momentum (OAM). Our metasurface-based AVF is composed of an upper acoustic metasurface (UAM) and a lower acoustic metasurface, of which the intrinsic topological charge (ITC) can be tuned by mechanically rotating the UAM along its central axis. Due to the critical order of the propagating vortex modes in waveguide, controlling the ITC of the AVF allows for the selective filtering of incoming multiplexed acoustic vortex beams based on the sound vortex diffraction in phase-gradient metasurface, which endows the vortex filter the capability that let the incident vortex of specific OAM pass through it. In the following demonstration, both in theory and experiment, we design the AVF and effectively filter the acoustic vortices with two opposite topological charges by simply altering the orientation angle of the UAM. Based on this, we further demonstrate its application in asymmetric acoustic wave transmission. Our work offers an approach to selectively filter the incident acoustic vortex, which improves the capability to control the acoustic OAM via metasurfaces.

Ray tracing model for long-range acoustic vortex wave propagation underwater

The use of vortex waves in multiple environments is of increasing interest for numerous applications including underwater acoustic communications, particle manipulations, and sonothrombolysis. Finite element methods are limited in the range for which the propagation of these vortex beams may be simulated. On the other hand, ray tracing programs simulate well over long ranges, though are generally limited in their ability to resolve the features of a propagating vortex. Methods for overcoming these difficulties in simulating the long-range propagation of such waves in inhomogeneous environments have been developed and employed, though their specific implementation has not been thoroughly discussed. This manuscript provides the methods by which existing ray tracing programs may be used to approximate the long-range propagation of acoustic vortex beams in complex environments.

Spatiotemporal Acoustic Vortex Beams with Transverse Orbital Angular Momentum.

Recently, the discovery of optical spatiotemporal (ST) vortex beams with transverse orbital angular momentum (OAM) has attracted increasing attention and is expected to extend the research scope and open new opportunities for practical applications of OAM states. The ST vortex beams are also applicable to other physical fields that involve wave phenomena, and here we develop the ST vortex concept in the field of acoustics and report the generation of Bessel-type ST acoustic vortex beams. The ST vortex beams are fully characterized using the scalar approach for the pressure field and the vector approach for the velocity field. We further investigate the transverse spreading effect and construct ST vortex beams with an ellipse-shaped spectrum to reduce the spreading effect. We also experimentally demonstrated the orthogonality relations between ST vortex beams with different charges. Our study successfully demonstrates the versatility of the acoustic system for exploring and discovering spatiotemporally structured waves, inspiring further investigation of exotic wave physics.

Generation of acoustic vortices and acousto-optic interactions with acoustic vortex beams.

We demonstrate the generation of acoustic vortex beams with the aid of an acoustic spiral phase plate and an acoustic grating having a fork-like bifurcation of its slits. The acoustic vortex beam generated in this manner influences the particles floating on a water surface and rotates them. Our experiments show that the intensity of the acoustic vortex beam increases from zero in its center to a certain maximal value near the shell of the beam. The phase of the beam changes by the angle 2π whenever the azimuthal angle is changed by the same value. We also study acousto-optic diffraction of the optical beam by the acoustic beam bearing a vortex and demonstrate that each of the appropriate diffraction maxima are split into two maxima. The latter correspond to diffraction by the elementary acoustic rays which are close to the opposite edges of the acoustic vortex beam. A dark region between the maxima appears due to destructive interference of the elementary optical rays originated from the diffraction by the opposite sides of the acoustic vortex beam, which are phase-shifted by π.

Broadband achromatic acoustic vortex generator based on integrated-resonant meta-atoms

Acoustic vortex generator based on metamaterials is of great importance in acoustic communication, particle manipulation, and other applications. However, most of these applications are limited by narrow bandwidth due to strong dispersion. In this work, we numerically and experimentally present a broadband achromatic acoustic vortex generator (BAAVG) based on integrated-resonant meta-atoms (IRMs). The proposed IRMs, composed of a straight channel with side-loaded resonators and a large cavity, demonstrate linear phase shifts with high transmission efficiency and guarantee the existence of broadband vortex resulting from constant phase difference between adjacent IRMs. Based on the proposed BAAVG, conversion from plane wave to high purity vortex (with average purity 0.9) is demonstrated from 2187 to 2787 Hz. Good agreement between numerical and experimental results verifies the effectiveness of the proposed BAAVG. The high vortex purity of BAAVG exhibits broadband conversion properties quantitatively compared with traditional acoustic vortex generator. Our study provides an avenue to construct broadband achromatic acoustic vortex generators, which advances the capability of designing broadband acoustic vortex beams for versatile applications.

Sound Waveguiding by Spinning: An Avenue toward Unidirectional Acoustic Spinning Fibers

Although optical fiber has revolutionized modern life, creating a corresponding fiber as a waveguide for airborne sound has proven difficult, due to the scarcity of high-index acoustic materials. In this study the authors utilize spinning of the air inside a metamaterial tube to create an acoustic fiber. Their simulations demonstrate nonreciprocal propagation resulting from the acoustic equivalent of the Zeeman effect, which leads to robust waveguiding in the proposed fiber. This contribution may open avenues for studying the physics of acoustic vortex beams and nonreciprocal phenomena.

Inverse-designed single-phase elastic metasurfaces for underwater acoustic vortex beams

During the past decade, the passive metasurfaces enabling vortex beams carrying orbital angular momentum have drawn great interest in the fields of electromagnetic and acoustic wave. In the underwater environment, however, the elastic metasurface with a certain bandwidth is more difficult to be realized due to the strong fluid-solid interaction and rigorous impedance matching. To overcome this challenge, an inverse-design strategy is proposed to systematically construct a single-phase underwater metasurface for converting the incident plane waves into vortex beams on demand. All topology-optimized metasurface elements, possessing both the positive/negative effective dynamic properties for the full phase control, can support the desired phase modulation and high transmission. We numerically and experimentally demonstrate the acoustic functionality of converting the plane waves in free space to the first-order vortex modes. The present passive single-phase metasurface by inverse design may offer opportunities for underwater acoustic communication.

The development trends and research fronts in orbital angular momentum technology: A bibliometric analysis

Orbital angular momentum (OAM) technology, refers to Laguerre-Gaussian (LG) beams, twisted beams, vector/vortex beams, acoustic vortex beams and fractional vortex beams. It is an emerging and promising technology to improve the communication capacity, spectral efficiency, and anti-jamming capability due to its helical phase fronts and infinite orthogonal states. Although the OAM research began in the 1990s, the developing trends, current status, issues and characteristics through a systematic observation have not yet been performed. This paper presents a knowledge-based evolution of OAM research published in the Web of Science (WoS) from 2011 to 2021 using bibliometric analysis in Citepspace. The results demonstrate that the bandwidth, efficiency, gain, divergence, phase quantization, bulky and complex feeding structures, misalignment, distortion, interferences atmospheric turbulence and diffraction were the key issues found in the OAM technology. The main research hotspots and categories, influential authors, leading journals, best institutions of OAM show a strong bias in favor of their functions and technology developments. The research on OAM was mainly performed by the counties that have developed the 5G and now moving towards 6G communications like China, USA and South Korea. This study would serve as an inclusive guide on the future research trends and status especially for the OAM researchers.

Mode converter of vortex beams by phase-gradient acoustic metagratings

Recently, acoustic vortices with orbital angular momentum (OAM) have attracted great attention and interest owing to their extensive applications. Based on the generalized Snell’s law (GSL), acoustic vortex beams can be observed by constructing different types of acoustic metasurfaces immersed into cylindrical waveguides. However, owing to the restrictions of the phase gradient of metasurface structures and the critical order of propagating vortex beams, there exist great challenges in realizing complex modulation of vortex beams and designing advanced multifunctional sound devices. Here, we theoretically study and experimentally demonstrate the extended GSL in designing a mode converter of vortex beams based on phase-gradient acoustic metagratings (AMs). We demonstrate that the extended GSL can not only accurately predict the transmitted or reflected orders of vortex beams created by a series of AMs with arbitrary phase gradient ξ, but also effectively overcome the limitations of both the phase gradient and the critical order. More importantly, we design a type of mode converter of vortex beams and realize the mode conversion of output vortex beams by simply modulating the orders of input vortex beams. To present it, we experimentally demonstrate the switch between the output vortex beams with the orders of −2 and 2 through the same phase-gradient AM by using the input vortex beams with the orders of 1 and −1, respectively, which can be separately predicted by the GSL and the extended GSL. Our work paves a way for the complex modulation of acoustic vortex beams and the design of multifunctional OAM-based sound devices with practical applications.

Phase-Dislocation-Mediated High-Dimensional Fractional Acoustic-Vortex Communication.

With unlimited topological modes in mathematics, the fractional orbital angular momentum (FOAM) demonstrates the potential to infinitely increase the channel capacity in acoustic-vortex (AV) communications. However, the accuracy and stability of FOAM recognition are still limited by the nonorthogonality and poor anti-interference of fractional AV beams. The popular machine learning, widely used in optics based on large datasets of images, does not work in acoustics because of the huge engineering of the 2-dimensional point-by-point measurement. Here, we report a strategy of phase-dislocation-mediated high-dimensional fractional AV communication based on pair-FOAM multiplexing, circular sparse sampling, and machine learning. The unique phase dislocation corresponding to the topological charge provides important physical guidance to recognize FOAMs and reduce sampling points from theory to practice. A straightforward convolutional neural network considering turbulence and misalignment is further constructed to achieve the stable and accurate communication without involving experimental data. We experimentally present that the 32-point dual-ring sampling can realize the 10-bit information transmission in a limited topological charge scope from ±0.6 to ±2.4 with the FOAM resolution of 0.2, which greatly reduce the divergence in AV communications. The infinitely expanded channel capacity is further verified by the improved FOAM resolution of 0.025. Compared with other milestone works, our strategy reaches 3-fold OAM utilization, 4-fold information level, and 5-fold OAM resolution. Because of the extra advantages of high dimension, high speed, and low divergence, this technology may shed light on the next-generation AV communication.

Acoustic radiation torque on an interface reflecting acoustic vortex beams

Torque exerted by acoustic waves to rotate objects can result from asymmetry such as the case of an averaged torque on a Rayleigh disk exerted by an ordinary acoustic field. In the absence of asymmetry, acoustic torque can occur by viscous absorption either in the object or in the adjacent boundary layer of the object when using specific types of acoustic fields such as acoustic vortex beams carrying twisted wave fronts [Zhang and Marston, Phys. Rev. E 84, 065601 (2011)] or two orthogonal standing wave fields [Zhang and Marston, J. Acoust. Soc. Am. 136, 2917 (2014)]. Recent study suggested a torque on a flat water-air interface exerted by an ultrasonic vortex beam obliquely incident and reflecting off the interface [Zou, Lirette, and Zhang, Phys. Rev. Lett. 125, 074301 (2020)]. This torque is not associated with viscous absorption and instead results from variation of orbital angular momentum carried by the vortex beam during the reflection. The torque would otherwise disappear in normal incidence or using ordinary beams. The investigations advance understanding of radiation torque physics and approaches for non-contact manipulations using acoustic waves.

Beam alignments based on the spectrum decomposition of orbital angular momentums for acoustic-vortex communications

Given the enhanced channel capacity of wave chirality, acoustic communications based on the orbital angular momentum (OAM) of acoustic-vortex (AV) beams are of significant interest for underwater data transmissions. However, the stringent beam alignment is required for the coaxial arrangement of transceiver arrays to ensure the accuracy and reliability of OAM decoding. To avoid the required multiple measurements of the traditional orthogonality based algorithm, the beam alignment algorithm based on the OAM spectrum decomposition is proposed for AV communications by using simplified ring-arrays. Numerical studies of the single-OAM and OAM-multiplexed AV beams show that the error of the OAM spectrum increases with the translation distance and the deflection angle of the transceiver arrays. To achieve an ideal arrangement, two methods of the single-array translation alignment and the dual-array deflection alignment are developed based on the least standard deviation of the OAM spectrum (SD-OAM). By decreasing the SD-OAM towards zero using transceiver arrays of 16 transmitters and 16 receivers, accurate beam alignments are accomplished by multiple adjustments in three dimensions. The proposed method is also demonstrated by experimental measurements of the OAM dispersion and the SD-OAM for misaligned beams. The results demonstrate the feasibility of the rapid beam alignment based on the OAM spectrum decomposition by using simplified transceiver ring-arrays, and suggest more application potentials for acoustic communications.

Acoustic vortex beam reflection, refraction, and orbital Hall effect

Some of our recent work concerned the interactions of twisted wave fronts of acoustic vortex beams with boundaries and inhomogeneities of media. Experimental measurements and numerical simulations were conducted to investigate vortex beam reflection from a water-air interface and vortex beam refraction when travelling in an inhomogeneous medium or through an acoustic metasurface. The results reveal spatial evolution of the wave fields and the carried angular momenta. In particular, the investigation observed a reversal of orbital angular momentum in the reflected wave field [Zou et al., Phys. Rev. Lett. 125(7), 074301 (2020)], and a shift of beam center and an asymmetric vortex pattern in the refracted wave field in the direction transverse to the plane of incidence [Fan et al., Phys. Rev. Res. 1(3), 032014 (2019); 3(1), 013251 (2021)]. The shift relates to conservation of angular momenta and coupling of the twisted wave fronts with the refraction and is termed as acoustic orbital Hall effect.

Implementation of an ultrasonic fluid manipulation experiment on the International Space Station

The International Space Station (ISS) provides a unique opportunity to investigate acoustic radiation force interactions with a minimal interference from gravity. There are many design considerations to make when creating an experiment that is suited for use on board the ISS. NASA’s primary concerns with all experiments implemented in the ISS are with the safety of the crew and the success of the mission. Acoustic experiments are surprisingly rare on the ISS and present additional unique design challenges. This is especially true for experiments operated under water. Our work implements a high-intensity acoustic vortex beam under water for single droplet tractoring and capture operated inside the Microgravity Science Glovebox (MSG) on the ISS. The experiment payload consists of a signal generator, an amplifier module, and a sealed water chamber. A secondary fluid immiscible with water is injected into the chamber for acoustic manipulation, and the results of the experiment are recorded as high-definition video.