Non-reciprocal Acoustic Devices Research Articles

Realisation of broadband two-dimensional nonreciprocal acoustics using an active acoustic metasurface.

Nonreciprocal acoustic devices have been shown to be able to control incident waves propagating in one direction whilst allowing incident waves propagating in the opposite direction to be transmitted without modification. Nonreciprocal sound transmission, typically, has been achieved by introducing nonlinearities or directional biasing through fluid motion or spatiotemporal modulation of resonant cavities. However, the spatial arrangement of these approaches creates preferential characteristics in one direction such that the direction of the nonreciprocal behaviour is fixed and, thus, they are not straightforwardly reconfigurable. To address this issue, it has previously been revealed that feedforward wave-based active controllers can be employed to drive a single subwavelength active unit cell to achieve broadband nonreciprocal sound transmission or absorption in a one-dimensional linear acoustic system. Extending this concept, this paper investigates how the feedforward wave-based active controller can be used to drive an array of subwavelength active unit cells forming a metasurface to achieve broadband nonreciprocal sound absorption over a two-dimensional plane. Through simulation and experimental studies, this paper shows that active wave-based absorption control systems can achieve broadband nonreciprocal sound absorption when the incident waves are generated by normally and obliquely positioned primary sources.

Giant nonreciprocity of surface acoustic waves induced by an anti-magnetostrictive bilayer

Lack of nonreciprocity is one of the major drawbacks of solid-state acoustic devices, which has hindered the development of microwave-frequency acoustic isolators and circulators. Here, we report a giant nonreciprocal transmission of shear-horizontal surface acoustic waves (SH-SAWs) on a LiTaO3 substrate coated with a negative–positive magnetostrictive bilayer structure of Ni/Ti/FeCoSiB. Although the static magnetic moments of two layers are parallel, SH-SAWs can excite optical-mode spin waves much stronger than acoustic-mode ones at relatively low frequencies via magnetoelastic coupling. The measured magnitude nonreciprocity exceeds 40 dB (or 80 dB/mm) at 2.333 GHz. In addition, maximum nonreciprocal phase accumulation reaches 188° (376°/mm), which is desired for an effective SAW circulator. Our theoretical model and calculations provide an insight into the observed phenomena and demonstrate a pathway for further improvement of nonreciprocal acoustic devices.

Active nonreciprocal structural control on a flexible plate

Reciprocity is an acoustic property that describes the symmetry of sound transmission between two points. However, this property is undesirable in certain applications, and this has led to significant interest in the development of nonreciprocal acoustic devices that achieve one-way sound transmission. These devices typically achieve nonreciprocal sound transmission by introducing nonlinearities or directional biasing. Previously proposed nonreciprocal acoustic devices generally have limitations; for example, they may not be fully adaptable, they can introduce signal distortions such as additional harmonics, or they can only exhibit nonreciprocal behaviour over a narrow bandwidth. To overcome these challenges, previous work has demonstrated how an wave-based active control system can be used to drive an array of acoustic sources to achieve reversible and broadband non-reciprocal behaviour. This paper aims to extend this concept to a linear wave-based active structural-acoustic control system that uses an array of structural actuators to control the individual wave components to achieve broadband nonreciprocal transmission through a flexible plate in a three-dimensional environment. The performance of the wave-based active controller has been investigated at a range of incident angles and the limits of this controller have been identified.

Nonreciprocal acoustic transmission through dynamic multilayer structures

A time Floquet transfer-matrix method for the description of acoustic wave propagation through dynamic stratified structures, modulated by another, low-frequency, pump acoustic wave, is reported. The method is applied to a specific example of a GaAs/AlAs periodic superlattice, subject to a spatiotemporal modulation induced by an evanescent pump wave with frequency in the lowest acoustic band gap of the structure. By means of systematic numerical calculations, we provide compelling evidence for the occurrence of significant nonreciprocal transmission of an acoustic signal with frequency in a high-order acoustic band gap, through inelastic multiple-scattering processes. Our results indicate a promising route to design nonreciprocal acoustic devices.

Realisation of nonreciprocal transmission and absorption using wave-based active noise control

Nonreciprocal acoustic devices typically break reciprocity by introducing nonlinearities or directional biasing. However, these devices are generally not fully adaptable and often use resonant cavities, which only exhibit nonreciprocal behaviour over a narrow bandwidth. Therefore, to overcome these challenges, this paper investigates how wave-based active control can be used to achieve broadband nonreciprocal behaviour in a one-dimensional environment. Wave-based controller architectures are described for both transmission and absorption control and, through simulation and experimental implementations, it is shown that they can achieve broadband nonreciprocal behaviour. Importantly, the direction of nonreciprocal behaviour can be straightforwardly reversed.

Nonreciprocal Sound Propagation via Cascaded Time-Modulated Slab Resonators

In this work, we demonstrate nonreciprocal acoustic wave transmission in two-cascaded Fabry-Perot-like slab resonators undergoing time modulation of their effective density. A phase difference is introduced into the time modulation between the two coupled resonators to produce spatial bias to access unidirectional wave propagation. A theoretical model based on plane-wave expansion and the transfer-matrix method is developed to study the Fabry-Perot-based system. The theoretical model allows rapid and precise characterization of the acoustic dispersion, with results in excellent agreement with finite-element-based numerical simulations. We demonstrate acoustic nonreciprocity behavior for the fundamental acoustic mode and further show that, with proper optimization, the coupled Fabry-Perot-based device can achieve nonreciprocal acoustic frequency conversion. This work could inspire the design of simple and compact nonreciprocal acoustic devices for efficient wave control.

Wave-based control for nonreciprocal acoustics using a planar array of secondary sources

There has been significant interest in the design of nonreciprocal acoustic devices that allow acoustic waves to be perfectly transmitted in one direction, whilst the acoustic waves propagating in the opposite direction are blocked or reflected. Previously proposed nonreciprocal acoustic devices have broken the symmetry of transmission by introducing nonlinearities or resonant cavities. However, these nonreciprocal acoustic devices typically have limitations, such as signal distortions and the bandwidth over which nonreciprocal behaviour can be achieved is narrow. This paper will investigate how active control can be used to minimise the transmitted and reflected waves independently to achieve nonreciprocal sound transmission and absorption using a planar array of secondary sources in a two-dimensional environment. The advantage of the proposed active control system is that it is fully adaptable, which means that the directivity of nonreciprocal behaviour can also be reversed. The performance of the proposed wave-based active control system is investigated for a range of angles of incidence and its performance limitations are explored.

Acoustic Metagrating Circulators: Nonreciprocal, Robust, and Tunable Manipulation with Unitary Efficiency

Nonreciprocal signal operation is highly desired for various acoustic applications, where protection from unwanted backscattering can be realized so that transmitting and receiving signals are processed in a full-duplex mode. Here we present the realization of a class of nonreciprocal circulators based on simply structured acoustic metagratings, which consist only of a few solid cylinders and a steady fluid flow with low velocity. These innovative metagratings are intelligently designed via a diffraction analysis of the linearized potential flow equation and a genetic-algorithm-based optimization process. Unitary reflection efficiency between desired ports of the circulators are demonstrated through full-wave numerical simulations, confirming nonreciprocal and robust circulation of the acoustic signal over a broad range of flow velocity magnitude and profile. Our design provides a feasible degree of tunability, including switching from reciprocal to nonreciprocal operation and reversing the handedness of the circulator, presenting a convenient but efficient approach for the realization of nonreciprocal acoustic devices from wavelength-thick metagratings. It may find applications in various scenarios including underwater communication, energy harvesting, and acoustic sensing.

Semiconductor Fluorinated Carbon Nanotube as a Low Voltage Current Amplifier Acoustic Device

Acoustoelectric effect (AE) in a non-degenerate fluorinated single walled carbon nanotube (FSWCNT) semiconductor was carried out using a tractable analytical approach in the hypersound regime , where q is the acoustic wavenumber and is the electron mean-free path. In the presence of an external electric field, a strong nonlinear dependence of the normalized AE current density , on ( is the electron drift velocity and is the speed of sound in the medium) was observed and depends on the acoustic wave frequency, , wavenumber q, temperature T and the electron-phonon interactions parameter, . When , decreases to a resonance minimum and increases again, where the FSWCNT is said to be amplifying the current. Conversely, when , rises to a maximum and starts to decrease, similar to the observed behaviour in negative differential conductivity which is a consequence of Bragg’s reflection at the band edges at T=300K. However, FSWCNT will offer the potential for room temperature application as an acoustic switch or transistor and also as a material for ultrasound current source density imaging (UCSDI) and AE hydrophone devices in biomedical engineering. Moreover, our results prove the feasibility of implementing chip-scale non-reciprocal acoustic devices in an FSWCNT platform through acoustoelectric amplification.

Acoustic radiation pressure for nonreciprocal transmission and switch effects

Systems capable of breaking wave transmission reciprocity have recently led to tremendous developments in wave physics. We report herein on a concept that enables one-way transmission of ultrasounds, an acoustic diode, by relying on the radiation pressure effect. This effect makes it possible to reconfigure a multilayer system by significantly deforming a water-air interface. Such a reconfiguration is then used to achieve an efficient acoustic transmission in a specified direction of propagation but not in the opposite, hence resulting in a highly nonreciprocal transmission. The corresponding concept is experimentally demonstrated using an aluminum-water-air-aluminum multilayer system, providing the means to overcome key limitations of current nonreciprocal acoustic devices. We also demonstrate that this diode functionality can even be extended to the design and operations of an acoustic switch, thus paving the way for new wave control possibilities, such as those based on acoustic transistors, phonon computing and amplitude-dependent filters.

Non-Reciprocal Acoustic Transmission in a GaN Delay Line Using the Acoustoelectric Effect

This letter reports on first-time demonstration of non-reciprocal acoustic transmission in a gallium nitride (GaN) delay line structure. The split of forward (S21) and backward (S12) transmissions is observed by applying a dc electric field through the active area with a two-dimensional electron gas (2DEG) sheet. The non-reciprocity (~20.7 dB/mm max. in this letter) varies by tuning the gate voltage to deplete the 2DEG sheet carrier density, which agrees with model prediction for the interaction between 2DEG and surface acoustic waves. Our preliminary results prove the feasibility of implementing chip-scale non-reciprocal acoustic devices in a GaN platform through acoustoelectric amplification.

Design of one-way acoustic energy flow devices using Bayesian network classifiers

Linear acoustic metamaterials (AMM) displaying dynamic negative stiffness or density have been the subject of current research to design exotic devices such as acoustic super-lenses and cloaks. More recently, nonlinear AMM have been of interest as one means of creating non-reciprocal acoustic devices [Liang et al., Nat. Mater. 9 (2010)]. The efficiency of those types of non-reciprocal acoustic devices strongly depends on the material nonlinearity of one of its components and is therefore limited by existing fluids containing contrast agents. This work addresses this limitation by exploring the top-down design of a nonlinear AMM with non-resonant subwavelength structures that display non-monotonic pressure-volume strain response to generate an AMM with large parameters of nonlinearity. A simple one-way device consisting of a frequency-selective acoustic mirror and a nonlinear medium is described using a nonlinear multiscale model and a nonlinear propagation model that includes quadratic and cubic nonlinearity. Bayesian network classifiers (BNC) map regions of high performance at the device scale to each design level and intersect the high performance space across levels to identify multilevel solutions. Three design levels are considered: one-way performance, nonlinear acoustic propagation in the effective medium, and effective properties of the AMM based on subwavelength structure.

Subwavelength ultrasonic circulator based on spatiotemporal modulation

Enabling efficient nonreciprocal acoustic devices is challenging, yet very desirable for a variety of applications, including acoustic imaging, underwater communications, energy concentration and harvesting, signal processing, and noise control. We discuss the theory and design of a fully linear compact acoustic circulator based on spatiotemporal modulation of the effective acoustic index, providing a compact and practical way to realize large sound circulation at any desired frequency. Our proposal enables tunable isolation levels of over 40 dB, with insertion losses as low as 0.3 dB, in a noise-free, integrable, frequency scalable device whose total size does not exceed $\ensuremath{\lambda}/6$.

Non-reciprocal acoustic devices based on spatio-temporal angular-momentum modulation

Acoustic devices that break reciprocity, for instance acoustic isolators or circulators, may find exciting applications in a variety of fields, including imaging, acoustic communication systems, and noise control. Non-reciprocal acoustic propagation has typically been achieved using non-linear phenomena, which require high input power levels and introduce distorsions. In contrast, we have recently demonstrated compact linear isolation for audible airborne sound by means of angular momentum bias [Fleury et al., Science 343, 516 (2014)], exploiting modal splitting in a ring cavity polarized by an internal, constantly circulating fluid, whose motion is imparted using low-noise CPU fans. We present here an improved design with no moving parts, which is directly scalable to ultrasonic frequencies and fully integrable. Instead of imparting angular momentum in the form of a moving medium as in our previous approach, we make use of spatio-temporal acoustic modulation of three coupled acoustic cavities, a strategy that can be readily implemented in integrated ultrasonic devices, for instance, using piezoelectric effects. In this new paradigm, the required modulation frequency is orders of magnitude lower than the signal frequency, and the modulation efficiency is maximized. This constitutes a pivotal step towards practically realizing compact, linear, noise-free, tunable non-reciprocal acoustic components for full-duplex acoustic communications and isolation.