Acoustic Topological Insulator Research Articles

Robustness analysis and prediction of topological edge states in topological elastic waveguides

Topological elastic waveguides constructed using acoustic topological insulators have garnered significant attention due to their exceptional wave modulation properties. While the existence of these edge states is guaranteed by topology, their robustness to defects is unclear. In this paper, topological edge states based on the acoustic pseudo-spin Hall effect are constructed, and the robustness of the topological edge states is quantitatively studied by analyzing displacement fields of phononic crystal (PnC) plates with various defects. Our robustness assessment considers nearly all possible defect scenarios, focusing on the influence of defects on three primary indicators: transmittance, maximum displacement and its specific location on the PnC plate. The results indicate that the topological edge states formed by this structure are highly robust to defects with varying rotation angles, but exhibit limited robustness to defects of different dimensions or positions. Furthermore, a Light Gradient Boosting Machine (LightGBM) model is employed to predict the displacement along the wave transmission path in the presence of diverse lattice defects. The model emerges as an accurate predictor of displacement distribution changes, and thus can provide potential optimization strategies for topologically elastic waveguide-based energy harvesting systems and self-powered sensors.

Valley edge states with opposite chirality in temperature dependent acoustic media

The valley degree of freedom in phononic crystals and metamaterials holds immense promise for manipulating acoustic and elastic waves. However, the impact of acoustic medium properties on valley edge state frequencies and their robustness to one-way propagation in valley topological phononic crystals remains unexplored. While significant attention has been devoted to scatterer design embedded in honeycomb lattices within acoustic and elastic media to achieve valley edge states and topologically protected nontrivial bandgaps, the influence of variations in acoustic medium properties, such as wave velocity and density affected by environmental temperature, has been overlooked. In this study, we investigate the effect of valley edge states and topological phases exhibited by topological phononic lattices in a temperature-dependent acoustic medium. We observe that a decrease in wave velocity and density, influenced by changing environmental temperature, shifts the topological valley edge states to lower frequencies. Therefore, alongside phononic lattice design, it is crucial to consider the impact of acoustic medium properties on the practical application of acoustic topological insulators. This issue becomes particularly significant when a topological phononic crystal is placed in a wave medium that transitions from incompressible to compressible, where wave velocity and density are no longer constant. Our findings offer a novel perspective on investigating topological insulators in variable acoustic media affected by changing thermodynamic and fluid properties.

Pseudospin-dependent acoustic topological edge and corner states in silica aerogel metamaterialsa).

Fueled by the concepts of topological insulators, analogous topological acoustics offer an alternative approach to manipulate sound. Theoretical proposals for subwavelength acoustic topological insulators are considered to be ideal effective parameters or utilizeing artificial coiling-space metamaterials. However, the corresponding realization using realistic soft metamaterials remains challenging. In this study, we present the design of an acoustic subwavelength second-order topological insulator using nanoscale porous solid material, silica aerogel, which supports pseudospin-dependent topological edge and corner states simultaneously. Through simulations and experiments, we demonstrate that silica aerogel can function as a soft acoustic metamaterial at the subwavelength scale. By embedding silica aerogel in an air matrix to construct a honeycomb lattice, a double Dirac cone is obtained. A topological phase transition is induced by expanding or contracting the supercell, resulting in band inversion. Additionally, we propose topologically robust acoustic transmission along the one-dimensional edge. Furthermore, we discover that the proposed sonic crystal sustains zero-dimensional corner states, which can efficiently confine energy at subwavelength corners. These findings offer potential for the realization of subwavelength topological acoustic devices using realistic soft metamaterials.

Enhanced surrounding sound signal acquisition for arbitrary source location by the acoustic higher-order topological insulator

Enhanced surrounding sound signal acquisition for arbitrary source location by the acoustic higher-order topological insulator

Observation of an acoustic topological Euler insulator with meronic waves

Topological band theory has conventionally been concerned with the topology of bands around a single gap. Only recently non-Abelian topologies that thrive on involving multiple gaps were studied, unveiling a new horizon in topological physics beyond the conventional paradigm. Here, we report on the first experimental realization of a topological Euler insulator phase with unique meronic characterization in an acoustic metamaterial. We demonstrate that this topological phase has several nontrivial features: First, the system cannot be described by conventional topological band theory, but has a nontrivial Euler class that captures the unconventional geometry of the Bloch bands in the Brillouin zone. Second, we uncover in theory and probe in experiments a meronic configuration of the bulk Bloch states for the first time. Third, using a detailed symmetry analysis, we show that the topological Euler insulator evolves from a non-Abelian topological semimetal phase via. the annihilation of Dirac points in pairs in one of the band gaps. With these nontrivial properties, we establish concretely an unconventional bulk-edge correspondence which is confirmed by directly measuring the edge states via. pump-probe techniques. Our work thus unveils a nontrivial topological Euler insulator phase with a unique meronic pattern and paves the way as a platform for non-Abelian topological phenomena.

Multifrequency corner states in acoustic second-order topological insulators

Multifrequency corner states in acoustic second-order topological insulators

Subwavelength acoustic topology frequency band regulation based on symmetric site-space folded resonant cavities

Acoustic topological insulators, as a type of acoustic metamaterial, possess special acoustic wave manipulation capabilities. However, an acoustic topological insulator based on Bragg scattering requires its lattice constant to be equivalent to the wavelength. This means that constructing acoustic topological insulators efficiently in the low-frequency range is a challenge. To this end, this paper proposes a method to construct sub-wavelength acoustic topological insulators by adding spatially folded resonant cavities at symmetric locations. It can easily reduce unidirectional transmission frequency bands with topological protection to subwavelength scales. On this basis, it is even possible to realize effective control of the unidirectional transmission band over a great range by adjusting the length parameter of the folded resonant cavity. The control range can reach more than 4000Hz. Experimental results from finite element simulations further verify that this approach does not affect the topologically protected edge states and the topologically unidirectional acoustic transmission properties. This work provides a method for the construction of low-frequency acoustic topological insulators, as well as an efficient method for the accurate regulation of unidirectional transmission frequency bands over a wide range of frequencies.

Acoustic Higher-Order Topological Insulators Induced by Orbital-Interactions.

The discovery of higher-order topological insulator metamaterials, in analogy with their condensed-matter counterparts, has enabled various breakthroughs in photonics, mechanics, and acoustics. A common way of inducing higher-order topological wave phenomena is through pseudo-spins, which mimic the electron spins as a symmetry-breaking degree of freedom. Here, this work exploits degenerate orbitals in acoustic resonant cavities to demonstrate versatile, orbital-selective, higher-order topological corner states. Type-II corner states are theoretically investigated and experimentally demonstrated based on tailored orbital interactions, without the need for long-range hoppings that has so far served as a key ingredient for Type-II corner states in single-orbital systems. Due to the orthogonal nature of the degenerate p orbitals, this work also introduces a universal strategy to realize orbital-dependent edge modes, featuring high-Q edge states identified in bulk bands. These findings provide an understanding of the interplay between acoustic orbitals and topology, shedding light on orbital-related topological wave physics, as well as its applications for acoustic sensing and trapping.

Multilevel resonant acoustic topological insulators based on three-dimensional extensions

Multilevel resonant acoustic topological insulators based on three-dimensional extensions

Acoustic valley Hall topological insulators with controllable band gap

In recent years, numerous studies have investigated the impact of imperfections on topological insulators, and certain defects have been observed. Former topological insulators have a limited range of topological transport bands and an irregular lattice structure, which hampers their usefulness in technological applications. Here, it’s implemented the topological transport of sonic waves in a phononic crystal array structure. This paper has shown that changing the rotating scatterer's angle can alter the topological phase, resulting in lattice band inversion. This establishes a topological channel that enables acoustic conduction. It is feasible to create multifarious channel structures and devise acoustic transmission systems with varying positions. The introduction of rotating scatterers enables the regulation of the influx of acoustic waves into targeted channels. And the robustness of its edge states was verified by using the finite element method. These outcomes furnish the sound wave control mechanism with a novel formation and urge the development of programmable sound wave regulation. Based on these properties, the technique of regulating topological wave propagation through the use of a topological insulator can also be extended to other systems, including filters and acoustic devices.

Frequency Band-Controllable Acoustic Topological Insulators Based on Local Dimensional Coordination.

The potential value of acoustic topological insulators is of wide interest because of their special topology-protected edge-state unidirectional transmission properties. In this article, a frequency band-controlled acoustic topological insulator based on local dimensional coordination is designed. It is capable of controlling the unidirectional transmission frequency range by controlling the position of the band gap within a certain range. The energy band structure and eigenmodes are analyzed by using the finite element method. The range of control of the band gap position by the local dimension length is derived. The control of the band gap over the unidirectional propagation frequency is experimentally verified. This work provides a way to precisely control the unidirectional transmission frequency of topology-protected edge states in a specific band, opening up new possibilities for their potential value.

Acoustic higher-order topological insulators protected by multipole chiral numbers

Acoustic higher-order topological insulators protected by multipole chiral numbers

Tunable bifunctional acoustic logic gates based on topological valley transport

Valley degree of freedom has attracted great interest in the realization of topological edge states in acoustic systems owing to its rich valley-contrasting physics and great potential applications. However, the practice of valley acoustic topological insulators (ATIs) in designing tunable multifunctional devices without changing their structures still remains a great challenge. Here, we show that the antisymmetric and symmetric distribution nature of valley edge states in the valley ATIs with two different domain walls can be utilized to design tunable robust acoustic logic gates (ALGs). We experimentally demonstrate two types of tunable bifunctional ALGs (denoted as ALG-I and ALG-II), in which ALG-I is composed of a single domain wall, and ALG-II is constructed by a bent topological waveguide containing two domain walls. For ALG-I, the functions of logical inclusive OR and logical exclusive OR (denoted as OR and XOR, respectively) can be switched by actively tuning the phases of two input sound sources without changing the structure. For ALG-II, the logic functions OR and XOR can be implemented through the left and right incidences, respectively, of a pair of sound sources. Similarly, the switching of the logic functions OR and XOR on both sides of ALG-II can be realized by simply adjusting the phases of two sound sources. The designed ALGs have the advantages of simple structure, high robustness, as well as active tunability, leading to a wide range of potential applications in integrated acoustics, acoustic communications, and information processing.

Phase-change in topological chiral phononic crystal for directional coupling switch

Recently, acoustic valley Hall topological insulators have become a cutting-edge area of acoustic physics, where the topological phase transition in phononic crystals shows the presence of band inversion through the Dirac point in the momentum space. We developed a 2D hexagonal lattice chiral phononic crystal using reconfigurable construction by extending one side of the original rectangular rods. When the variation of the side length was from left to right, the topological phase transition is triggered by reopening the Dirac degeneracies beyond high-symmetry points in the first Brillouin zone. We numerically showed valley edge state’s propagation through the interface bent toward distinct chiral topological phononic crystals. Moreover, we assembled 2 × 2 cross-waveguides with a defect cavity based on double heterostructure interfaces. The simulated results verify that the phase change is achieved by the directional coupling switching. This research possibly paves the way for exploiting valley edge states to design the complex acoustic waveguide.

Observation of an acoustic non-Hermitian topological Anderson insulator

Observation of an acoustic non-Hermitian topological Anderson insulator

Rainbow trapping for sound waves in one-dimensional topological insulator

Over the recent decade, topological insulators, originating from the condensed matter physics, have resided at the frontier in the field of acoustics owing to their novel topological properties for manipulating robust wave propagation, which have also opened an intriguing landscape for potential applications. At the meantime, gradually slowing down acoustic waves with metamaterials allows temporary storage of sound, leading to the exploration of so-called trapped rainbow. However, most of the current studies are reported in a topological trivial context with complex structures, and it is hitherto still a challenge to obtain the high-efficient acoustic rainbow trapping effect in a straightforward setup. Here, we propose an acoustic gradient topological insulator in the one-dimensional system to realize a highly efficient rainbow trapping device. Based on the acoustic analogous Su–Schrieffer–Heeger model, we tune the eigenfrequencies of the topological interface states through modulating the neck widths of Helmholtz resonators. The experimentally measured pressure spectra clearly show that the proposed structure could tightly trap the broad-band sound waves at the target spatial positions. Our proposal may provide versatile possibilities for the design of topological acoustic devices.

Programmable dual-band acoustic topological insulator with dynamically movable interface states

Topological acoustic interface states in one-dimensional (1D) acoustic topological insulators (ATIs) are zero-dimensional (0D) topological states localized at an interface. Unlike topological edge states that can propagate to deliver information in acoustic waveguides, the 0D topological interface states generally cannot serve as information carriers to deliver information from one location to another due to their intrinsic localization. Here, we design and demonstrate a 1D ATI with a movable interface, enabling the 0D topological acoustic interface states to deliver information from one location to another. The ATI design is based on two types of elemental building blocks—denoted as “1” and “0”—which are programmable. These elements of 1 and 0, when periodically arranged, can form topologically distinct crystals, whose interface hosts acoustic topological interface states in two bandgaps simultaneously. Since these two types of elements can switch from each other with external control, a programmable 1D dual-band ATI can be constructed. By programming coding sequences of 1 and 0 elements, we can observe dynamically movable 0D topological interface states riding on a moving interface along the 1D ATI in both bandgaps. Our work opens an avenue to develop topological acoustic devices with programmable and dynamic functions, which may have a variety of potential applications in the fields of energy trapping, topological pumping, information processing, and sound communication.

Pseudospin-layer coupled edge states in an acoustic topological insulator

The acoustic pseudospin edge states characterized by backscattering immunity and unidirectional transport provide a basis for designing devices with unconventional functions. In this Letter, we report pseudospin-layer coupled acoustic topological edge states realized by two layers of coupled honeycomb sonic crystals. With the additional layer degree of freedom, we define two distinctly different topological invariants that collectively determine the direction of the acoustic vortex and the layer polarization. We achieve an interesting phase diagram and explore the edge states between different phases. Additionally, we extend the topological edge states to the heterostructure and experimentally verify its capability to focus acoustic waves and convert layer polarization. Our work may provide a feasible platform for the manipulation of acoustic waves and could have promising applications in various areas, such as acoustic signal transmission and splitting.

Acoustic transport in higher-order topological insulators with Dirac hierarchy

Dirac cones (DCs) are an important band structure in topological insulators (TIs) for realizing topological phase transition, and they provide unique ways to artificially regulate wave transport. Herein, we proposed a simple method to achieve Dirac hierarchy in three-dimensional (3D) acoustic TIs with rich and controllable topological phase transitions. The split of multifold DCs in each bulk Dirac hierarchy induced boundary Dirac hierarchy, including topological surface states and topological hinge states. We successfully realized 3D higher-order topological insulators (HOTIs) that exhibited two-fold boundary Dirac hierarchy with hinge states and achieved energy transport along three independent directions based on hinge-to-hinge channels. The proposed method is not limited to single hinges, and it provides a new design idea for multidimensional sound transport, serving as the basis for controllable acoustic functional devices.

Designing Topological Acoustic Lattices via Electroacoustic Analogies

Abstract Topological acoustics has recently witnessed a spurt in research activity, owing to their unprecedented properties transcending typical wave phenomena. In recent years, the use of coupled arrays of acoustic chambers has gained popularity in designing topological acoustic systems. In their common form, an array of acoustic chambers with relatively large volume is coupled via narrow channels. Such configuration is generally modeled as a full three-dimensional system, requiring extended computational time for simulating its harmonic response. To this end, this article establishes a comprehensive mathematical treatment of the use of electroacoustic analogies for designing topological acoustic lattices. The potential of such analytical approach is demonstrated via two types of topological systems: (i) edge states with quantized winding numbers in an acoustic diatomic lattice and (ii) valley Hall transition in an acoustic honeycomb lattice that leads to robust waveguiding. In both cases, the established analytical approach exhibits an excellent agreement with the full three-dimensional model, whether in dispersion analyses or the response of an acoustic system with a finite number of cells. The established analytical framework is invaluable for designing a variety of acoustic topological insulators with minimal computational cost.