Acoustic topological insulator and robust one-way sound transport (1512.03273v1)

Abstract: Discovery of novel topological orders of condensed matters is of a significant interest in both fundamental and applied physics due to the associated quantum conductance behaviors and unique symmetry-protected backscattering-immune propagation against defects, which inspired similar fantastic effects in classical waves system, leading to the revolution of the manipulation of wave propagation. To date, however, only few theoretical models were proposed to realize acoustic topological states. Here, we theoretically and experimentally demonstrate a two dimensional acoustic topological insulators with acoustic analogue of quantum spin Hall Effect. Due to the band inversion mechanism near the double Dirac cones, acoustic one-way pseudospin dependent propagating edge states, corresponding to spin-plus and spin-minus, can be observed at the interface between two graphene-like acoustic crystals. We have also experimentally verified the associated topological immunity of such one-way edge states against the different lattice defects and disorders, which can always lead to inherent propagation loss and noise. We show that this unique acoustic topological phenomenon can offer a new promising application platform for the design of novel acoustic devices, such as one-way sound isolators, acoustic mode switchers, splitters, filters etc.

• The paper demonstrates that engineered acoustic crystals with a honeycomb lattice achieve topologically protected one-way sound transport via band inversion at double Dirac cones.
• The paper verifies that acoustic edge states exhibit immunity to backscattering from defects, confirmed through precise experimental transmission measurements.
• The paper implements a spin-dependent cross-waveguide splitter to validate robust, one-way propagation, suggesting promising applications in advanced acoustic devices.

Overview of Acoustic Topological Insulators and Robust One-Way Sound Transport

The research presented in this paper addresses the design and experimental realization of acoustic topological insulators (ATIs), drawing analogies with the quantum spin Hall Effect (QSHE) observed in electronic systems. The work demonstrates that acoustic waveguides constructed from graphene-like acoustic crystals can support robust one-way sound transport, leveraging the unique properties of band inversion mechanisms near double Dirac cones.

The investigation is set against the backdrop of topological insulators in condensed matter physics, where quantum conductance behaviors are protected by symmetry, allowing backscattering-immune propagation. This research extends the principles of topological insulators to classical wave systems, specifically acoustics, offering potential for innovative acoustic devices free from backscatter issues.

Key Findings

The research introduces the design of acoustic crystals capable of forming nontrivial topological waveguides. These structures enable the one-way propagation of sound along edges, unaffected by defects. Fundamental to this design are the properties of stainless steel rods arranged in a honeycomb lattice structure. By manipulating the radii of these rods, the team achieves a band inversion mechanism at specific lattice constants, facilitating the formation of four-fold degenerate acoustic states, or double Dirac cones, at the Brillouin zone center.

Key results include:

• Observed Topological Immunity: The acoustic edge states demonstrated immunity to backscattering from defects, including cavities, disorders, and sharp bends. This was verified experimentally by measuring transmission through the constructed waveguides.
• Spin-Dependent Transport: The waveguides allowed for the one-way transport of sound induced by pseudo-spin states, enabled by the hybridization of acoustic spin and spin-orbit coupling effects.
• Cross-Waveguide Splitter Implementation: By designing a cross-waveguide splitter, the researchers were able to experimentally demonstrate spin-dependent one-way propagation characteristics, further confirming the robustness of these acoustic topological states.

Implications

The implementation of ATIs has significant implications for the development of acoustic devices that require robust sound propagation. The inherent resistance to defects and backscattering opens new avenues for devices such as acoustic isolators, filters, and switches.

The paper also contributes to the broader understanding of topological phenomena in classical systems, potentially guiding future research into topologically protected states in other acoustic and photonic applications. Notably, the research suggests scalability from audible to ultrasonic frequencies, making it relevant for diverse applications, including underwater acoustics.

Prospects for Future Research

The findings present a platform upon which future exploration of topological states in acoustic systems can be constructed. Potential areas for further research include exploring different lattice configurations, materials, and frequency ranges to refine and broaden the applicability of these topological phenomena. Additionally, there may be opportunities to investigate dynamic control of acoustic topological states through external stimuli such as temperature and pressure variations.

In conclusion, the detailed investigation and experimental realizations outlined in this paper substantiate the feasibility of utilizing topological principles to manipulate acoustic wave transport, offering promising directions for both theoretical exploration and practical device innovations in acoustics.