Collaborative Research: Science and Engineering of Topological Acoustics and Mechanics

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Propagation, interference and scattering are basic manifestations of the wave nature of acoustic and other mechanical waves. For centuries, humans have used these properties to control and manipulate sound to a certain degree, for instance to realize musical instruments, music halls and whispering galleries. However, a new principle of organization of matter based on advanced topological concepts has been recently discovered in condensed matter physics. Scientists working in different branches of physics and engineering are motivated by these concepts. By exploiting topological constraints in the dispersion of suitably engineered composite material systems, it is possible to realize highly nonlocal responses with unusual stability to perturbations in their wave propagation characteristics. The aim of this project is to translating these concepts to acoustic and mechanical systems. The goals are to redefine the understanding of wave phenomena and to dramatically expand the ability to manipulate mechanical and acoustic waves. Results from this research will expand the engineering toolkit, improving the architecture of mechanical and acoustic devices, for instance by reducing undesirable interactions between different components, including transducers, receivers, and resonant elements. This approach will endow mechanical wave propagation with topological protection, enabling one-way guiding along arbitrarily shaped pathways without back-reflection, and making it robust to defects and disorder. Since this project bridges several disciplines, including material science, physics and engineering, its multi-disciplinary character will have positive educational impact. The project will widen the background and improve the preparation of students involved into this project, and, due to the broad overlap with diverse disciplines, including engineering of music and sound, it will broaden participation of underrepresented minorities in research and education.The idea of applying the concepts of topological order to sound and mechanical waves opens venues in a multitude of scientific fields of research, from basic science to applied physics and engineering. The research plan, inspired by the unique properties of topological robustness discovered in quantum systems, envisions topological acoustic waves that can be engineered in artificial acoustic lattices and synthetic elastic media, and that are immune to unwanted scattering and back-reflection caused by imperfections in device fabrication or impedance mismatch. The approaches to topological order for sound and mechanical waves exploit two advanced concepts based on synthetic gauge fields. The first approach relies on breaking time-reversal symmetry by applying an angular momentum bias based on mechanical or spatio-temporal modulation, emulating the effect of a dc magnetic field. The second approach relies on the principle of synthetic spin-orbital coupling, acting on a pseudo-spin engineered in mechanical systems with preserved time-reversal symmetry. Building upon these two mechanisms, the engineering of acoustic systems and devices with one-way and helical edge transport is advanced. Thanks to the inherent robustness against local defects and disorder, a variety of novel devices with topological protection will be engineered to steer sound and mechanical waves along arbitrary pathways in two and three dimensions, leading to increased bandwidth, multiplexing, reconfigurability and novel architectures for acoustic systems.