EFRI NewLAW: Topological acoustic metamaterials for programmable and high-efficiency one-way transport

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In this research, physicists, wave mechanicians and materials scientists will come together to design, fabricate and characterize novel metamaterials with nonreciprocal wave propagation attributes. The field of sound waves, which represent the oldest way of communication between human beings, is experiencing a revival in the context of modern material technology and engineering, with a myriad of applications ranging from medical imaging and echolocation to acoustic cloaking. A fundamental principle governing wave propagation in both fluid and solid media, called "reciprocity", is that the transmission rate of waves must be equal forward and backward between any two arbitrarily selected points in a medium. The ability to break this principle, that is, the realization of one-directional mechanical or acoustic "diodes" will enable the design of new classes of mechanical systems with novel functionalities, including adaptive sensors and vibration isolation devices. It will also provide a transformative contribution to the emerging field of material logic, a design paradigm where simple structural and material modules are used as the engineering building blocks to create mechanical devices of arbitrary complexity. The core conceptual ideas behind the research originate from the notion of "topological protection", which is exploited to obtain wave propagation properties that are robust against disorder and noise. The knowledge developed through this research will advance technologies, including but not limited to: a) mechanical computing, where acoustic logic ports can be seen as building blocks capable of carrying out an array of mathematical operations, and b) devices with novel thermal transport properties, exploiting the analogy between thermal phonons and mechanical vibrations at the spatial and temporal scales of mechanical devices. The research will also be disseminated to the broader community through a workshop with public tutorials, broaden the participation of underrepresented groups in STEM fields through enhancing existing programs at the different academic institutions and will result in YouTube videos to share the discoveries with the general public.This project will develop a fundamental understanding of how the phonon band topology governs the acoustic properties of systems experiencing spatial-temporal modulation, leading to a formal interpretation framework for this type of time-reversal symmetry breaking phenomena. The powerful toolkits to be developed through the project will allow for a rich variety of designs beyond the current boundaries of the use of topological states in acoustic wave control strategies. The research will span frontiers of condensed matter physics, wave mechanics, and materials science. The two main conceptual ideas to be used for the realization of topologically protected nonreciprocal wave propagation are: (1) program the phonon band structure in spatial-temporal modulated materials using principles of topological band theory, and (2) exploit the contrasting wave propagation properties of Maxwell lattices in their metal-like and topological insulator-like phases. The research team will advance the theoretical understanding of the intimate role played by topological protection in nonreciprocal wave propagation, use state-of-the-art fabrication techniques to realize prototypes of acoustic diodes at different scales using a variety of material platforms, and deploy laser-enabled wave reconstruction capabilities to characterize the nonreciprocal wave propagation phenomena in the fabricated metamaterial specimen. By developing and applying principles of topology and material logic design across the scales, the project will transform a set of simple mechanical components into a versatile platform for the next generation acoustic logic ports with programmable acoustic transport and time reversal symmetry breaking capabilities.