Acoustic metamaterials are structures with exotic acoustic properties, with promising applications in acoustic beam steering, focusing, impedance matching, absorption and isolation. Recent work has shown that the efficiency of many acoustic metamaterials can be enhanced by controlling an additional parameter known as Willis coupling, which is analogous to bianisotropy in electromagnetic metamaterials. The magnitude of Willis coupling in a passive acoustic meta-atom has been shown theoretically to have an upper limit, however the feasibility of reaching this limit has not been experimentally investigated. Here we introduce a meta-atom with Willis coupling which closely approaches this theoretical limit, that is much simpler and less prone to thermo-viscous losses than previously reported structures. We perform two-dimensional experiments to measure the strong Willis coupling, supported by numerical calculations. Our meta-atom geometry is readily modeled analytically, enabling the strength of Willis coupling and its peak frequency to be easily controlled.
A key concept underlying the specific functionalities of metasurfaces, i.e. arrays of subwavelength nanoparticles, is the use of constituent components to shape the wavefront of the light, on-demand. Metasurfaces are versatile and novel platforms to manipulate the scattering, colour, phase or the intensity of the light. Currently, one of the typical approaches for designing a metasurface is to optimize one or two variables, among a vast number of fixed parameters, such as various materials' properties and coupling effects, as well as the geometrical parameters. Ideally, it would require a multi-dimensional space optimization through direct numerical simulations. Recently, an alternative approach became quite popular allowing to reduce the computational cost significantly based on a deeplearning-assisted method. In this paper, we utilize a deep-learning approach for obtaining high-quality factor (high-Q) resonances with desired characteristics, such as linewidth, amplitude and spectral position. We exploit such high-Q resonances for the enhanced light-matter interaction in nonlinear optical metasurfaces and optomechanical vibrations, simultaneously. We demonstrate that optimized metasurfaces lead up to 400+ folds enhancement of the third harmonic generation (THG); at the same time, they also contribute to 100+ folds enhancement in optomechanical vibrations. This approach can be further used to realize structures with unconventional scattering responses.
Metagratings are a recently proposed class of metasurfaces for efficient manipulation of an impinging wavefront within a subwavelength layer. They avoid the requirement for fine discretization of gradient metasurfaces, and overcome their inherent limitations in efficiency. Here, we demonstrate experimentally the functioning principle of a reconfigurable acoustic metagrating for anomalous reflection with high efficiency, using coarse geometric design features. It is formed by a periodic array of C-shaped meta-atoms, which exhibit large Willis coupling, resulting in a controlled level of asymmetry in their scattering pattern. Our results reveal that the proposed acoustic metagrating can reroute an incident wave towards a large angle, beyond the limitations of gradient-phase approaches with nearly unitary reflection efficiency. The proposed designs offer a highly efficient tunable platform to control steering angle and operating frequency. In our experiments, an acoustic wave is successfully steered to the desired reflection direction by finite metagratings, demonstrating reconfigurability in angle and operating frequency.
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