Loss in acoustic metasurfaces: a blessing in disguise
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Prospective Article
Loss in acoustic metasurfaces: a blessing in disguise Nikhil JRK Gerard and Yun Jing
, Department of Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, NC 27695, USA
Address all correspondence to Yun Jing at [email protected] (Received 26 September 2019; accepted 24 October 2019)
Abstract From being an unfavorable consequence to finding itself as the intended imaginary part of a non-Hermitian system, loss has truly emerged as more of a friend than a foe in the context of acoustic metasurfaces. With the promising features of sub-wavelength geometries and the rapid advances in manufacturing techniques that can enable their realization, loss becomes a central topic of discussion. Further, the capability of introducing and tailoring loss allows it to serve as a new degree of freedom in passive wavefront shaping devices. In this review, the authors look back at the recent progress in the field of lossy acoustic metasurfaces. The background behind loss in deep sub-wavelength geometries and the instinctive responses to treat them and exploit them are overviewed, followed by more recent works that embrace and tailor their behavior for unconventional applications. The forthcoming years for acoustic metasurfaces thus hold several promising avenues for exploration, with loss as the protagonist.
Introduction Although “loss” is often perceived as something undesirable, the recent trends in acoustic metasurfaces have implied otherwise. Sound dissipation in intricate sub-wavelength microstructures is rather ubiquitous and addressing them is of utmost importance. Tapping into loss, however, lies in how its role in the system is perceived. While it could be regarded as just an adverse effect that must be minimized for unabated sound propagation, one could harness this as an efficient tool to achieve favorable sound absorption. With the dawn of non-Hermitian physics, on the other hand, researchers have also been inspired to treat loss as the imaginary parts in physical quantities that can be tailored to realize new wave behavior at exceptional points (EPs). Interestingly, either of these routes hold favorable outcomes for acoustic metasurfaces—for both conventional and unconventional functionalities. Over the past five years, the rational design of twodimensional materials for passive wavefront manipulation has emerged as a central topic in material physics.[1] In acoustics, such thin devices have been studied to showcase fascinating features, such as self-bending beams,[2] holographic rendering,[3–5] artificial Mie resonance,[6] retroreflection,[7,8] asymmetric transmission,[9,10] diffuse reflection,[11,12] and near-perfect absorption.[13–16] Such research has in a way redefined our understanding of classical physics, while proposing alternatives to renovate useful applications. In contrast to their electromagnetic counterparts, however, acoustic metasurfaces are required to tackle concerns with regard to the mechanical nature of the wave. One such concern is the presence of loss when soun
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