Abstract
An acoustic metagrating (MG) constructed with an inverse-design scheme can be used to realize anomalous reflection, but its efficiency is limited. In this work, we find that the evanescent guided modes are an important component of achieving perfect retroreflection in an MG. Considering the evanescent first-order guided mode, we derive an alternative inverse-design condition to produce perfect retroreflection and improved performance in the MG parameter space compared with the traditional method. We further investigate the corresponding contribution ratios of the guided modes to perfect retroreflection and find an orthogonal relationship between the contribution ratios of odd and even guided modes. These findings are confirmed using analytical calculations and numerical simulations.
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