A pure-mode performance is critical for elastic diodes, but difficult to achieve because of the rich polarization characteristics of elastic waves. In this work, we propose a type of elastic mode-converting diode for an efficient and pure-mode asymmetric transmission. The diode consists of two components, one purely for mode-selecting and the other solely for mode-converting. The former is made of fluid-like metamaterials, which only support the transmission of longitudinal waves in a certain frequency range with a nearly 100% power efficiency. The latter works as a transmodal Fabry–Perot interferometer, and can convert the wave energy between longitudinal and transverse modes solely and maximally. With the designed diode, a pure-mode one-way transmission with a power efficiency of nearly 90% is realized in the ultralow frequency range. We expect that this effective and efficient device can offer a wide range of options for manipulating elastic waves, including mode isolation, selection, conversion, rectification, etc.

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