Extreme reciprocal asymmetric elastic wave energy transport enabled by an ultra-thin, non-deflecting Willis structure

Signal rectification protects sensor systems that are vulnerable to external disturbances and enables diode-based signal control. Achieving such direction-dependent energy transport, however, typically requires breaking reciprocity through cumbersome devices, which limits practical feasibility. In addition, metamaterial- or diffraction-based approaches often rely on bulky structures or deflect the wave propagation direction, with wave deflection compromising signal fidelity. Therefore, realizing strong asymmetric energy transport in a compact, non-deflecting, and reciprocal device remains elusive. Here, we present a subwavelength mode-coupled Willis-type structure that enables highly asymmetric elastic wave transmission by fundamentally engineering the elastic constitutive relations. By identifying and enhancing the mode coupled Willis coefficients, which are otherwise weak, we establish a systematic route for controlling direction dependent intermodal energy exchange between longitudinal and shear wave modes. Leveraging this mechanism, we demonstrate unidirectional perfect transmission and reflection, in which complete mode converting transmission occurs for one sided incidence while total reflection arises for the opposite direction, all while preserving reciprocity. Ultrasonic experiments in a plate confirm unidirectional perfect mode-converting transmission and reflection for shear-horizontal wave incidence, in good agreement with numerical simulations. Unlike previous approaches, the proposed ultra-thin structure achieves strong asymmetric energy transport while preserving the signal direction and reciprocity, thereby enabling reliable signal communication. As potential applications, we further demonstrate elastic wave diodes and signal isolation, highlighting the practical utility of mode coupled Willis structures for directional wave control.