Transmissive metasurface for various elastic wave modes.
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The existence of multi-modes is the unique characteristic of elastic waves; the interaction between multiple modes makes elastics more interesting, but complicated at the same time. In this point, tailoring multi-mode waves demands corresponding effective parameters for each wave. I have focused on the transmissive elastic metasurface for in-plane modes, longitudinal and shear waves.
Here, the proposed metasurface unit can tune overall four effective parameters, longitudinal and shear effective mass and stiffness, by corresponding motions; the symmetric motion tunes the longitudinal wave, whereas the rotational motion tunes the shear wave. Based on this unit, the below functionalities could be achieved.
Since the above unit can simultaneously tune both waves, I could design the mode-selective elastic metasurface, which tailors the target wave, filtering out the undesired mode. i.e., L-MS fully tailors L. wave and reflects S. wave, whereas S-MS fully tailors S. wave and reflects L.wave.
Considering the mode coupling, I found that the total mode conversion can be achieved with various incident angles by turning the transmitted longitudinal wave into imperfect surface wave. By sufficiently large phase gradient, total mode conversion could be achieved with various incident angles from -20 to 20 degree.
Between different two media, a single layer symmetric metasurface cannot fully tailor the elastic waves; here, suggest two solutions can be adoptedfor impedance-matched wave tailoring between different media: asymmetric metasurface or double-layer metasurface.
Perfectly Controllable Nonreciprocity by Active Willis metastructures
Reciprocity refers to a solid principle that the same wave output occurs along one and the opposite directions within a homogeneous propagation path. Here, Breaking reciprocity has been an challenging subject in the field of wave research. To this objective, I implemented the sensing-actuating based active metastructure system for perfect control on the nonreciprocity based on the constructive/destructive interferences. Beyond the realization of the any desired nonreciprocal scatterings, the demonstration of the phenomena could be achieved by the Willis material system and the equivalent mass-spring system. i.e., the sensor-actuator pairs are analogous to the mass-spring system, which reveals the connection between nonreciprocity control and the virtual Willis materials. For the validations, the numerical examples are provided.
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