Depolarization of mechanical waves by elastodynamic metasurface

While the manipulation of wave polarization is commonplace in electromagnetism, the physical means to do so in homogeneous elastic media used to remain an open question. Here, we report the concept of micro-structured surfaces with inner resonance in the field of elastodynamics. Such a metasurface can tune the boundary conditions at specific frequencies and achieve the depolarization of mechanical waves without introducing heterogeneities in the medium itself. The principle relies on the anisotropic behaviour of an array of resonators with deep-subwavelength spacing: The localization of Bragg interferences in a boundary layer enables to homogenize the resonant array into an unconventional surface impedance, the frequency-dependence and anisotropy of which lead to depolarization. The concept of elastodynamic metasurfaces is put into practice experimentally and results bear testament to its efficacy and robustness to control the polarization of reflected waves. Such metasurfaces are easily realized, analytically predictable, and very adjustable, opening new possibilities in vibration control and tomography techniques, from nano- to geophysical scale.