Mechanical Mode Engineering with Orthotropic Metamaterial Membranes

Metamaterials are structures engineered at a small scale with respect to the wavelength of the excitations they interact with. These structures behave as artificial materials whose properties can be chosen by design, mocking and even outperforming natural materials, and making them the quintessential tool for manipulation of wave systems. In this paper it is shown how the acoustic properties of a silicon nitride membrane can be affected by nanopatterning. The degree of asymmetry in the pattern geometry induces an artificial anisotropic elasticity, resulting in the splitting of otherwise degenerate mechanical modes. The artificial material introduced has a maximum Ledbetter-Migliori anisotropy of 1.568, favorably comparing to most bulk natural crystals. With an additional freedom in defining arbitrary asymmetry axes by pattern rotation, the described approach can be useful for fundamental investigation of material properties as well as for devising improved sensors of light, mass, or acceleration based on micromechanical resonators.