Experimentally validated broadband self-collimation of elastic waves

One extraordinary phenomenon exhibited by phononic crystals (PnCs) is that they can manipulate oblique incident waves in a target direction via proper refraction, commonly called wave self-collimation. Although prior studies have demonstrated self-collimation of electromagnetic and acoustic waves, only a few studies have examined elastic waves in this context. If a PnC is designed to represent rectangular-like and/or parallel-line-like equi-frequency contour (EFCs), elastic waves can be self-collimated in either horizontal or vertical directions. However, in the existing literature, there is no theoretical rationale for realizing rectangular-like and/or parallel-line-like EFCs; besides, broadband self-collimation of elastic waves has not yet been explored. Therefore, we propose to derive the material property requirement for an anisotropic PnC to exhibit rectangular-like EFCs (a range of low frequencies) and parallel-line-like EFCs (a range of high frequencies) in the case of elastic waves. Furthermore, by inherently adjoining two frequency ranges of the rectangular-like and parallel-line-like EFCs at a certain frequency, broadband self-collimation covering the entire range of frequencies can be achieved. With the designed anisotropic PnC that fulfills the proposed material property requirement, self-collimation for S₀ Lamb waves is numerically simulated and experimentally demonstrated over a broad range of frequencies.