Simultaneous negative reflection and refraction and reverse-incident right-angle collimation of sound in a solid-fluid phononic crystal

The square lattice phononic crystal (PnC) has been used extensively to demonstrate metamaterial effects. Here, positive and negative refraction and reflection are observed simultaneously due to the presence of Umklapp scattering of sound at the surface of PnC and square-like equifrequency contours (EFCs). It is found that a shift in the EFC of the third transmission band away from the center of the Brillouin zone results in an effectively inverted EFC. The overlap of the EFC of the second and third band produce quasimomentum-matching conditions that lead to multi-refringence phenomena from a single incident beam without the introduction of defects into the lattice. Additionally, the coupling of a near-normal incident wave to a propagating almost perpendicular Bloch mode is shown to lead to strong right-angle redirection and collimation of the incident acoustic beam. Each effect is demonstrated both numerically and experimentally for scattering of ultrasound at a 10-period PnC slab in water environment. (C) 2022 Acoustical Society of America.

Automated summary

The study demonstrates various metamaterial effects using a square lattice phononic crystal (PnC), including positive and negative refraction and reflection. By shifting the equifrequency contour (EFC) away from the center of the Brillouin zone, an effectively inverted EFC is achieved, leading to multi-refringence phenomena. Additionally, strong right-angle redirection and collimation of the incident acoustic beam are achieved through the coupling of different modes. These effects are validated through numerical simulations and experiments.