Inherent Negative Refraction on Acoustic Branch of Two Dimensional Phononic Crystals

TL;DR

This study experimentally demonstrates inherent negative refraction in the acoustic passbands of a simple two-dimensional phononic crystal with isotropic materials, enabling advanced wave control at low frequencies.

Contribution

It reveals that negative refraction is an inherent property of isotropic phononic crystals with compliant inclusions, not requiring anisotropy or complex structures.

Findings

Negative refraction observed in shear and longitudinal modes.

Refractive phenomena enable beam splitting, focusing, and filtering.

Phenomena occur at low frequencies with simple unit cells.

Abstract

Guided by theoretical predictions, we have demonstrated experimentally the existence of negative refraction on the lowest two (acoustic) passbands (shear and longitudinal modes) of a simple two dimensional phononic crystal consisting of an isotropic stiff (aluminum) matrix and square- patterned isotropic compliant (PMMA) circular inclusions. At frequencies and wave vectors where the refraction is negative, the effective mass density and the effective stiffness tensors of the crystal can be positive-defnite, and that, this is an inherent property of phononic crystals with an isotropic stiff matrix containing periodically distributed isotropic compliant inclusions. The equi-frequency contours and energy ux vectors as fuctions of the phase-vector components, reveal a rich body of refractive properties that can be exploited to realize, for example, beam splitting, focusing, and frequency filtration on the lowest passbands of the crystal where the dissipation is the least. By proper selection of material and geometric parameters these phenomena can be realized at remarkably low frequencies (large wave lengths) using rather small simple two-phase unit cells. Keywords: Doubly periodic phononic crystals, acoustic branch negative refraction, beam splitting, focusing, imaging, frequency filtration at large wave lengths