Compact acoustic metamaterial based on the 3D Mie resonance of a maze ball with an octahedral structure

Acoustic metamaterials (AMs) offer ever-expanding possibilities for manipulating sound waves. Potential applications include diagnostic medical imaging, super-absorption, acoustic sensing, and acoustic stealth. In spite of recent progress, the investigation of AMs with a three-dimensional (3D) response is lagging behind, in particular for those that exhibit an isotropic response. Here, we demonstrate a highly compact subwavelength maze-like multi-shell plastic sphere, which generates Mie resonances with isotropic monopolar and anisotropic dipole, quadrupole, and octupole modes at low frequencies for airborne sound, based on an octahedral structure. Eigenmode analysis shows that the proposed maze ball exhibits a negative bulk modulus at the monopole Mie resonance frequency in the absence of viscous losses, which is a signature of strong transmission blocking. With a diameter of 0.17λ and a volume filling factor of 13.5%, a constructed single 3D maze ball reduces the experimentally-measured transmitted acoustic energy by 67%, limited mainly by viscous losses. With optimized fabrication, the proposed 3D Mie resonator should provide a versatile approach for the manipulation of sound waves on a subwavelength scale, and lead to the realization of practical 3D metamaterial devices.