Robust 2D/3D multi-polar acoustic metamaterials with broadband double negativity

Acoustic negative-index metamaterials show promise in achieving superlensing for diagnostic medical imaging. In spite of the recent progress made in this field, most acoustic metamaterials (AMMs) suffer from deficiencies such as low spatial symmetry, sophisticated labyrinth topologies and narrow-band features, which hamper their applications for symmetric subwavelength imaging. To overcome the hurdle of designing practical negative-index metamaterials, in this paper, we propose a novel category of robust multi-cavity metamaterials and reveal their common double-negative mechanism enabled by multi-polar (dipole, quadrupole and octupole) resonances in both two-dimensional (2D) and three-dimensional (3D) scenarios. In particular, we discover explicit relationships governing the double-negative frequency bounds from equivalent circuit analogy. For the first time, we construct a simple, highly-symmetric and intuitionistic 3D AMM by exploiting the multi-cavity topological features. This entails the broadband single-source and double-source subwavelength imaging, which is demonstrated and verified by 2D and 3D superlens both numerically and experimentally. Moreover, the analogical 3D superlens can ensure the subwavelength imaging in all directions. The proposed multi-polar resonance-enabled robust metamaterials and design methodology open horizons for easier manipulation of subwavelength waves and realization of practical 3D metamaterial devices.