  Acoustic negative-index metamaterials show promise in achieving superlensing
for diagnostic medical imaging. In spite of the recent progress made in this
field, most metamaterials suffer from deficiencies such as low spatial
symmetry, sophisticated labyrinth topologies and narrow-band features, which
make them difficult to be utilized for symmetric subwavelength imaging
applications. Here, we propose a 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. Moreover, broadband single-source and double-source
subwavelength imaging is realized and verified by 2D and 3D superlens. More
importantly, 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.
