  Isotropic negative index metamaterials (NIMs) are highly desired,
particularly for the realization of ultra-high resolution lenses. However,
existing isotropic NIMs function only two-dimensionally and cannot be
miniaturized beyond microwaves. Direct laser writing processes can be a
paradigm shift toward the fabrication of three-dimensionally (3D) isotropic
bulk optical metamaterials, but only at the expense of an additional design
constraint, namely connectivity. Here, we demonstrate with a proof of-principle
design that the requirement connectivity does not preclude fully isotropic
left-handed behavior. This is an important step towards the realization of bulk
3D isotropic NIMs at optical wavelengths.
