  We theoretically investigate the optical properties of parity-time
(PT)-symmetric three dimensional metamaterials composed of strongly-coupled
planar plasmonic waveguides. By tuning the loss-gain balance, we show how the
initially isotropic material becomes both asymmetric and unidirectional. The
highly tunable optical dispersion of PT -symmetric metamaterials provides a
foundation for designing an entirely new class of three-dimensional bulk
synthetic media, with applications ranging from sub-diffraction-limited optical
lenses to non-reciprocal nanophotonic devices.
