  Inspired by the parity-time symmetry concept, we show that a judicious
spatial modulation of gain and loss in epsilon-near-zero metamaterials can
induce the propagation of exponentially-bound interface modes characterized by
zero attenuation. With specific reference to a bi-layer configuration, via
analytical studies and parameterization of the dispersion equation, we show
that this waveguiding mechanism can be sustained in the presence of moderate
gain/loss levels, and it becomes leaky (i.e., radiative) below a gain/loss
threshold. Moreover, we explore a possible rod-based metamaterial
implementation, based on realistic material constituents, which captures the
essential features of the waveguiding mechanism, in good agreement with our
theoretical predictions. Our results may open up new possibilities for the
design of optical devices and reconfigurable nanophotonics platforms.
