  Here we re-examine one of the most basic quantities in optics: the refractive
index. Considering propagation in a plane, we first develop a general formalism
for calculating the direction dependent refractive index in a general
bi-anisotropic material. From this we derive the general condition for
achieving zero refractive index in a given direction. We show that when the
zero-index direction is complex valued the material supports waves that can
propagate in only one sense, e.g. in only a clockwise direction. We dub such
materials complex axis nihility (CAN) media. Our condition shows that there are
an infinite family of both time reversible and time irreversible homogeneous
electromagnetic media that support unidirectional propagation for a particular
polarization. We give examples showing that scattering from such media results
in the complete exclusion of partial waves with one sign of the angular
momentum, and that interfaces between such media generally support one-way
interface states. As well as giving new sets of material parameters, our simple
condition reproduces many of the findings derived using topology, such as
unidirectional propagation in gyrotropic media, and spin-momentum locking of
evanescent waves.
