  Surface polaritons, e.g., surface plasmon polaritons, are invaluable tools in
nanophotonics. However, considerable plasmon loss narrows the application
regime of plasmonic devices. Here we reveal some general conditions for
lossless surface polaritons to emerge at the interface of a gain and a loss
media. The gain medium does not only compensate the energy loss, but also
modifies surface wave oscillation mechanisms. A new type of surface polaritons
induced by the sign switch of the imaginary part of the permittivity across the
interface is discovered. The surface polaritons exhibit spin Hall effect due to
spin-momentum locking and unique Berry phase. The spin Hall coefficient changes
the sign across the parity-time symmetric limit and becomes quantized for
perfect metal-dielectric interface and for dielectric-dielectric interface with
very large permittivity contrast, carrying opposite topological numbers. Our
study opens a new direction for manipulating light with surface polaritons in
non-Hermitian optical systems.
