  One of the principal goals in the design of photonic crystals is the
engineering of band gaps and defect states. Drawing on the concepts of
band-structure topology, I here describe the formation of exponentially
localized, topologically protected midgap states in photonic systems with
spatially distributed gain and loss. When gain and loss are suitably arranged
these states maintain their topological protection and then acquire a
selectively tunable amplification rate. This finds applications in the beam
dynamics along a photonic lattice and in the lasing of quasi-one-dimensional
photonic crystals.
