  We investigate laser emission at the interface of a topological and trivial
phases with loss and gain. The system is described by a Su-Schrieffer-Heeger
model with site-dependent hopping parameters. We study numerically and
analytically the interface states. The ground state is described by the
Jackiw-Rebbi mode with a pure imaginary energy, reflecting the non-Hermiticity
of the system. It is strictly localized only at the A sites. We also find a
series of analytic solutions of excited states based on SUSY quantum mechanics,
where the A and B sites of the bipartite lattice form SUSY partners. We then
study the system containing loss and gain with saturation. The Jackiw-Rebbi
mode is extended to a nonlinear theory, where B sites are also excited. The
relative phases between A and B sites are fixed, and hence it will serve as a
large area coherent laser.
