  Topological photonics has emerged as a novel route to engineer the flow of
light. Topologically-protected photonic edge modes, which are supported at the
perimeters of topologically-nontrivial insulating bulk structures, have been of
particular interest as they may enable low-loss optical waveguides immune to
structural disorder. Very recently, there is a sharp rise of interest in
introducing gain materials into such topological photonic structures, primarily
aiming at revolu-tionizing semiconductor lasers with the aid of physical
mechanisms existing in topological physics. Examples of re-markable
realizations are topological lasers with unidirectional light output under
time-reversal symmetry breaking and topologically-protected polariton and
micro/nano-cavity lasers. Moreover, the introduction of gain and loss provides
a fascinating playground to explore novel topological phases, which are in
close relevance to non-Hermitian and parity-time symmetric quantum physics and
are in general difficult to access using fermionic condensed matter systems.
Here, we review the cutting-edge research on active topological photonics, in
which optical gain plays a pivotal role. We discuss recent realizations of
topological lasers of various kinds, together with the underlying physics
explaining the emergence of topological edge modes. In such demonstrations, the
optical modes of the topological lasers are deter-mined by the dielectric
structures and support lasing oscillation with the help of optical gain. We
also address recent researches on topological photonic systems in which gain
and loss themselves essentially influence on topological prop-erties of the
bulk systems. We believe that active topological photonics provides powerful
means to advance mi-cro/nanophotonics systems for diverse applications and
topological physics itself as well.
