Topological Amplification in Photonic Lattices.

We characterize topological phases in photonic lattices by unveiling a formal equivalence between the singular value decomposition of the non-Hermitian coupling matrix and the diagonalization of an effective Hamiltonian. Our theory reveals a relation between topological insulators and directional amplifiers. We exemplify our ideas with an array of photonic cavities which can be mapped into an AIII topological insulator. We investigate stability properties and prove the existence of stable topologically nontrivial steady-state phases. Finally, we show numerically that the topological amplification process is robust against disorder in the lattice parameters.