  The flourishing of topological photonics in the last decade was achieved
mainly due to developments in linear topological photonic structures. However,
when nonlinearity is introduced, many intriguing questions arise. For example,
are there universal fingerprints of underlying topology when modes are coupled
by nonlinearity, and what can happen to topological invariants during nonlinear
propagation? To explore these questions, here we experimentally demonstrate
nonlinearity-induced coupling to topologically protected edge states using a
photonic platform, and theoretically develop a general framework for
interpreting the mode-coupling dynamics in nonlinear topological systems.
Performed in laser-written photonic Su-Schrieffer-Heeger lattices, our
experiments reveal nonlinear coupling of light into a nontrivial edge or
interface defect channel otherwise not permissible due to topological
protection. Our theory explains well all the observations. Furthermore, we
introduce the concepts of inherited and emergent nonlinear topological
phenomena, and a protocol capable of unveiling the interplay of nonlinearity
and topology. These concepts are applicable for other nonlinear topological
systems, either in higher dimensions or beyond our photonic platform.
