Nontrivial coupling of light into a defect: the interplay of nonlinearity and topology

TL;DR

This paper demonstrates experimentally and theoretically how nonlinearity can induce coupling of light into topologically protected edge states in photonic systems, revealing new nonlinear topological phenomena.

Contribution

It introduces a general framework for understanding mode coupling in nonlinear topological systems and experimentally shows nonlinear coupling into protected edge states in photonic lattices.

Findings

Nonlinear coupling into topological edge states observed.

Theoretical framework explains mode-coupling dynamics.

Introduces concepts of inherited and emergent nonlinear topological phenomena.

Abstract

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.