Topological edge solitons in χ waveguide arrays.

We address the formation of χ topological edge solitons emerging in a topologically nontrivial phase in Su-Schrieffer-Heeger (SSH) waveguide arrays. We consider edge solitons, whose fundamental frequency (FF) component belongs to the topological gap, while the phase mismatch determines whether the second harmonic (SH) component falls into topological or trivial forbidden gaps of the spectrum for the SH wave. Two representative types of edge solitons are found, one of which is thresholdless and bifurcates from the topological edge state in the FF component, while the other exists above a power threshold and emanates from the topological edge state in the SH wave. Both types of soliton can be stable. Their stability, localization degree, and internal structure strongly depend on the phase mismatch between the FF and SH waves. Our results open up new prospects for the control of topologically nontrivial states by parametric wave interactions.