Experiments with nonlinear topological edge states in static and dynamically modulated Su–Schrieffer–Heeger arrays

Progress in observation of solitons in photonic topological insulators is discussed. Results are presented of experiments with nonlinear topological states in Su–Schrieffer–Heeger arrays—fabricated using the femtosecond writing technique—that are static, i.e., invariable in the direction of light propagation, and dynamically modulated (primarily periodically) in the direction of light propagation. Such objects are one of the simplest models of a topologically nontrivial structure. Solitons in topological insulators bifurcate with increasing laser beam power from linear edge states in the topological bandgap, inheriting their topological protection. The spatial localization of the soliton and the position of its propagation constant in the topological bandgap depend in a nonlinear medium on peak power and can be effectively controlled. Experimental observation of the switching of the edge topological modes in the bandgap between two closely spaced dimerized Su–Schrieffer–Heeger arrays is presented. The switching, whose rate depends on radiation intensity, can be completely arrested in a strongly nonlinear regime. In trimer waveguide arrays, whose spectrum in the topological phase features two simultaneously emerging topological bandgaps with edge states of different symmetries, two coexisting types of topological solitons exhibiting different degrees of stability were observed. We also discuss experimental observations of $\pi $-solitons—nonlinear topological Floquet states periodically reproducing their profiles in 1D- and 2D-dimensional Su–Schrieffer–Heeger arrays modulated in the direction of propagation of radiation.