Prediction and observation of topological modes in fractal nonlinear optics

TL;DR

This paper reviews recent theoretical and experimental advances in creating and observing topological corner modes and solitons in fractal nonlinear optical waveguides, specifically those based on the Sierpinski gasket structure.

Contribution

It summarizes the prediction and experimental realization of topological corner modes and their transformation into stable corner solitons in fractal HOTI optical waveguides.

Findings

Demonstration of corner modes in fractal HOTI waveguides

Transformation of corner modes into stable solitons via nonlinearity

Potential for new topological photonic devices

Abstract

This item from the News & Views category, to be published in Light: Science & Applications, aims to provide a summary of theoretical and experimental results recently published in Ref. [24], which demonstrate the creation of corner modes in nonlinear optical waveguides of the higher-order topological-insulator (HOTI) type. Actually, these are second-order HOTIs, in which the transverse dimension of the topologically protected edge modes is smaller than the bulk dimension (it is 2, in the case of optical waveguide) by 2, implying zero dimension of the protected modes, that are actually realized as corner or defect ones. Work [24] reports prediction and creation of various forms of the corner modes in a HOTI with a fractal transverse structure, represented by the Sierpinski gasket (SG). The self-focusing nonlinearity of the waveguide's material transforms the corner modes into corner solitons, almost all of which are stable. The solitons may be attached to external or internal corners created by the underlying SG. This N&V item offers an overview of these new findings reported in Ref. [24] and other recent works, and a brief discussion of directions for the further work on this topic.