Topological Floquet bound states in the continuum

TL;DR

This paper demonstrates the existence of topological Floquet bound states in the continuum within a honeycomb waveguide array with a refractive index gradient, revealing their formation, localization, and robustness.

Contribution

It introduces a novel mechanism for creating Floquet BICs in a honeycomb waveguide array with a refractive index gradient, highlighting their topological nature and robustness.

Findings

Floquet BICs emerge from spectrum crossings and avoided crossings.

Topological edge states show stronger localization than bulk states.

Edge Floquet states are robust against localized defects.

Abstract

A honeycomb array of helical waveguides with zigzag-zigzag edges and a refractive index gradient orthogonal to the edges may support Floquet bound states in continuum (BICs). The gradient of the refractive index leads to strong asymmetry of the Floquet-Bloch spectrum. The mechanism of creation of such Floquet BICs is understood as emergence of crossings and avoided crossings of the branches supported by spatially limited stripe array. The whole spectrum of a finite array is split into the bulk branches being continuation of the edge states in the extended zone revealing multiple self-crossings and bulk modes disconnected from the gap states by avoided crossing. Nearly all states in the system are localized due to the gradient, but topological edge states manifest much stronger localization than other states. Such strongly localized Floquet BICs coexist with localized Wannier-Stark-like bulk modes. Robustness of the edge Floquet states is confirmed by their passage through a localized edge defect in the form of a missing waveguide.