Inverse design of broadband and lossless topological photonic crystal waveguide modes.

Topological photonic crystal waveguides can create edge states that may be more robust against fabrication disorder and can yield propagation modes below the light line. We present a fully three-dimensional method to modify state-of-the-art designs to achieve a significant bandwidth improvement for lossless propagation. Starting from an initial design with a normalized bandwidth of 7.5% (13.4 THz), the modification gives more than 100% bandwidth improvement to 16.2% (28.0 THz). This new design is obtained using an automatic differentiation enabled inverse design and a guided mode expansion technique to efficiently calculate the band structure and edge state modes.