An inverse-designed topological waveguide crossing on valley-Hall photonic crystals

Waveguide crossings are essential in integrated photonics, enabling light to cross paths without significant loss or interference. They are crucial for interconnecting photonic components in complex circuits, facilitating high-density integration. However, conventional waveguide crossings face challenges related to sensitivity of fabrication errors and large footprints. To address these issues, topological photonics could, in principle, be introduced into the design of robust and compact waveguide crossing. However, how to realize the topological waveguide crossing is still a challenge. In this work, we report the realization of an inverse-designed topological waveguide crossing in a 2.5 × 2.5 μm² footprint. The topological waveguide crossing achieves high transmission and low crosstalk (<-20 dB) within the telecommunication band. By integrating this crossing with topological beam splitters and a thermo-optic phase shifter, we also design the topological 2 × 2 Mach-Zehnder interferometer with a 25 × 50 μm² footprint. Numerical simulations show that our designed devices are robust against perturbations, and possess high extinction ratios. Our design offers a promise for reconfigurable optical interconnects, large-scale quantum circuits, and ultra-dense programmable photonic integrated circuits.