Directly coupled low reflection loss valley topology defect cavity

Defect cavities have been extensively studied for their ability to efficiently manipulate light transmission. However, integrating defect cavities into conventional photonic crystal waveguides typically incurs significant reflection loss, resulting in high energy dissipation in on-chip devices. To address this challenge, we incorporate topological concepts into the defect cavity design and propose a topological defect cavity (TDC) based on valley photonic crystals. Benefiting from the defect-immunity of topological waveguides, the TDC-topological waveguide directly coupled system enables low reflection loss transmission under non-resonant conditions, while allowing flexible tuning of the cavity's resonant frequency. These characteristics make this TDC an excellent platform for on-chip functional device design. As a demonstration, we design a multifunctional logic device combining this TDC and a power splitter, capable of delivering diverse output states as needed. This work holds potential for applications in integrated micro-nano photonics.