Generation principle for robust and regularly spaced resonant modes in a nontopological photonic crystal ring resonator

A topological valley photonic crystal cavity supports robust multiple resonant modes and is therefore widely applied in broadband Purcell enhancement, laser, channel filter, and sensor. So far, its robustness has been conventionally attributed to the different topological properties of the valley photonic crystals that make up the interface. In this work, we clarify that the distribution of the circular polarization topological singularities is instrumental in generating robust and regularly spaced resonant modes. We demonstrate a photonic crystal ring resonator, constructed entirely by a deformed line-defect waveguide. Through simulations, we validate that the ring resonator retains the most advantages of a valley photonic crystal ring resonator. This work can instruct the development of a high-performance and robust photonic crystal cavity.