Topological corner states in a silicon nitride photonic crystal membrane with a large bandgap

The theory of band topology has inspired the discovery of various topologically protected states in the regime of photonics. It has led to the development of topological photonic devices with robust property and versatile functionalities, like unidirectional waveguides, compact power splitters, high-Q resonators, and robust lasers. These devices mainly rely on the on-chip photonic crystal (PhC) in Si or III-V compound materials with a fairly large bandgap. However, the topological designs have rarely been applied to the ultra-low-loss silicon nitride (SiN) platform which is widely used in silicon photonics for important devices and integrated photonic circuits. It is mainly hindered by the relatively low refractive index. In this work, we revealed that a rhombic PhC can open a large bandgap in the SiN slab, and thus support robust topological corner states stemming from the quantization of the dipole moments. Meanwhile, we propose the inclination angle of rhombic lattice, as a new degree of freedom, to manipulate the characteristics of topological states. Our work shows a possibility to further expand the topological protection and design flexibility to SiN photonic devices.