Interface Topology Driven Loss Minimization in Integrated Photonics: THz Ultrahigh-Q Cavities and Waveguides.

Compact, robust, and ultralow-loss on-chip photonic devices are essential for densely integrated photonic chips. Conventional designs struggle to achieve these properties due to their inherent trade-offs among compactness, robustness, and low loss. Topological valley photonic crystals (VPCs) offer a promising solution, as their valley-vortex-protected edge states are capable of robustly guiding light through sharp bends and structural perturbations with negligible loss. Notably, exhaustive control over all loss channels is crucial for minimizing undesired losses. However, the intrinsic loss mechanisms in valley edge states remain largely unexplored, severely limiting their full potential. Here, we unveil that radiation is the dominant loss mechanism in valley edge states and propose a new methodology of interface topology driven bandgap and wavevector engineering to thoroughly suppress their radiation losses in chip-scale waveguides and cavities. The methodology minimizes the radiation losses of topological cavities by ≈10-fold through readily tailoring interface geometry without compromising their compactness, ultimately achieving a measured loaded Q-factor of 31231.2 in the terahertz (THz) frequency regime. Furthermore, the enhanced Q-factor significantly strengthens light-matter interactions, enabling efficient spectral modulation with ultralow-power photoexcitation. These findings enable topological VPCs to realize ultra-compact and ultralow-loss devices across diverse frequencies, unleashing their full potential for robust, low-power, and densely integrated photonic chips.