Ultralow‐Backscattering Induced Mode Splitting in Chipscale Valley Photonic Topological Cavities

Topological valley photonic crystals hold substantial potential for on‐chip integration owing to their backscattering protection against sharp corners and fabrication imperfections. Topological interfaces enable compact and flexible photonic routing critical for the miniaturization, scalability and energy efficiency of photonic chips. However, such reciprocal topological photonic devices inevitably experience backscattering loss, and effective in‐situ methods for quantifying the robustness remain unavailable. Here, we report that ultralow backscattering at sharp corners induces mode splitting in a topological cavity, serving as a self‐detection methodology for its quantitative evaluation. The overall backscattering loss at the level of 0.10 dB/bend over a broad topological bandgap of 37 GHz, with a minimum of 0.047 dB/bend near the K valley, demonstrates the near‐perfect photon transport of valley edge state through sharp corners. The high‐resolution terahertz spectral measurements in cavity‐waveguide coupled silicon chips confirm an extremely low loss of 0.09 dB/bend. Our work uncovers the universal mechanism of mode splitting and introduces a quantitative framework to measure the backscatteringloss dispersion in topological valley photonics. Mode splitting emerges as a new degree of freedom for spectral engineering in integrated topological devices, paving new ways toward multi‐channel communications, high‐sensitivity photonic sensing, and quantum information processing.