Leaky Topological Edge State Cavities for On‐Chip Terahertz Sensing

ABSTRACT Terahertz (THz) sensing on a chip is essential for realizing compact and integrated platforms for chemical and biological detection. However, achieving high sensitivity within a compact device footprint remains challenging due to the intrinsically weak interaction between THz waves and matter. Here, we propose and experimentally demonstrate an on‐chip topological edge state cavity (TESC) sensor that exploits engineered radiation leakage to enhance light‐matter interaction and thereby improve sensitivity. By tuning the in‐plane inversion symmetry of valley photonic crystals, the topological edge state is continuously shifted from below to above the light line, enabling a transition from guided to leaky modes. The guided TESC sensor exhibits a high loaded quality factor of 14 148, providing precise spectral resolution. Moreover, the leaky TESC sensor further enhances performance by harnessing radiation leakage and modal confinement to strengthen THz wave‐matter interactions, resulting in a 2.2‐fold increase in sensitivity compared to the guided TESC sensor and achieving a large thin‐film thickness sensitivity of 150.5 GHz mm − 1 . This synergy between topological protection and engineered radiation leakage establishes a powerful strategy for enhancing light‐matter interaction without compromising device compactness. Our results establish leakage‐engineered topological cavities as a robust platform for high‐performance on‐chip THz sensing and integrated photonic sensing systems.