On‐Chip Sensing with Slow Light Topological Cavities

Terahertz (THz) on‐chip sensing has emerged as a powerful approach for high spatial resolution, real‐time, label‐free, and non‐invasive detection. Valley photonic crystals (VPC)‐based waveguide‐cavity systems featuring topologically protected signal propagation with reduced scattering losses can pave the way for robust THz sensing platforms. However, achieving high detection sensitivity through strong light‐matter interactions within a compact device footprint remains challenging. Slow‐light effects enable spatial compression of optical energy and enhance light‐matter interaction time by reducing the group velocity, thereby improving sensitivity within a reduced cavity dimension. In this work, we present an on‐chip slow‐light VPC waveguide‐cavity system designed for high‐detection sensitivity. We demonstrate that slow‐light engineering effectively compensates for the reduced physical dimensions of the slow‐light cavities by exploiting dispersion and modal control. This approach shortens the round‐trip path length by up to 1.76 times compared with the non‐slow‐light cavities, thereby enhancing light‐matter interactions within a smaller cavity footprint. Through polyimide thin‐film sensing experiments, we demonstrate that slow‐light VPC waveguide‐cavity systems exhibit up to a threefold increase in sensitivity and threefold reduction in thickness resolution compared to systems without slow‐light effects. Our work offers a novel platform for next‐generation, compact, and high‐performance THz sensing.