Sensitivity‐Tunable Complex Permittivity Sensing Enabled by Bound States in the Continuum in a Waveguide‐Resonator System

ABSTRACT Bound states in the continuum (BICs), featuring strong field confinement and theoretically infinite quality factors, offer a powerful mechanism for enhancing light–matter interactions. Here, we propose and experimentally demonstrate a universal strategy to construct and merge two types of BICs in a compact metallic rectangular waveguide‐cuboid resonator system in the microwave regime. Leveraging the high‐ quasi‐BICs, a sensitivity‐tunable complex permittivity sensor is developed, with sensing sensitivity adjustable through waveguide‐resonator structural parameters. A first‐principles analysis establishes a quantitative relationship between the transmission coefficient and the material's complex permittivity, enabling simultaneous extraction of the real part and loss tangent. The sensor's accuracy is validated using commercial microwave dielectric substrates with well‐defined permittivities, and its tunable sensitivity is further confirmed through cross‐check experiments. Compared with representative microwave permittivity sensors, the proposed approach achieves over 90% improvement in frequency detection resolution and nearly fivefold enhancement in normalized sensitivity. Benefiting from its ultrahigh quality factor, tunable sensitivity, and first‐principles‐guided design, the sensor can be extended to different frequencies and structural configurations, while allowing convenient sample replacement. These features make it highly suitable for RF substrate characterization, laboratory‐scale material screening, and other high‐sensitivity microwave sensing applications.