Quasi-BIC high-index resonators for liquid characterization and analysis

Capabilities to monitor the purity and mixture composition of liquids with the aid of low-cost portable devices can grant essential advantages in maintaining personal health safety. The overwhelming majority of consumer wireless devices operate at relatively small operational bandwidth, thus not allowing for retrieving material composition via dispersion characteristics. To mitigate the bandwidth limitations, resonant methods, granting precision in a small frequency window, might be of use. Here, we demonstrate a liquid sensor able to provide 90.5 kHz/RIU sensitivities owing to a resonator, supporting high-quality factor quasi-bound states in the continuum. The sensor's architecture encompasses a high-permittivity ceramic resonator and a capillary wrapped around it. The volumetric design increases the overlap between the electromagnetic mode and the liquid under test while maintaining resonant conditions within a relatively narrow frequency band. To demonstrate the capabilities of the proposed method, the UHF RFID band was considered, and temperature dependence of the distilled water permittivity was retrieved. Interfacing standalone low-cost electromagnetic sensors with widely available consumer-level wireless devices offers promising opportunities that contribute to the paradigm shift toward IoT.