Integrated Quasi-Optical Terahertz Liquid Sensor Leveraging Mode-Parity-Dependent Interaction with a Capillary-Confined Analyte.

The integration of terahertz (THz) sensing technology into compact, on-chip platforms is essential to the advancement of high-precision chemical and biomedical analysis, promising to bring analytics closer to the point of care and to enable in situ analysis of industrial processes. This study presents an integrated quasi-optical THz liquid sensor that features a longitudinal cavity in a silicon slab waveguide, in which a capillary-confined analyte interacts with guided slab modes on resonance. The sensor design leverages mode-parity-dependent field distributions: even-parity resonances exhibit strong analyte-field interaction, whilst odd-parity modes remain largely unaffected by the presence of the analyte, enabling intrinsic self-calibration. The device is fabricated using deep reactive ion etching of high-resistivity silicon and monolithically integrates all required components. Experimental measurements with water and isopropanol demonstrate alternating resonance peaks with distinct sensitivity to refractive index and absorption, validated by linear shifts in frequency and transmission loss. The self-calibrating feature allows for real-time compensation of system fluctuations towards automated continuous monitoring applications. These findings establish the sensor's capability for simultaneous, precise material characterization and calibration, highlighting its potential for in-line process monitoring and other high-bandwidth sensing applications.