A Differential THz/MW Sensor for Characterizing Liquid Samples Based on CSRs

A differential on-chip sensor loaded with circular spiral resonators (CSRs) for charactering liquid samples is proposed in this article. The proposed on-chip sensor is working at terahertz (THz) frequency band, and the sensor can be fabricated by integrated passive device (IPD) technology. With comparison to square and octagonal spiral resonators, the CSR has a higher quality factor and a stronger confinement of electrical field, which are beneficial to improve the measurement sensitivity. The proposed sensor has a differential structure, and two CSRs are equally distributed on both sides of the microstrip line, and one resonator is regarded as reference, and another is utilized as test. The frequency shift and notched magnitude of transmission coefficient will be changed by loading liquid samples with different complex permittivities; thus, the relevant mathematical relationship between S-parameters and permittivity can be established. However, as the cost of fabricating on-chip sensor by IPD technology is too expensive, printed circuit board (PCB) technology is used to fabricate the proposed sensor and to verify the feasibility of the sensor. It should be noted that the on-chip sensor operates at THz band, while the PCB-based sensor operates at microwave (MW) band. The experiment shows that the proposed CSRs-based MW sensor has a high sensitivity of about 0.28%. When the real permittivity changes by one unit, the resonant frequency shifts for the microwave sensor and THz sensor are about 0.445 and 1.45 GHz, i.e., $\text {0.{445}}$ and $1.45 {\text {GHz}/}\varepsilon _{r}^{\prime} $ , respectively. Obviously, the on-chip THz sensor has a higher resolution than the microwave sensor. All in all, the proposed differential microwave CSR-based sensor is a good candidate in the field of microfluidic measurement.