Accurate permittivity characterization at terahertz frequencies is important for material analysis and device design, yet it remains challenging for small-volume samples and compact test structures. In this work, a terahertz permittivity sensor based on a spoof surface plasmon polariton (SSPPs) transmission line coupled to a backside split-ring resonator (SRR) is proposed and numerically studied. The SSPPs line is patterned on the top side of the substrate, while the SRR is etched on the backside, with the sample loaded into the SRR gap. The SSPPs mode penetrates through the substrate and excites the SRR, producing a pronounced transmission notch. Changes in the sample permittivity modulate the effective capacitance of the resonator, resulting in a monotonic shift in the notch center frequency. For relative permittivities from 1 to 8, the notch center frequency decreases from 152.1 GHz to 117.8 GHz, corresponding to a total shift of 34.3 GHz and an average sensitivity of about 4.90 GHz/εr. The minimum S21 remains within approximately −23.80 to −21.56 dB, while the Q-factor stays in the range of 94.33–108.23, indicating good spectral readability. Tolerance analysis further shows that the resonance frequency is sensitive to critical structural dimensions and layer alignment, and practical implementation is therefore more suitable for single-device calibrated frequency-shift sensing. These results demonstrate the feasibility of the proposed dual-layer SSPPs–SRR configuration for compact permittivity sensing in the terahertz regime.
Zeng et al. (Fri,) studied this question.
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