Design and implementation of a slow-wave structure for reducing mutual coupling in THz transmission lines toward ultra-high Q-factor sensing enhancement

This paper presents a compact slow-wave (SW) grating structure for simultaneous mutual coupling suppression in terahertz (THz) transmission lines and high-performance refractive index sensing. Unlike conventional shielding techniques that increase circuit footprint or make excessive insertion loss, the proposed subwavelength periodic grating (period Λ = 30 μm, tooth height h = 20 μm) leverages spoof surface plasmon polariton (SSPP) excitation and slow-wave factor (SWF ≈ 3.5) to compress the effective wavelength and tightly confine electromagnetic fields. The grating reduces forward crosstalk below −40 dB over 0.1–2 THz and preserves an insertion loss of about −3 dB when placed between two parallel THz microstrip lines. Thus, THz biosensor operating at 1.07 THz is realized by integrating the same SW grating which also provides isolation into a slotted transmission line containing a cross-shaped resonator. The grating enhances the quality factor (Q-factor) from 35 to 2150 by suppressing radiation leakage and enabling a trapped-mode resonance similar to a bound state in the continuum (BIC). For biomaterials with refractive indices from 1.0 to 2.0, the sensor achieves a sensitivity of 142 GHz/RIU and a figure of merit (FOM) up to 201 RIU −1 (unloaded) and 9.7–17.5 RIU −1 under analyte loading. The design uniquely combines crosstalk suppression and high-Q sensing within a compact size (150 μm × 150 μm), is realizable with standard i-line lithography, and offers a passive, low-loss platform for high-resolution THz biosensing and highly integrated THz circuits.