Demonstration of a planar multimodal periodic filter at THz frequencies

TL;DR

This work addresses the challenge of implementing planar THz filters with flexible multimode operation by designing a periodic band-stop filter that alternates CPS and the odd-mode of a FGPCPW on a thin SiN membrane. The authors develop a cascaded unit-cell theory using ABCD matrices, validated by full-wave simulations and THz time-domain measurements, achieving a center frequency of $f_c=0.8$ THz with a bandwidth around $Δf\approx 0.07$–$0.1$ THz. The experimental results closely match the theoretical and simulated predictions, with expected conductor-loss and grating-radiation effects accounted for. A key contribution is showing how multimode coupling enables potential reconfigurability (e.g., converting to a band-pass by exciting the even-mode), suggesting practical integration with phase shifters, hybrids, and active elements for THz signal processing.

Abstract

This paper presents a planar multimodal periodic filter that is constructed from alternating sections of coplanar stripline and the odd-mode of a finite-ground plane coplanar waveguide constructed on a 1 um silicon nitride substrate to facilitate operation at THz frequencies. The multimode configuration differs from standard single-mode periodic filters and enables flexible designs and the possibility for active control of the filter characteristics. For this proof-of-concept, we present the relevant theory and design procedures required to develop a band-stop filter that has a center frequency of fc = 0.8 THz and a bandwidth of df = 0.07 THz. We find good agreement between theory, simulation, and experiment.

Figures (13)

• Figure 1: (a) Overall structure. (b) Cross section of the CPW sections. (c) Definition of the parameters associated with the unit cell.
• Figure 2: Mode illustrations. (a) CPS mode. (b) CPW odd mode. (c) CPW even mode. The conductors and substrates are overlaid for visibility, their thicknesses are not to scale ($\approx$10$\times$ thinner than visualized).
• Figure 3: CPS-to-CPW$^o$ mode coupling at 0.8 THz. (a) $W_m$ = 5 µ m. (b) $W_m$ = 45 µ m. (c) $W_m$ = 70 µ m. (d) $|S_{21}^{o}|^2$ for CPS-to-CPW$^o$ vs $W_m$.
• Figure 4: (a) Characteristic impedance using QS expressions and full-wave simulation at 0.8 THz with $S$ = 80 µ m, $W$ = 45 µ m, $H_s$ = 1 µ m, $H_a$ = 200 nm, and $\varepsilon_r = 7.6$. The impedance at 0.8 THz with $W_m$ = 45 µ m are $Z_{\text{CPS}} = 291 \Omega$ and $Z_{\text{CPW}}^o = 254 \Omega$. (b) The corresponding fractional bandwidth from (\ref{['eqn:frac_bandwidth']}) and bandwidth where $f_c = 0.8$ THz. At $W_m$ = 45 µ m, the bandwidth is 0.07 THz.
• Figure 5: Dispersion diagram for the unit cell.