[논문 리뷰] A Terahertz Bandpass Filter Using a Capacitive Transition Circuit and a Spoof Surface Plasmon Polariton Waveguide

Mohsen Haghighat, Levi Smith|arXiv (Cornell University)|2026. 03. 25.

Plasmonic and Surface Plasmon Research인용 수 0

한 줄 요약

제안 및 실험적으로 1 THz에서 스푸프 표면 플램슨 극판 파동(SSPP) 웨이브가이드와 코플래너 스트립(CPS) 공급 및 커패시티브 트랜지션 회로를 통합한 테라헤르츠 대역 통과 필터 기반으로 0.3 THz 대역폭을 달성.

ABSTRACT

This paper presents a novel terahertz (THz) bandpass filter (BPF) based on a spoof surface plasmon polariton (SSPP) waveguide with a center frequency of 1 THz and a 3 dB bandwidth of 0.3 THz. The proposed BPF comprises cascaded high-pass and low-pass elements. The high-pass element is a capacitive gap in the SSPP transition circuit, and the low-pass element is the SSPP waveguide itself. We find that the measurement results, including cut-off frequencies, align well with the theoretical predictions and simulations. To the authors' knowledge, the proposed SSPP BPF is the first of its kind.

연구 동기 및 목표

• Motivate and demonstrate a THz SSPP-based bandpass filter that combines a CPS feedline with a single-conductor SSPP waveguide.
• Achieve a defined passband around 1 THz with a 0.3 THz bandwidth using a cascaded high-pass (capacitive gap) and low-pass (SSPP) approach.
• Show experimental validation at THz frequencies on a thin SiN membrane to mitigate loss and dispersion.

제안 방법

• Use a cascaded high-pass and low-pass design where the high-pass element is a capacitive gap in the SSPP transition circuit and the low-pass element is the SSPP waveguide.
• Design the SSPP unit cell and transition circuit via eigenmode simulations to extract dispersion and band-edge frequencies.
• Model the upper cut-off frequency with an analytical dispersion relation for a symmetric 1D groove-like SSPP structure and validate with numerical simulations.
• Fabricate the device on a 1 µm SiN membrane with a 200 nm conductor thickness and integrate photo-conductive switches for THz generation/detection.
• Characterize the device using a modified THz time-domain spectroscopy setup with PCS-based Tx/Rx to measure S-parameters and spectral responses.

Figure 1 : Fabricated THz SSPP BPF on thin Si-N membrane with CPS feedlines and transition circuits for excitation and Tx/Rx PCS for THz generation and detection.

실험 결과

연구 질문

• RQ1 Can a single-conductor SSPP waveguide, integrated with a CPS feedline through a capacitive transition circuit, realize a THz bandpass filter?
• RQ2 How do geometric parameters (notably Hn in the SSPP stub design) affect the upper cut-off and overall bandpass performance?
• RQ3 What passband and out-of-band rejection can be achieved experimentally at around 1 THz?
• RQ4 What are the trade-offs between insertion loss, coupling, and bandwidth in the CPS-to-SSPP transition at THz frequencies?

주요 결과

• Center frequency of 1 THz with a 3 dB bandwidth of 0.3 THz (0.87–1.17 THz).
• Passband insertion loss around 1.5–5–7 dB depending on modeling assumptions and fabrication, with simulations indicating ~1.5 dB in the passband for the fabricated structure.
• Out-of-band rejection measured as 15–20 dB in the stopbands (lower and upper) due to measurement noise and weak signal levels, with reasonable agreement to simulations.
• Upper cut-off frequency controlled primarily by the stub height Hn in the SSPP unit cell; smaller Hn yields higher cut-off frequencies.
• Demonstrates integration of a single-conductor SSPP with a CPS feedline on a thin 1 µm SiN membrane to enable THz SSPP-based filtering.
• Experimental results qualitatively align with simulations, validating the concept of THz SSPP-based bandpass filtering on a MEMS-like thin membrane.

Figure 2 : Cross section of CPS feedlines on the thin Silicon Nitride membrane.