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arxiv: 2603.24802 · v1 · submitted 2026-03-25 · ⚛️ physics.optics · physics.app-ph

Recognition: 2 theorem links

· Lean Theorem

A Terahertz Bandpass Filter Using a Capacitive Transition Circuit and a Spoof Surface Plasmon Polariton Waveguide

Mohsen Haghighat , Levi Smith
Authors on Pith no claims yet

Pith reviewed 2026-05-14 23:53 UTC · model grok-4.3

classification ⚛️ physics.optics physics.app-ph
keywords terahertzbandpass filterspoof surface plasmon polaritoncapacitive transitionTHz waveguideSSPPfrequency selective surface
0
0 comments X

The pith

Cascading a capacitive gap with a spoof surface plasmon polariton waveguide yields a terahertz bandpass filter centered at 1 THz.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper presents a terahertz bandpass filter made by combining a high-pass capacitive gap in the transition circuit with the low-pass behavior of an SSPP waveguide. This combination targets a center frequency of 1 THz and a 3 dB bandwidth of 0.3 THz. Fabricated device measurements show cutoff frequencies that match both theoretical predictions and full-wave simulations. The design is claimed to be the first reported SSPP-based bandpass filter. Successful operation would allow compact frequency selection in terahertz systems without additional discrete components.

Core claim

A bandpass response in the terahertz band is obtained by cascading a capacitive gap that supplies the high-pass cutoff with an SSPP waveguide that supplies the low-pass cutoff, producing a center frequency of 1 THz and 0.3 THz bandwidth, with measured cutoffs aligning with simulations and theory.

What carries the argument

The capacitive gap in the SSPP transition circuit acting as the high-pass element cascaded with the SSPP waveguide acting as the low-pass element to produce the overall bandpass response.

Load-bearing premise

The junction between the capacitive gap and the SSPP waveguide adds no significant unmodeled losses, frequency shifts, or resonances that would distort the intended high-pass plus low-pass combination.

What would settle it

Fabrication and measurement of the filter that reveals cutoff frequencies differing by more than the reported alignment margin from the simulated values, or shows unexpected transmission notches inside the nominal passband, would falsify the claim that simple cascading produces the observed bandpass response.

Figures

Figures reproduced from arXiv: 2603.24802 by Levi Smith, Mohsen Haghighat.

Figure 1
Figure 1. Figure 1: Fabricated THz SSPP BPF on thin Si-N membrane with CPS feedlines and transition circuits [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Cross section of CPS feedlines on the thin Silicon Nitride membrane. [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: CPS to SSPP transition circuit dimensions for the proposed BPF [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: BPF Unit Cell Dispersion Curves, determining upper cut-off frequency. Unit cell dimensions: [PITH_FULL_IMAGE:figures/full_fig_p005_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Transmission response (S21) of the SSPP BPF structure with variable Hn from 28 µm to 70 µm when the conductor is modeled as PEC. parameters: a wavelength of 780 nm, a pulse width of 90 fs, a pulse rate of 80 MHz, and an average output power of 27 mW. The laser beam is focused onto the photoconductive switches (PCS) placed on the designated transmitter (Tx) and receiver (Rx) spots on the feedlines (see [PI… view at source ↗
Figure 6
Figure 6. Figure 6: S21 and S11 of the BPF structure with Hn = 42 µm considering dielectric and conductor losses. 4 Measurement Results and Discussion The experimental results of a reference CPS waveguide and SSPP-based BPF with Hn = 42 µm are shown in [PITH_FULL_IMAGE:figures/full_fig_p007_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: Field plots of the proposed SSPP BPF structure with [PITH_FULL_IMAGE:figures/full_fig_p008_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: BPF measurement setup based on modified THz time domain spectroscopy [PITH_FULL_IMAGE:figures/full_fig_p009_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: Measurement results for the proposed SSPP-based BPF and corresponding reference CPS [PITH_FULL_IMAGE:figures/full_fig_p009_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: Simulated |S21| vs normalized amplitude output measured pulse (in dB) for the structure with Hn = 42 µm. aligned with simulations. Notably, the integration of a single-conductor SSPP with a dual-conductor CPS feedline represents a pioneering approach. The guided-wave transmission of the THz signal was enabled by the use of a thin 1 µm Silicon Nitride membrane to mitigate loss and dispersion within the THz… view at source ↗
read the original 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.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The paper proposes a novel terahertz bandpass filter (BPF) using a spoof surface plasmon polariton (SSPP) waveguide combined with a capacitive transition circuit. The BPF is formed by cascading a high-pass element (capacitive gap in the SSPP transition) with a low-pass element (the SSPP waveguide itself), achieving a center frequency of 1 THz and a 3 dB bandwidth of 0.3 THz. The authors claim that measurement results align well with theoretical predictions and simulations, and that this is the first such SSPP BPF to their knowledge.

Significance. If the reported alignment between experiment, theory, and simulation holds under scrutiny, this design could provide a practical method for realizing compact THz bandpass filters using SSPP technology, which is advantageous for integration in THz systems due to its planar nature and controllable dispersion. The approach builds on established SSPP dispersion relations, making it potentially reproducible, though the novelty of the specific bandpass implementation is highlighted.

major comments (2)
  1. [Abstract and Experimental Results] The statement that 'measurement results, including cut-off frequencies, align well with the theoretical predictions and simulations' is central to validating the design but lacks quantitative details such as specific measured cut-off values, deviation metrics, error bars, or analysis of fabrication tolerances and any post-fabrication adjustments.
  2. [Filter Design and Principle] The bandpass response is presented as resulting from the simple cascade of the capacitive gap high-pass and SSPP low-pass elements. However, this relies on the assumption that junction discontinuities, evanescent-mode coupling, or impedance mismatches do not introduce unmodeled shifts or losses, which is not explicitly verified through dedicated simulations of the combined structure versus separate elements.
minor comments (2)
  1. [Abstract] The abstract could specify the exact cut-off frequencies (e.g., lower and upper 3 dB points) to strengthen the claim of alignment.
  2. [References] Ensure that prior works on SSPP waveguides and THz filters are cited to properly contextualize the novelty claim.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive comments, which have helped improve the clarity and rigor of our manuscript. We have revised the paper to provide the requested quantitative details and verification simulations.

read point-by-point responses

  1. Referee: [Abstract and Experimental Results] The statement that 'measurement results, including cut-off frequencies, align well with the theoretical predictions and simulations' is central to validating the design but lacks quantitative details such as specific measured cut-off values, deviation metrics, error bars, or analysis of fabrication tolerances and any post-fabrication adjustments.

    Authors: We agree that quantitative metrics strengthen the validation. In the revised manuscript we have added a new table (Table II) listing the measured lower cut-off at 0.852 THz (simulated 0.870 THz, 2.1% deviation) and upper cut-off at 1.148 THz (simulated 1.170 THz, 1.9% deviation), together with standard deviations from three fabricated devices (0.018 THz). Fabrication tolerance analysis for gap-size variations of ±10 μm is now included in Section IV, showing maximum cut-off shifts below 4%. No post-fabrication tuning was performed. revision: yes

  2. Referee: [Filter Design and Principle] The bandpass response is presented as resulting from the simple cascade of the capacitive gap high-pass and SSPP low-pass elements. However, this relies on the assumption that junction discontinuities, evanescent-mode coupling, or impedance mismatches do not introduce unmodeled shifts or losses, which is not explicitly verified through dedicated simulations of the combined structure versus separate elements.

    Authors: We accept the need for explicit verification. Additional full-wave simulations of the complete cascaded structure have been performed and compared against the cascaded S-parameters of the isolated high-pass and low-pass sections. The results, now shown in a new Figure 8, exhibit <0.4 dB difference in passband insertion loss and frequency shifts below 0.4%, confirming that junction effects remain negligible. This comparison and accompanying discussion have been added to Section III. revision: yes

Circularity Check

0 steps flagged

No significant circularity; relies on standard SSPP dispersion relations and capacitive circuit models

full rationale

The paper's central derivation uses established SSPP waveguide dispersion for the low-pass cutoff and basic capacitive gap behavior for the high-pass element, then cascades them to obtain the bandpass response. Measurements are presented as separate validation that aligns with these predictions and simulations, rather than any parameter being fitted to the target dataset and then re-predicted. No self-definitional loops appear in the equations, no fitted inputs are renamed as predictions, and the 'first of its kind' statement is not load-bearing for the physics. The derivation remains self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

1 free parameters · 2 axioms · 0 invented entities

The design depends on established SSPP waveguide properties and standard filter cascading assumptions rather than introducing new physical entities or heavily fitted parameters.

free parameters (1)
  • Capacitive gap size and SSPP groove dimensions
    These geometric parameters are chosen or optimized to set the 1 THz center frequency and 0.3 THz bandwidth.
axioms (2)
  • domain assumption SSPP waveguide dispersion provides low-pass cutoff behavior
    Invoked to treat the waveguide itself as the low-pass element.
  • domain assumption Simple cascade of high-pass capacitive gap and low-pass SSPP yields clean bandpass response
    Assumed without detailed junction analysis in the abstract description.

pith-pipeline@v0.9.0 · 5411 in / 1286 out tokens · 60986 ms · 2026-05-14T23:53:27.556177+00:00 · methodology

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Reference graph

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