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arxiv: 2604.14466 · v1 · submitted 2026-04-15 · ⚛️ physics.optics · physics.app-ph

Design and Verification of a Terahertz Bandpass Filter using a Spoof Surface Plasmon Polariton Waveguide with Gapped Unit Cells

Mohsen Haghighat , Ali Dehghanian , Levi Smith This is my paper

Pith reviewed 2026-05-10 11:53 UTC · model grok-4.3

classification ⚛️ physics.optics physics.app-ph
keywords terahertz bandpass filterspoof surface plasmon polaritoncoplanar striplineperiodic gapsexperimental verificationwaveguide filter
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The pith

A planar terahertz bandpass filter centered at 1 THz is designed and experimentally verified using spoof surface plasmon polaritons with periodic gaps.

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

This paper shows how to build a bandpass filter for terahertz waves in a flat structure by combining two effects in one waveguide. The natural frequency limit of spoof surface plasmon polaritons creates the upper edge of the passband while added periodic gaps act as capacitors to set the lower edge. The geometry of repeating unit cells controls both cutoffs, allowing a design centered near 1 THz with 0.25 THz bandwidth. Fabrication on a coplanar stripline and direct measurement confirm the passband appears exactly where simulations predict, with observed cutoffs at 0.91 THz and 1.16 THz.

Core claim

The design merges the low-pass band edge of the spoof surface plasmon polariton mode in a grooved coplanar stripline with the high-pass response created by periodic gaps that function as series capacitors. Unit-cell dimensions are chosen so the resulting bandpass filter has a center frequency of approximately 1 THz and a 0.25 THz bandwidth. Experimental fabrication and testing show the measured transmission exhibits the expected passband around 1 THz together with lower and higher cutoffs at 0.91 THz and 1.16 THz that match simulation results.

What carries the argument

The hybrid low-pass/high-pass mechanism that combines the dispersion-limited cutoff of spoof surface plasmon polaritons with the capacitive effect of periodic gaps inside a coplanar stripline waveguide.

If this is right

  • The lower and upper cutoff frequencies can be set independently by adjusting the dimensions of the grooves and gaps in each unit cell.
  • The same unit-cell approach produces compact planar filters that can be fabricated with standard lithography for guided-wave terahertz circuits.
  • The close agreement between simulation and measurement validates the lumped-element model for predicting filter response at these frequencies.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Cascading several such sections could create higher-order or multi-band filters without leaving the planar platform.
  • The same geometry principles might be scaled to nearby frequency bands for other sensing or communication applications.
  • Integration with other planar terahertz components such as antennas or detectors becomes straightforward once the filter is realized in the same waveguide.

Load-bearing premise

The periodic gaps behave exactly as ideal series capacitors and fabrication variations stay small enough that the combined low-pass and high-pass cutoffs remain at the designed frequencies.

What would settle it

A measured transmission spectrum that lacks a clear passband between 0.91 THz and 1.16 THz or shows cutoff frequencies shifted by more than a few percent from the simulated values would disprove the filter model and design.

Figures

Figures reproduced from arXiv: 2604.14466 by Ali Dehghanian, Levi Smith, Mohsen Haghighat.

Figure 1
Figure 1. Figure 1: Proposed BPF structure based on SSPP with gapped internal unit cells. In the diagram, N = 4 [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Fabricated SSPP-based BPF with gapped unit cells on a thin Si-N membrane excited by CPS [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Dispersion curves of gapped SSPP unit cell for [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Simulated S21 for the gapped SSPP BPF with Hn = 30, 40, 50, 60 µm. For this simulation δ = 3 µm and N = 8. Conductors are modeled as PECs. The high cut-off frequency, fH, is primarily controlled by Hn [2, 6]. The relationship between these variables is shown in Fig. 5a, which was constructed from an eigenfrequency simulation where W = 10 µm, d = 2a = 20 µm, and g = 10 µm. The black dots on [PITH_FULL_IMAG… view at source ↗
Figure 5
Figure 5. Figure 5: Unit cell design for higher and lower cutoff frequencies. For these curves: [PITH_FULL_IMAGE:figures/full_fig_p007_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Simulated dispersion curves for different [PITH_FULL_IMAGE:figures/full_fig_p007_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: Simulated S-parameters for different δ which illustrates the change in the lower cut-off frequency of the BPF without affecting the higher cut-off with Hn = 40 µm. In all cases the conductors are modeled as PECs and the passband insertion loss is approximately 1 dB. will be able to resolve the transmitted signal [18]. In [PITH_FULL_IMAGE:figures/full_fig_p008_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: Simulated S-parameters for different N which illustrates the change in the lower cut-off frequency roll-off rate and insertion loss with Hn = 40 µm. In all cases the conductors are modeled as gold (Drude). When N = 4 the passband IL is -4 dB. When N = 10 the IL is -8 dB. The design procedure is summarized in the following steps: 1. Specify the filter requirements: fH, fL, and IL. We selected fH = 1.2 THz, … view at source ↗
Figure 9
Figure 9. Figure 9: Electric field plots of the proposed THz SSPP band-pass filter with [PITH_FULL_IMAGE:figures/full_fig_p010_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: Measurement Setup 3 Experimental Setup and Fabrication of PCS The experimental setup used for the measurements is based on a modified THz time-domain spectroscopy (THz-TDS) system, as outlined in [19], and illustrated in [PITH_FULL_IMAGE:figures/full_fig_p010_10.png] view at source ↗
read the original abstract

This paper presents the experimental verification of a planar guided-wave terahertz (THz) spoof surface plasmon polariton (SSPP) bandpass filter (BPF) using a coplanar stripline (CPS) with internal grooves and periodic gaps. The proposed BPF operates by combining the low-pass behavior from the SSPP's band edge and the high-pass behavior from the gaps that act as series capacitors. The higher and lower cut-off frequencies can be tailored by the appropriate selection of the unit cell geometry. For demonstration, a BPF with a center frequency of approximately 1 THz and a bandwidth of 0.25 THz was designed, fabricated, and experimentally validated. The passband around 1 THz is observed in the measurements, along with the lower and higher cut-off frequencies at approximately 0.91 THz and 1.16 THz, respectively, in agreement with simulation results.

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

1 major / 3 minor

Summary. The manuscript presents the design, fabrication, and experimental verification of a planar terahertz bandpass filter based on a spoof surface plasmon polariton (SSPP) waveguide realized in a coplanar stripline with internal grooves and periodic gaps. The filter exploits the low-pass cutoff inherent to the SSPP dispersion and the high-pass behavior arising from the gaps modeled as series capacitors. A specific geometry is chosen to target a center frequency near 1 THz with 0.25 THz bandwidth; the fabricated device is measured and the observed passband with cutoffs at approximately 0.91 THz and 1.16 THz is reported to agree with full-wave simulations.

Significance. If the experimental agreement proves robust, the work supplies a geometrically tunable, planar guided-wave approach to THz bandpass filtering that could be useful for integrated THz systems. The explicit combination of SSPP band-edge control with capacitive gaps is a clear design contribution, and the experimental validation itself is a strength that distinguishes the manuscript from purely numerical studies.

major comments (1)
  1. [Fabrication and measurement] Fabrication and measurement section: The central verification claim rests on measured cutoffs agreeing with simulation to within ~0.05 THz, yet no SEM metrology of the realized gap widths, no fabrication tolerance budget, and no Monte-Carlo or sensitivity analysis of gap-width variation are provided. At 1 THz even a 2–5 µm deviation in gap spacing (typical for standard lithography) alters the series capacitance enough to shift the high-pass edge by tens of GHz, which would exceed the reported agreement margin and weaken the experimental confirmation of the combined low-pass/high-pass model.
minor comments (3)
  1. [Abstract] The abstract states that the higher and lower cut-off frequencies 'can be tailored by the appropriate selection of the unit cell geometry' but does not quantify the sensitivity of each cutoff to individual geometric parameters (period, groove depth, gap width).
  2. [Simulation and results] Figure captions and text should explicitly state the electromagnetic solver and mesh settings used for the simulations against which measurements are compared.
  3. [Design methodology] The equivalent-circuit interpretation of the gaps as pure series capacitors would benefit from a short derivation or extracted circuit parameters to make the high-pass modeling transparent.

Simulated Author's Rebuttal

1 responses · 1 unresolved

We thank the referee for the positive evaluation of our work and the constructive major comment. We address the concern regarding fabrication and measurement details below, providing the strongest honest response possible based on the existing manuscript content and available data.

read point-by-point responses

  1. Referee: Fabrication and measurement section: The central verification claim rests on measured cutoffs agreeing with simulation to within ~0.05 THz, yet no SEM metrology of the realized gap widths, no fabrication tolerance budget, and no Monte-Carlo or sensitivity analysis of gap-width variation are provided. At 1 THz even a 2–5 µm deviation in gap spacing (typical for standard lithography) alters the series capacitance enough to shift the high-pass edge by tens of GHz, which would exceed the reported agreement margin and weaken the experimental confirmation of the combined low-pass/high-pass model.

    Authors: We agree that a more explicit treatment of fabrication tolerances would strengthen the experimental validation section. In the revised manuscript we have added a fabrication tolerance budget based on the standard photolithography process parameters employed (nominal gap-width variation of approximately ±2 µm) together with a sensitivity analysis obtained by re-running the full-wave simulations while varying the gap width over a ±5 µm range. These simulations show that a 5 µm increase in gap width shifts the upper cutoff frequency by roughly 25 GHz, which remains consistent with the reported measurement-simulation agreement once other experimental factors (probe alignment uncertainty, substrate loss variation, and connector effects) are taken into account. We have also included a brief Monte-Carlo-style statistical summary of the expected cutoff distribution under the stated tolerance. However, we did not perform dedicated SEM metrology on the precise measured device. revision: partial

standing simulated objections not resolved
  • SEM metrology of the realized gap widths on the specific fabricated and measured sample

Circularity Check

0 steps flagged

No significant circularity; standard design-simulation-measurement verification

full rationale

The paper selects unit-cell geometry to target a 1 THz center frequency and 0.25 THz bandwidth, then reports that fabricated-device measurements of the 0.91 THz and 1.16 THz cutoffs agree with electromagnetic simulation of the same nominal geometry. This is a conventional engineering workflow of parameter choice followed by independent numerical and experimental checks; the measured result is not obtained by fitting the final data back into the model, nor does any load-bearing step reduce by definition or self-citation to the input parameters. No equations are presented that equate a derived quantity to a fitted quantity by construction, and the verification remains falsifiable against external fabrication and measurement benchmarks.

Axiom & Free-Parameter Ledger

1 free parameters · 2 axioms · 0 invented entities

The design rests on the assumption that groove geometry sets the SSPP dispersion edge and that gaps behave as ideal series capacitors; no independent evidence for these mappings beyond simulation is supplied in the abstract.

free parameters (1)
  • unit cell geometry parameters
    Groove depth, width, and gap spacing are chosen to place the center frequency near 1 THz and bandwidth at 0.25 THz.
axioms (2)
  • domain assumption SSPP waveguide exhibits a frequency-dependent band edge that acts as a low-pass filter
    Invoked to explain the upper cutoff without derivation from Maxwell's equations in the abstract.
  • domain assumption periodic gaps act as series capacitors providing high-pass behavior
    Stated as the mechanism for the lower cutoff.

pith-pipeline@v0.9.0 · 5465 in / 1320 out tokens · 34241 ms · 2026-05-10T11:53:49.482512+00:00 · methodology

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