We present a genetic algorithm (GA)-based inverse design framework for synthesizing high-performance planar terahertz (THz) filters integrated with coplanar striplines (CPSs). The method efficiently explores high-dimensional design spaces to generate filter geometries matching user-defined S-parameter magnitude and phase responses, while enforcing structural connectivity for compatibility with terahertz system-on-chip (TSoC) platforms. To accelerate optimization, filter performance is evaluated using the ABCD matrix method, providing a significant computational advantage over full-wave simulations. Final validation is performed through finite element method (FEM) simulations. As a proof of concept, we design band-stop filters with center frequencies of 0.6, 0.8, and 1.0 THz, each with a 150 GHz target bandwidth, and demonstrate tunable rejection depths within a constant physical footprint. Optimization is guided by minimizing the root-mean-square error (RMSE) between simulated and target S-parameters.

In this paper, we report a proof-of-concept terahertz bandstop filter constructed from split-ring resonators. The design comprises nine split-ring resonators, organized into three groups of three same radius. The radius of the groups is 13 µm, 14 µm, and 15 µm, respectively, and are placed between the conductors of a coplanar stripline. Each group of split-ring resonators results in notch filters with different center frequencies. The combined bandstop center frequency is 1.09 THz and the -3 dB bandwidth is 0.36 THz. The transmission response of the filter is measured using a modified terahertz time-domain spectrometer and we find agreement between theory, simulation, and experiment. This work demonstrates the viability of using varying-radii split-ring resonators as sub-wavelength filter elements for terahertz systems.

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.

This study presents a terahertz (THz) bandpass filter that is constructed from periodic subwavelength split rings developed from spoof surface plasmon polariton (SSPP) on coplanar stripline (CPS)-SSPP with internal grooves. The bandpass filter achieves high-frequency rejection through SSPP characteristics and blocks low frequencies using capacitive gaps. A bandpass filter with a center frequency of 1 THz was simulated and showed lower and upper 3 dB cut-off frequencies at approximately 0.8 THz and 1.2 THz, respectively. The simulated passband insertion loss considering all of the loss factors was 4 dB with out-of-band rejection of higher than 30 dB.

This paper presents the design and experimental verification of a terahertz (THz) spoof surface plasmon polariton (SSPP) waveguide using a coplanar stripline (CPS) with internal corrugations and is compared against an external corrugation configuration. Internal corrugations are selected to reduce the insertion loss by improving the mode conversion efficiency of the transition circuit. We examine this effect using simulation and then experimentally confirm that the SSPP mode was excited for two different corrugation depths, 55 µm, and 65 µm. We found that the associated SSPP band-edge frequency changed from 0.89 THz to 0.72 THz which is consistent with the simulations.

At terahertz (THz) frequencies, there are few experimental works that demonstrate the viability of all-pole network synthesis filters to obtain desired frequency characteristics (i.e., Chebyshev, Butterworth, Bessel, etc.) using planar waveguides. This capability has been proven with non-planar waveguides, but has yet to be demonstrated using planar waveguides, which are desirable due to their integration capabilities similar to printed circuit board or monolithic microwave integrated circuit structures. In this paper, we use network synthesis methods from microwave engineering to fabricate three integrated planar low-pass filters for THz applications that have the same cut-off frequency ( f c = 0.8 THz) but different orders (N = 3, 4, 5). We measure their response to a THz-bandwidth excitation pulse and find that the experimental results exhibit increasing roll-off rates with increasing filter order without incurring significant pulse distortion, which is aligned with theory.

This paper demonstrates an on-chip anhydrous D-glucose sensor based using a coplanar stripline (CPS) on a thin (1 um) silicon nitride membrane at terahertz (THz) frequencies. A thin layer ( 10 um) of D-glucose was placed in close proximity to the CPS and the transmission response was measured using a modified THz-TDS setup. The D-glucose introduces frequency-dependent changes to the effective permittivity of the CPS resulting in a modified spectral response at the receiver. Measurement results show absorption signatures at 1.42 THz and 2.07 THz corresponding to the first two significant resonances beyond 1 THz for D-glucose allowing for label-free detection. The frequency-dependent attenuation coefficient was estimated by simulation for several D-glucose layer thicknesses using a modified Lorentz model. Measurement results align with simulations and other literature that use free-space THz radiation. This work verifies on-chip THz sensing of D-glucose and presents a pathway towards on-chip sensing of other materials at THz frequencies.

There is a growing interest in spoof surface plasmon polariton (SSPP) structures at terahertz (THz) frequencies for applications such as filtering, sensing, and communications. However, to date, there are limited experiments that confirm SSPP characteristics at THz frequencies. The majority of literature focuses on simulation or verification by device scaling to Gigahertz (GHz) frequencies where standard vector network analyzers are readily available. This paper presents the first experimental verification of SSPP characteristics at THz frequencies in a guided wave system using coplanar strip (CPS) feedlines. Specifically, we design three SSPP structures with varying band-edge frequencies (1.04 THz, 0.63 THz, and 0.53 THz), then fabricate and verify the low-pass transmission characteristics using a modified THz-time-domain spectrometer (THz-TDS) system. We find strong agreement between simulation, theory, and experiment.

This paper demonstrates an on-chip anhydrous D-glucose sensor using a coplanar stripline (CPS) on a thin (1 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\upmu$$\end{document}m) silicon nitride membrane at terahertz (THz) frequencies. A thin layer (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\approx$$\end{document} 10 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\upmu$$\end{document}m) of D-glucose was placed in close proximity to the CPS and the transmission response was measured using a modified THz-TDS setup. The D-glucose introduces frequency-dependent changes to the effective permittivity of the CPS resulting in a modified spectral response at the receiver. Measurement results show absorption signatures at 1.42 THz and 2.07 THz corresponding to the first two significant resonances beyond 1 THz for D-glucose allowing for label-free detection. The frequency-dependent attenuation coefficient was estimated by simulation for several D-glucose layer thicknesses using a modified Lorentz model. Measurement results align with simulations and other literature that use free-space THz radiation. This work verifies on-chip THz sensing of D-glucose and presents a pathway toward on-chip sensing of other materials at THz frequencies.

This paper presents a proof-of-concept terahertz (THz) spoof surface plasmon polariton (SSPP) bandpass filter operating at sub-THz frequencies (near 300 GHz band) for 6G communication applications. The filter operates by integrating a transition circuit and SSPP waveguide into a coplanar strip feedline. The passband insertion loss at 260 GHz is 6.8 dB considering the conductor and substrate losses. The out-of-band rejection is more than 20 dB for low frequencies and more than 30 dB for high frequencies. The filter demonstrates a 3 dB bandwidth of around 90 GHz, covering the frequency range from 230 to 320 GHz.

This study presents an on-chip glucose sensor using a coplanar strip (CPS) waveguide on a thin (approximate to 1 mu m) Silicon Nitride (Si3N4) membrane which operates at terahertz (THz) frequencies. A thin (approximate to 10 mu m) glucose layer was drop-coated on the backside of the Si3N4 membrane and a modified THz-TDS was used to measure the transmission response. The introduction of glucose altered the propagation constant of the wave, thereby influencing the spectral response based on the material's absorption characteristics. Experimental results revealed a resonance at 1.42 THz, aligning with the first notable absorption peak beyond 1 THz for glucose. The sensor's functionality was also characterized using simulations using a modified Lorentz model to represent a layer of glucose beneath the substrate. The measurement outcomes aligned with the simulation results, establishing the proposed method's capability for on-chip integrated THz sensing of glucose and other materials.

In this paper, we experimentally verify the performance of a Terahertz (THz) spoof surface plasmon polariton (SSPP) device using a co-planar strip (CPS) transmission line. The CPS-SSPP structure investigated here was initially proposed by Guo et al. in [1], but the authors were unable to verify the performance in the THz region due to equipment limitations. We validate their work by using methods commonly used in our research laboratory by fabricating the CPS-SSPP structure on a thin $(1 \mu \mathrm{m})$ Silicon Nitride membrane to minimize the radiation loss and dispersive effects. After noting inconsistencies incurred by the different substrate conditions via simulation, we find the experiment results agree. Also, for the first time, we demonstrate a guided wave SSPP stopband at a frequency greater than 1.0 THz.

At terahertz (THz) frequencies there are few experimental works which demonstrate filter synthesis to obtain a desired filter response (i.e., Chebyshev, Butterworth, Bessel, etc.). Currently, the majority of literature perform THz filter analysis, that is, characterizing the filter response after design procedure. In this paper, we apply filter synthesis methods from microwave engineering to design several integrated planar low-pass filters fc = 0.8 THz). We find that the transmission characteristics align with theory and simulation.

This paper presents the demonstration of an on-chip integrated Terahertz (THz) Apodized Bragg grating (TABG) which functions as band-stop filter with a center frequency of 0.8 THz and a bandwidth of 200 GHz. For experimentation, we integrate the TABG into our THz System-on-Chip to enable wideband (DC - 1.5 THz) device characterization. Using this methodology, we measure the signal transmission through the TABG and find the experimental results align with simulation and theory provides a rejection of approximately 20 dB across the stop-band.

A guided-wave THz System-on-Chip THz System-on-Chip (TSoC) is emerging as an attractive alternative to the routine free-space THz systems to reduce physical bulk, propagation loss, pulse dispersion and cost of free-space THz systems. Recently, our research group succeeded in demonstrating a novel waveguided TSoC based on the coplanar stripline Coplanar Stripline (CPS) transmission lines on a 1 µm-thin Silicon Nitride membrane. The novelty of this membrane-based platform was bonding the transmitter and receiver directly on the transmission line to eliminate the radiation loss by the routine THz optics. Besides, the delicate thin-membrane dramatically reduces the dielectric loss of the platform which results in low-loss and low-dispersion THz-bandwidth pulses. In this paper, we introduce the design, theoretical model and the simulation results of a novel THz Apodized Bragg Grating (TABG) based on a periodic reflection of consecutive different impedance sections of CPS transmission lines on the membrane platform. This structure was introduced based on theoretical modelling and the simulation results. The CPS-TABG consists of 20 λ/2-length cascaded sections. The central frequency of the bandstop filter was selected around 0.8 THz to avoid signal distortion of the received terahertz pulse.

Terahertz membrane-based system-on-chip devices provide unique access to concentrated fields surrounding the transmission line. We scan a needle in proximity of a CPS line and measure reflected sub-picosecond pulses. This provides useful design insight and potential application in nonlinear terahertz spectroscopy.

Complex terahertz (THz) System-on-Chip (TSoC) circuits require ultra-wideband low-loss low-dispersion interconnections between building-block components of various dimensions and characteristics. Tapered transmission lines, which enable the gradual transformation of both physical dimensions and characteristic impedance, are a convenient basis for these interconnections. In this paper, we quantify both experimentally and through simulation, the efficacy of transmission-line tapers connecting two different coplanar-strip transmission-line configurations, for frequencies up to 2.0 THz and with 25 GHz spectral resolution. We demonstrate tapers that enable transitioning from a small device-constrained transmission-line dimension (10 μm line width) to a lower-loss (20-40 μm line width) dimension, as a method to reduce the overall attenuation, and outline design constraints for tapered sections that have minimal detrimental impact on THz pulse propagation.

Recently we demonstrated the fabrication and testing of a variety of RF-engineered passive transmission-line-based components designed for operation at terahertz frequencies and fabricated on thin (1 μ m) silicon-nitride membranes. In this work we measure the transmission response of a coplanar-strip transmission line loaded with split-ring resonators up to 2.5 THz. We observe three dominate modes within the measured frequency range; the predicted LC resonator mode at 0.510 THz, a higher-order LC resonator mode at 1.03 THz, and a higher-order dipole mode at 1.85 THz. The LC resonator mode is investigated using a modified version of the standard lumped element model which incorporates the transmission line between adjacent meta-atoms using ABCD matrices.

A membrane-based coplanar-stripline (CPS) transmission-line platform has recently enabled implementation of diverse THz system-on-chip (TSoC) components. In this paper, we demonstrate an elliptic-function THz low-pass filter (TLPF) using cascaded λ/4 resonators between the right-angle bending of a CPS transmission line defined on a 1 μm-thin membrane. We investigated the effect of bending the CPS transmission line with different angles that introduces a frequency response similar to a simple LC low-pass filter (LPF) and facilitates the design of a desired roll-off performance using traditional methods. ANSYS HFSS was used to provide a full-wave analysis and characterize the effective parameters of the TLPF with a designed cutoff-frequency around 0.6 THz. Using 7 sections of right-angle CPS bending with total length 1.4 mm, we demonstrate experimentally an elliptic-function TLPF characterized by a low-ripple at passband, a roll-off transition with zero transmission near the cutoff frequency and a wide stopband with -60 dB rejection.

Recently, research has focused on developing efficient wave-guided THz system-on-chip (TSoC) components to reduce physical bulk, loss and cost of free-space THz systems. We recently demonstrated a TSoC platform using a coplanar-stripline (CPS) transmission-line on a 1 μm-thin membrane to generate and detect THz-bandwidth pulses with low loss and low dispersion up to 1.5 THz. In this paper, we demonstrate experimentally an in-phase THz power divider (TPD) at frequency 0.65 THz using the CPS transmission line defined by photolithography on a thin membrane. Measured pulses show close agreement with simulation results. The spectral power density of the measured THz-bandwidth pulses at the output ports are identical at the frequency of 0.65 THz with less than 1 dB power imbalance over a wide spectrum up to 1 THz.