Stepan Lucyszyn

This article presents a cost-effective method for verifying (sub-)terahertz scattering ( $S$ -)parameter measurements using a vector network analyzer (VNA) with frequency extension modules. The approach reduces the need for physical standards with rigorous dimensional characterization by using full-wave electromagnetic (EM) simulation to create a “simulation-driven reference model” with a predictable uncertainty envelope. This method utilizes the known manufacturing tolerances of generic waveguide sections to generate a pass/fail verification bound. Two simple, calculable waveguide devices (a cross-connected line and reduced-aperture sections) are characterized in the WM-570 (330–500 GHz) band. Excellent agreement is achieved between the EM-simulated predictions and VNA-measured $S$ -parameters, with the measurements falling within the calculated uncertainty bounds. This work demonstrates that combining simple hardware with EM simulation is a reliable and low-cost approach for validating $S$ -parameter measurements and calibrations at terahertz frequencies.

The IEC 60153-2 and IEEE 1785.1 international standards for rectangular metallic waveguides are currently undergoing revision. Textbook derivations of the associated equations for the metal-pipe rectangular waveguide attenuation constant to be given in the revised standards are presented. These derivations provide important mathematical traceability for these equations, which has previously not been proven in the open literature for these standards. Furthermore, the historical approximations used in the original equations are identified and now corrected here. Results using the newly derived equations are demonstrated, with comparison to experimental results in the WM-380 (500 GHz to 750 GHz) band.

In this research, we explore the few-shot object detection application for identifying concealed objects in sub-terahertz security images, using fine-tuning based frameworks. To adapt these machine learning frameworks for the (sub-)terahertz domain, we propose an innovative pseudo-annotation method to augment the object detector by sourcing high-quality training samples from unlabeled images. This approach employs multiple one-class detectors coupled with a fine-grained classifier, trained on supporting thermal-infrared images, to prevent overfitting. Consequently, our approach enhances the model’s ability to detect challenging objects (e.g., 3D-printed guns and ceramic knives) when few-shot training examples are available, especially in the real-world scenario where images of concealed dangerous items are scarce.

The low-cost thermal infrared ‘THz-Torch’ concept (referred to here as ‘THz-T’) was first introduced over a decade ago, and since then the associated ‘over the THz horizon’ thermal infrared (10-100 THz) implementation technologies have continued to advance. While short range secure wireless communications links have received a great deal of attention, material spectroscopy has only briefly been introduced in a short conference abstract. Here, for the first time, we explore in depth the basic concepts behind THz-T spectroscopy. Moreover, when compared to the unvalidated results within our previous work, we demonstrate an enhanced experimental THz-T spectrometer. A detailed thermal noise power link budget model for both the transmission and reflection modes of operation have been undertaken and independently validated. As a proof of principle, a diverse array of different material types has been characterized. This includes glass sheets, semiconductor wafers, ceramic plate, plastic tape, plastic sheets, as well as polymer and cotton paper banknotes. THz-T technology has the advantages of hardware simplicity and low cost non-destructive testing for ubiquitous applications.

In this research, we explore the Few-Shot Object Detection application for identifying concealed objects in sub-terahertz security images, using fine-tuning based frameworks. To adapt these machine learning frameworks for the (sub-)terahertz domain, we propose an innovative pseudo-annotation method to augment the object detector by sourcing high-quality training samples from unlabeled images. This approach employs multiple one-class detectors coupled with a fine-grained classifier, trained on supporting thermal-infrared images, to prevent overfitting. Consequently, our approach enhances the model’s ability to detect challenging objects when few-shot training examples are available.

This paper presents the state-of-the-art in polymer-based 3-D printing of metal-pipe rectangular waveguides (MPRWGs) with the first reported terahertz filters, all operating within the WR-2.2 band (325 to 500 GHz): a 5 mm-long thru line, two 399 GHz single-cavity resonators and two 403 GHz bandpass filters (BPFs). Our thru line exhibits a measured average insertion loss of only 0.9 dB, with a worst-case return loss of 13.3 dB, across the band. The single-cavity resonators, without and with corner rounding compensation (CRC) are investigated with the use of an RLC equivalent circuit model. The uncompensated resonator exhibits a 2.3% frequency downshift and an increase of 10.8 GHz in its 3 dB bandwidth. The compensated resonator exhibits a 2.2% frequency upshift and an increase of only 2.2 GHz in its 3 dB bandwidth; clearly demonstrating that CRC helps to mitigate against increased coupling into the resonators, as a result of manufacturing limitations with low-cost 3-D printing. Finally, the 3 ^rd order Butterworth and Chebyshev MPRWG BPFs both have a measured passband insertion loss of only 1.0 dB. The Butterworth filter exhibits a 0.8% passband frequency upshift and worst-case return loss of 16.6 dB; while the Chebyshev filter exhibits a 1.2% passband frequency downshift and worst-case return loss of 10.4 dB. With our low-cost polymer-based 3-D printing technology, we have demonstrated measured performances that are better than those using metal-based 3-D printing in the WR-2.2 band and this may, in the not too distant future, challenge components manufactured using traditional machining technologies.

This paper demonstrates the current state-of-the-art in low-cost, low loss ruggedized polymer-based 3-D printed G-band (140 to 220 GHz) metal-pipe rectangular waveguide (MPRWG) components. From a unique and exhaustive up-to-date literature review, the main limitations for G-band split-block MPRWGs are identified as electromagnetic (EM) radiation leakage, assembly part alignment and manufacturing accuracy. To mitigate against leakage and misalignment, we investigate a ‘trough-and-lid’ split-block solution. This approach is successfully employed in proof-of-concept thru lines, and in the first polymer-based 3-D printed 90° twist and symmetrical diaphragm inductive iris-coupled bandpass filters (BPFs) operating above 110 GHz. An inexpensive desktop masked stereolithography apparatus 3-D printer and a commercial copper electroplating service are used. Surface roughness losses are calculated and applied to EM (re-)simulations, using two modifications of the Hemispherical model. The 7.4 mm thru line exhibits a measured average dissipative attenuation of only 12.7 dB/m, with rectangular-to-trapezoidal cross-sectional distortion being the main contributor to loss. The 90° twist exhibits commensurate measured performance to its commercial counterpart, despite the much lower manufacturing costs. A detailed time-domain reflectometry analysis of flange quality for the thru lines and 90° twists has also been included. Finally, a new systematic iris corner rounding compensation technique, to correct passband frequency down-shifting is applied to two BPFs. Here, the 175 GHz exemplar exhibits only 0.5% center frequency up-shifting. The trough-and-lid assembly is now a viable solution for new upper-mm-wave MPRWG components. With this technology becoming less expensive and more accurate, higher frequencies and/or more demanding specifications can be implemented.

This work demonstrates the integration of a low-pass filter into a 90 $^{\circ}$ waveguide twist at $W$ -band (75–110 GHz), manufactured using polymer-based 3-D printing. For the first time, a 1-D periodic electromagnetic bandgap (EBG) structure is incorporated within a waveguide twist. Unlike conventional filters, implemented using irises and septa, EBG structures employing hollow cavities are structurally robust and mechanically insensitive to 3-D printing. The measured average passband insertion loss is only 0.48 dB at the $W$ -band while using our unconventional split-block solution ( $H$ -plane $a$ -edge split with raised lips). A nontwist thru filter has also been demonstrated, as a reference. Our approach demonstrates the potential for the low-cost manufacture of compact and high-performance multifunctional integrated waveguide components at millimeter-wave frequencies.

Due to a production error, in the above letter [1] , on page 1, the sentence “The optical microphotograph of the plane view for the $322\times 876\times 1630\,\,\mu \text{m}$ test chip is shown in Fig. 1(b)” should have been presented as: