Abstract-In this paper, the design and measurement of a 3D-printed low-loss asymptotically single-mode hollow-core terahertz Bragg fiber is reported, operating across the frequency range from 0.246 to 0.276 THz. The HE11 mode is employed as it is the lowest loss propagating mode, with the electromagnetic field concentrated within the air core as a result of the photonic crystal bandgap behavior. The HE11 mode also has large loss discrimination compared to its main competing HE12 mode. This results in asymptotically single-mode operation of the Bragg fiber, which is verified by extensive simulations based on the actual fabricated Bragg fiber dimensions and measured material parameters. The measured average propagation loss of the Bragg fiber is lower than 5 dB/m over the frequency range from 0.246 to 0.276 THz, which is, to the best of our knowledge, the lowest loss asymptotically single-mode all-dielectric microstructured fiber yet reported in this frequency range, with a minimum loss of 3 dB/m at 0.265 THz.Index Terms-Bragg fiber, electromagnetic propagation, millimeter wave technology, photonic crystals, three-dimensional printing.

Abstract-This paper reports on a miniaturized lab-on-awaveguide liquid-mixture sensor, achieving highly-accurate nanoliter liquid sample characterization, for biomedical applications. The nanofluidic-integrated millimeter-wave sensor design is based on near-field transmission-line technique implemented by a single loop slot antenna operating at 91 GHz, fabricated into the lid of a photolaser-based subtractive manufactured WR-10 rectangular waveguide. The nanofluidic subsystem, which is mounted on top of the antenna aperture, is fabricated by using multiple Polytetrafluoroethylene (PTFE) layers to encapsulate and isolate the liquid sample during the experiment, hence, offering various preferable features e.g. noninvasive and contactless measurements. Moreover, the sensor is reusable by replacing only the nanofluidic subsystem, resulting a cost-effective sensor. The novel sensor can measure a liquid volume of as low as 210 nanoliters, while still achieving a discrimination accuracy of better than 2% of ethanol in the ethanol/deionized-water liquid mixture with a standard deviation of lower than 0.008 from at least three repeated measurements, resulting in the highest accurate ethanol and DI-water discriminator reported to date. The nanofluidic-integrated millimeter-wave sensor also offers other advantages such as ease of design, low fabrication and material cost, and no life-cycle limitation of the millimeter-wave subsystem.Index Terms-biomedical liquid mixtures, nanofluidic, millimeter-wave sensor, transmission line method, W-band.

This paper presents a 220-320-GHz hemispherical lens antenna fabricated using photopolymer-based additive manufacture and directly fed by the standard WR-3 rectangular waveguide without any additional waveguide extension. The microfabrication process is based on digital light processing rapid prototyping using the Monocure 3DR3582C resin-based photocurable polymer. This gives various key advantages, including ease of antenna fabrication, manufacturing speed, and cost-effectiveness due to its rapid fabrication capability. Even though the photopolymer is found to have a loss tangent of 0.034 at 320 GHz, the all-polymer lens antennas still achieve a fractional bandwidth of 37%, covering the whole 220-320-GHz WR-3 waveguide band with a measured gain of approximately 16 dBi at 0 • over the whole band. A measured return loss of better than 14 dB is achieved from 220 to 320 GHz with a half-power beamwidth of approximately 12 • , which is relatively constant over the whole WR-3 band. INDEX TERMS Lens antenna, digital light processing, terahertz antennas.

This paper proposes a novel miniaturization technique of quarter-wave transformers (QWTs), implemented using multi-section transmission lines (MSTLs), based on the quarter-wave-like transformer (QWLT) theory. Multi-section QWLT characteristics are derived analytically and solved via appropriate optimization algorithms for associated transmission-line parameters. For an illustration purpose, two-and three-section QWLT prototypes with 50% physical size reduction from the corresponding QWT size operating at 2.4 GHz are fabricated using microstrips and tested. It is found that these prototypes yield acceptable return loss at 2.4 GHz without significant bandwidth reduction, comparing to the QWT result.

This work presents, for the first time, an in-situ self-aligned fluidic-integrated microwave sensor for characterizing NaCl contents in NaCl-aqueous solution based on a 16-GHz bandpass combline cavity resonator. The discrimination of the NaCl concentration is achievable by determining amplitude differences and resonant frequency translations between the incident and reflected microwave signals at the input terminal of the cavity resonator based on the capacitive loading effects of the comb structure inside the cavity introduced by the NaCl solution under test. Twelve NaCl-aqueous liquid mixture samples with different NaCl concentrations ranging from 0% to 20% (0-200 mg/mL), which are generally exploited in most industrial and biomedical applications, were prepared and encapsulated inside a Teflon tube performing as a fluidic channel. The Teflon tube is subsequently inserted into the cavity resonator through two small holes, fabricated through the sidewalls of the cavity, which can be used to automatically align the fluidic subsystem inside the combline resonator considerably easing the sensor setup. Based on at least five repeated measurements, the NaCl sensor can discriminate the NaCl content of as low as 1% with the measurement accuracy of higher than 96% and the maximum standard deviation of only 0.0578. There are several significant advantages achieved by the novel NaCl sensors, e.g. high accuracy, traceability and repeatability; ease of sensor setup and integration to actual industrial and biomedical systems enabling insitu and real-time measurements; noninvasive and noncontaminative liquid solution characterization as well as superior sensor reusability due to a complete physical separation between the fluidic and microwave subsystems.

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This article presents, for the first time, new design and fabrication techniques for Hollow Substrate Integrated Waveguides (HSIWs), demonstrated in the nominal frequency from 21 to 31 GHz, for use in wireless communication applications such as 5G, IoT and robotics. The design and fabrication techniques introduced in this paper feature: 1) the use of low-cost rapid prototyping additive manufacturing based on polymer jetting (PJ), and 2) the use of commercially available through-substrate copper via transitions. In contrast to the conventional SIW designs and fabrications, this new approach does not rely on through-substrate via fabrication, hence avoiding some difficult manufacturing steps, such as through-substrate etching, via formation and via metallization, which are considered complex and expensive to implement. The 3D printed HSIWs in this article can achieve a propagation loss of lower than 1.56 Np/m (13.55 dB/m), which is considered one of the results with the lowest propagation loss achieved to date, when compared to the state-of-the-art.