This paper presents a wideband terahertz (THz) mixer based on a thin-film antenna-coupled high-temperature superconducting (HTS) YBa ₂ Cu ₃ O _7−x (YBCO) step-edge Josephson junction. The HTS mixer enables the flexible harmonic mixing operation at multiple THz bands with the same microwave local oscillator (LO) source, and features very wide intermediate-frequency or instantaneous bandwidth. In order to optimize the frequency down-conversion performance of the mixer, systematic electromagnetic design and analysis have been carried out to improve the power coupling of THz radiation as well as wideband transmission of microwave signals. Experimental characterization of a fabricated device prototype has demonstrated that the mixer exhibits good performance at both the 200 GHz and 600 GHz bands. Detailed measurement results including the DC characteristics, LO pumping requirement, frequency response, mixing linearity and conversion gain are presented in this paper.

This paper presents a dual-band on-chip antenna coupled high- $T_\rm{c}$ superconducting (HTS) Josephson-junction sub-terahertz (THz) fourth-harmonic mixer. The antenna utilizes a couple of different structured twin-slots to enable the resonant radiations at two frequencies, and integrates a well-designed coplanar waveguide (CPW) network for achieving good radiation coupling and signal isolation characteristics. The electromagnetic (EM) simulations show that coupling efficiencies as high as -4 dB and -3.5 dB are achieved for the 160-GHz and 640-GHz operating frequency bands, respectively. Based on this dual-band antenna, a 640-GHz HTS fourth-harmonic mixer is developed and characterized in a range of operating temperatures. The mixer exhibits a measured conversion gain of around -18 dB at 20 K and -22 dB at 40 K respectively. The achieved intermediate-frequency (IF) bandwidth is larger than 23 GHz. These are the best results reported for HTS harmonic mixers at comparable sub-THz frequency bands to date.

Wireless communication at terahertz (THz) frequencies is viewed as one of the potentials for future 6G wireless systems. However, the path on secure data transmission by terahertz links is facing many complex challenges. Of that, scattering-induced eavesdropping threat remains one of the most critical but less explored. With the updating of carrier frequency into terahertz range, the wavelength becomes comparable to surface roughness of typical objects in our daily life. Surface scattering, which are negligible in current wireless networks, becomes serious and would lead to desperate signal eavesdropping. For a non-line-of-sight (NLOS) link achieved by surface reflection, how do the scattering affect link security at physical layer? Such direct information is crucial for inspection of security threats and proposing of possible solutions. Using metallic wavy surfaces, we characterize possibilities of signal eavesdropping caused by bistatic scattering, by several unmodulated channels at different frequencies. We also demonstrate a real-time data transmission for this eavesdropping configuration, with a data rate of 4 gigabits per second by 16 quadrature amplitude modulation. We observe that successful signal interception from a NLOS link happens, even when transceiver designs incorporating back-scatter measure is considered. This is likely to be a common security threat for any terahertz communication systems in which NLOS channels are employed to achieve a wider user coverage.

Wireless communication and sensing are moving from microwave, millimeter-wave into the terahertz (THz) frequency regime to meet the fast growing demand of ultrahigh data-rate communications and super resolution imaging. Faced with severe atmospheric absorption attenuation and the lack of power efficient transmitting source at the higher band, ultrasensitive and cost-effective receiver frontend technology is required for advanced THz wireless systems. To date, the most sensitive heterodyne mixers, the key components of frontend receiver systems, are based on low-temperature superconducting (LTS) materials that operate at liquid helium (4.2 K) temperature range, requiring expensive and bulky cryogenic cooling systems thus hindering them from commercial applications such as wireless communications and sensing. In this paper, we present a 340-GHz double-sideband (DSB) fundamental mixer based on thin-film antenna-coupled high-temperature superconducting (HTS) Josephson junction that operates at a much higher temperature range attainable with smaller and cheaper cryocoolers. Based on our innovative work in terms of advanced device circuit and on-chip antenna designs, accurate parametric simulation analyses, and Josephson junction parameter optimizations, the reported mixer exhibits a measured noise temperature of 470 K and 780 K at operating temperatures of 20 K and 40 K respectively at 340 GHz, a performance significantly higher than any HTS THz mixers reported to date.

This paper presents a quad-polarization reconfigurable multi-faceted conformal array of wide coverage range and high gain that is suitable for unmanned aerial vehicle (UAV) communication applications. The design features a sparse array layout comprised of three conformal subarrays, where each subarray includes nine quad-polarization reconfigurable radiating elements. The radiating element is a single-port excited patch radiator loaded by edge slots for size miniaturization and central shorting pin for biasing simplification, which is integrated with a low-complicity reconfigurable feeding network consisting of relatively few PIN-diodes and biasing lines. Using such miniaturized and high-efficiency element, a polarization reconfigurable conformal array protype is designed, fabricated and experimentally characterized. Simulation and measurement results have shown that, the presented conformal array exhibits a 3-dB axial-ratio bandwidth of up to 1.3% at around 1.5 GHz, and -10-dB impedance bandwidth of 7.7% for two circular-polarization modes and 7.3% for two linear-polarization modes, respectively. Besides, the realized gain is over 14 dBi for all reconfigurable polarization modes over a relatively wide beam coverage range of 50° × 180°.