Wideband THz Mixers and Components for the Next Generation of Receivers for Radioastronomy

Doctoral thesis, 2024

In recent decades, there has been a growing interest in THz research, leading to substantial improvements in technology and the emergence of new applications. In particular, the ever-evolving field of radio astronomy instrumentation has been pushing the limits of millimeter and sub-millimeter technology boundaries, redefining the state-of-the-art for wideband low-noise receivers. The technological roadmaps for radio astronomy applications, such as  “The ALMA 2030 Wideband Sensitivity Upgrade”, set the requirements for the next generation of heterodyne receivers. Among these requirements, it establishes the need for a wider IF bandwidth and the possibility of covering multiple existing RF bands with a single receiver, e.g., combining ALMA bands 6 (211–275 GHz) and 7 (275–373 GHz), i.e., a ~56% fractional bandwidth. To build receivers with such a large fractional bandwidth, each of their components must be able to cover the required bandwidth with minimal insertion loss, or equivalently, add the minimum noise to the system. In particular, it is essential to focus on the components that are critical for the performance of such receivers, such as front-end waveguide components and the mixer chip. This thesis addresses this need and focuses on the design, simulation, fabrication, and characterization of ultra-wideband THz devices for the next generation of radio astronomy receivers.

The thesis starts by presenting the development of waveguide passive components key to future ultra-wideband receivers, such as waveguide twists and power dividers. Waveguide twists are essential interconnection parts in most polarization-sensitive THz receivers that make use of orthomode transducers. Since compactness and low insertion loss are critical requirements, step-twists have become a promising solution. This work introduces novel designs for step twists covering the frequency ranges of 120-220 GHz and 210-375 GHz, i.e., 44% and 56% fractional bandwidth with 20 dB return loss, respectively. In the first design, the experimental verification showed an insertion loss of 0.4 dB, while the second demonstrated an insertion loss as low as 0.3 dB. Additionally, the thesis investigates waveguide power dividers, a fundamental component in the development of 2SB receivers for LO injection. It presents a waveguide power divider that incorporates a substrate-based element into a waveguide structure to enhance the output port’s isolation and matching in the frequency range of 150-220 GHz, i.e. 38% fractional bandwidth.

THz mixers are implemented with thin-film technology. As a consequence, the waveguide-to-substrate transitions have a fundamental role in the performance and bandwidth of such systems. In this work, a waveguide-to-slotline superconducting transition based on substrateless finlines is proposed. Moreover, for the majority of modern mixers with Superconductor Insulator Superconductor (SIS) technology, the microstrip line topology is the most suitable. Hence, this work presents the development of a broadband slotline to microstrip transition based on Marchand baluns. Both transitions were experimentally verified at cryogenic temperatures. Remarkably, each of these transitions achieved a fractional bandwidth of ~56%, while the substrateless finline transition demonstrated an insertion loss of 0.5 dB, the Marchand Balun showed an insertion loss as low as 0.3 dB. The integration of the substrateless finline and the Marchand balun transitions served as the first approach to a platform for the development of an ultra-wideband SIS mixer. This platform evolved into the SIS mixer design for 210-375 GHz introduced in this thesis. The mixer chip represents a significant shift from traditional design approaches since the dielectric substrate is removed and replaced with a micromachined metallic substrate which integrates a metallic finline. The micromachined substrate is employed as a technological platform for the Nb-Al/AlN-Nb SIS junctions, the RF matching circuitry, and the IF output filter. The mixer features a designed IF bandwidth of 4-16 GHz.  Furthermore, this thesis demonstrates the feasibility of micromachined metallic substrates as a technological platform for SIS devices.