Dr. Boon Kok Tan

Abstract:

We compare the design of two 0.85 THz SIS mixers fed with a radial probe antenna aligned to the E-Plane of the input full-height rectangular waveguide connected to a drilled smooth-walled horn. Both designs employ the same 0.5 µm2 hybrid Nb/AlN/NbN tunnel junction technology, sandwiched between a NbTiN ground and aluminium wiring layer fabricated on top of a 40 µm quartz substrate. The two designs is differed by how we tune out the unwanted junction capacitance for broadband performance. The first design uses the commonly-used twin-junction tuning scheme; whilst the second design utilises an end-loaded scheme. We successfully achieve close to 2× the double sideband quantum noise performance for both schemes, but the twin-junction design is less sensitive to fabrication accuracy of planar circuit components utilised. However, the end-loaded design offers a much better IF bandwidth performance, almost twice wider than the twin-junction design. The need for an ultra-wide IF bandwidth mixer is becoming more pressing and important for the future and up-coming upgrades of various millimetre (mm) and sub-mm astronomical instruments, hence we conclude that the end-loaded design is a better solution for the THz heterodyne mixing applications.

Abstract:

In this letter, we report the near quantum-limited performance of a 230 GHz endfire superconductor-insulator-superconductor (SIS) mixer utilizing a Nb/Al-AlOx/Nb trilayer. An important feature of this mixer is its use of a unilateral finline for the waveguide-to-planar circuit transition, which allows for a wide radiofrequency (RF) bandwidth, a simple waveguide structure with easy alignment, and for the mixer chip to be aligned along the optical axis. Each of these factors is beneficial in the construction of large-format focal plane arrays. We tested the new finline mixer from 210 to 260 GHz in a liquid helium cryostat at ∼ 4 K. The best recorded noise temperature was approximately twice the quantum limit, which is comparable to conventional radial probe mixers. This suggests that endfire SIS mixers can be used in large format arrays, comprising 100s or even 1000s of SIS mixing elements, while retaining state-of-the-art quantum mixing performance.

Abstract:

Background: Kinetic Inductance Travelling Wave Parametric Amplifiers (KITWPAs) are a new variant of superconducting amplifier that can potentially achieve high gain with quantum-limited noise performance over broad bandwidth, which is important for many ultra-sensitive experiments. In this paper, we present a novel modelling technique that can better capture the electromagnetic behaviour of a KITWPA without the translation symmetry assumption, allowing us to flexibly explore the use of more complex transmission line structures and better predict their performance.

Methods: In order to design a KITWPA with optimal performance, we investigate the use of different superconducting thin film materials, and compare their pros and cons in forming a high-gain low-loss medium feasible for amplification. We establish that if the film thickness can be controlled precisely, the material used has less impact on the performance of the device, as long as it is topologically defect-free and operating within its superconducting regime. With this insight, we propose the use of Titanium Nitride (TiN) film for our KITWPA as its critical temperature can be easily altered to suit our applications. We further investigate the topological effect of different commonly used superconducting transmission line structures with the TiN film, including the effect of various non-conducting materials required to form the amplifier.

Results: Both of these comprehensive studies led us to two configurations of the KITWPA: 1) A low-loss 100 nm thick TiN coplanar waveguide amplifier, and 2) A compact 50 nm TiN inverted microstrip amplifier. We utilise the novel modelling technique described in the first part of the paper to explore and investigate the optimal design and operational setup required to achieve high gain with the broadest bandwidth for both KITWPAs, including the effect of loss.

Conclusions: Finally, we conclude the paper with the actual layout and the predicted gain-bandwidth product of our KITWPAs.

Abstract:

Travelling wave parametric amplifiers (TWPAs) made from highly nonlinear reactive superconducting thin films have been demonstrated to be a potentially viable quantum-noise-limited amplifier technology for various fundamental physics platforms, including microwave/mm/sub-mm astronomy, dark matter search experiments, neutrino mass experiments and qubit readout. To date, only a limited number of successful kinetic inductance TWPA devices have been reported, with the majority fabricated from the same material, niobium titanium nitride (NbTiN), although in principle any highly nonlinear superconducting film can be used for kinetic inductance TWPA fabrication. Here, we present a detailed analysis of titanium nitride (TiN) transmission lines, to ascertain their suitability for use as kinetic inductance TWPAs. We will experimentally characterise our transmission line structures at cryogenic temperatures and compare the results with electromagnetic simulations. This characterisation and analysis would allow us to understand the advantages and limitations of TiN films, and whether they are suitable for applications as kinetic inductance TWPAs.

Abstract:

We report the development of an on-chip, compact dual-polarisation Superconductor-Insulator-Superconductor (SIS) receiver covering the millimetre frequency band from 190 GHz to 290 GHz. All the required components for the receiver are integrated on-chip using planar circuit technology, except the feedhorn. We use a 4-probe orthomode transducer (OMT) to couple the radio frequency (RF) and local oscillator (LO) and to split the incoming signal into two RF polarisation states before they are routed to the twin junction Nb/A10x/Nb mixers via a planar crossover, branch-line hybrids and bandpass filters. The entire receiver chip, only 4.0 mm by 4.1mm in size, is mounted in a split block where the feedhorn and the magnetic biasing are located on the front side, and the intermediate frequency (IF) and DC biasing connectors are on the rear side, leaving the four adjacent sides of the block unobscured for a 2-dimensional extension into a multi-pixel array. In this paper, we present the receiver and mixer block design and preliminary experimental results obtained from testing the receiver.