Dr. Boon Kok Tan

Abstract:

Supra-THz heterodyne mixers generally have higher conversion loss compared to the millimetre-wave mixers. Hence, the overall receiver noise temperature becomes increasingly dominated by the first-stage semiconductor low noise amplifier (LNA), which still struggles to achieve quantum-limited noise performance. Here, we aim to develop a Josephson junction travelling wave parametric amplifier (JTWPA) that can achieve high gain over broad bandwidth but with better noise performance to replace these Intermediate Frequency (IF) amplifiers. JTWPAs are typically considered not suitable for astronomical receivers due to their low power handling capability. However, the critical current of the Josephson junctions (JJ) can be easily engineered to match the output power of the front-end detectors. Nevertheless, this may results in the requirement of a higher number of JJs as the junction inductance is in reverse relation with the critical current. Therefore, in this paper, we aim to explore the different design parameters required for developing a JTWPA with a dynamic range compatible for readout a Superconductor-Insulator-Superconductor (SIS) mixer, as an example. Here, we present two JTWPA models that are suitable for the objective, one requiring 3,142 Nb/Al-AlOx/Nb junctions with a maximum gain of 23 dB, and the other with a lower gain at 16 dB but requires only 1,317 JJs. We then compare the SIS receiver noise performance utilising these JTWPAs with that of using a conventional high gain High Electron Mobility Transistor (HEMT) amplifier. We show that we can improve the receiver sensitivity significantly by either cascading two 23 dB gain JTWPA or using a combination of a 16 dB gain JTWPA and a HEMT amplifier. We conclude that the former option more suitable for large detector array applications as it completely replaces the high heat dissipation HEMT amplifiers; while the latter option is favourable at this stage for low pixel count application as it is easier to fabricate a lower number of junctions JTWPA.

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.