Dr. Boon Kok Tan

Abstract:

In this paper, we present a method to measure the effective physical temperature of the tunnel junction in a terahertz superconductor-insulator-superconductor (SIS) mixer when illuminated with a strong local oscillator (LO) power that results in local hot spot around the tunnel junction. We observed that the gap voltage of the pumped current-voltage (IV) curve is suppressed when the LO pumping level is increased, indicating that the junction physical temperature is increased beyond the mixer block temperature. We quantified this extra heating effect by recovering the effective junction temperature through comparing the gap voltage of the pumped IV curves measured at a fixed block temperature, with the unpumped IV curve measured at varying block temperatures. We found that the heat trapped in the tunnel junction can be as high as 1.7 K when the mixer stabilised at 3.3 K is pumped at only 21% of the gap current.

Abstract:

We present a new approach to develop a Josephson Junction Travelling Wave Parametric Amplifier (JTWPA) that could potentially minimise the gain-ripple effect. Our design consists of a 50 Ω superconducting niobium coplanar waveguide (CPW) periodically loaded with series of Josephson junctions (JJs) to provide the non-linearity required for wave mixing. The embedded JJs alter the characteristic impedance of the transmission line abruptly, therefore creating the stopbands in the transmission (S21) for the suppression of higher harmonics and provide the means for dispersion engineering required to achieve exponential gain. The simulated gain profile of the amplifier shows that we can obtain a minimum of 15 dB gain from 4–12 GHz, close to a 100% bandwidth performance. More importantly, the characteristic impedance of the main linear transmission line remains unaffected by the strong pump, therefore ensuring the TWPA remains impedance matched to the input and output ports. This can reduce the unwanted gain undulation that inflict the optimal performance of the TWPA.

Abstract:

We describe the performance of a superconductor- insulator-superconductor (SIS) mixer operating in the frequency range of 780-950 GHz. Unlike most SIS mixers, the tunnel junction employs two different superconductors, a niobium nitride top and a niobium bottom electrode sandwiching an aluminum nitride barrier layer, fabricated on a niobium titanium nitride ground plane. The mixer was tested in a pulse tube cryostat, with all the optical components, in the signal path, mounted inside the vacuum environment to avoid attenuation of the RF signal as it propagates from the hot/cold loads to the mixer. With this setup, we have measured an RF-corrected noise temperature of ~220 K. In this article, we focus on investigating the influence of local oscillator (LO) power heating on the performance of the terahertz mixer. The increase in the junction's physical temperature can be observed experimentally by noting the suppression of the gap voltage in the pumped current-voltage (I-V ) curve as the LO pumping level is increased. Similar observation has already been reported, and attempts were made to estimate the effective temperature of the device using equations of heat transfer between the mixer chip layers. Here, we present an experimental method of quantifying this effect by recovering the effective temperature of the junction through comparing the pumped I-V curves at different pumping levels and fixed bath temperature, with the unpumped I-V curves obtained at varying bath temperatures. We also estimate, for the first time, the effect of heating on the noise temperature as a function of bath temperature and frequency. We show that for typical experimental parameters, the LO heating can increase the double-sideband receiver noise temperature by as much as 20%, and that in the frequency range of the measurements, the effective temperature of the junction at fixed LO power increases linearly with frequency at a rate of 0.5 K/100 GHz.

Abstract:

We present a new concept for constructing a compact linear 1 × n multi-beam phased array transceiver system that can be scaled to operate from radio to millimetre wavelength. In this design, all the components required to form the phased array are fabricated on a single printed circuit board, where both the input antenna array and the readout arrays are integrated on the same platform. This is made possible by using a novel planar power splitter/combiners technology, which allows the input signal from each antenna to be split simultaneously through these components arranged in horizontal, and re-combined vertically to form individual beam. This compact single-platform phased array system could be important for many applications that required compact and light-weight design such as 4G/5G wireless telecommunications, inter-satellite (CubeSat) or satellite-base station communication links, space-based remote sensing, vehicle transceiver system and astronomical receivers.

Abstract:

We present a new software package, called QMix, for simulating the behavior of superconductor/insulator/ superconductor (SIS) mixers. The software uses a harmonic balance procedure to calculate the ac voltage across the SIS junction and multitone spectral domain analysis to calculate the quasiparticle tunneling currents. This approach has two major advantages over other simulation techniques: 1) it can include an arbitrary number of higher order harmonics, and 2) it can simulate the effect of multiple strong nonharmonic frequencies. This allows the QMix software to simulate a wide range of SIS mixer operation, including the effects of harmonics in the local-oscillator (LO) signal and gain saturation with respect to the RF signal power. In this article, we compare the simulated results from QMix to the measured performance of a 230-GHz SIS device, both to validate the software and to investigate the experimental data. To begin, we simulated the conversion gain of the mixer and we found excellent agreement with the experimental results. We were also able to recreate the broken photon step effect by adding higher and lower order harmonics to the LO signal. We then simulated a range of incident RF signal powers in order to calculate the gain saturation point. This is an important metric for SIS mixers because we require linear gain for reliable measurements and calibration. In the simulated results, we found both gain compression and gain expansion, which is consistent with other studies. Overall, these examples demonstrate that QMix is a powerful software package that will allow researchers to simulate the performance of SIS mixers, investigate experimental results, and optimize mixer operation. We have made all of the QMix software open-source and we invite others to contribute to the project.