Dr. Boon Kok Tan

Abstract:

We present preliminary measured responses of a planar superconducting on/off switch centred at the 220 GHz. The superconducting switch, comprising three niobium nitride (NbN) bridges, deposited across the slotline section of a backto-back unilateral finline, made of a similar superconducting material. The transmission characteristics of the superconducting switch illuminated by a sub-millimetre source were measured using a superconductor-insulator-superconductor (SIS) chip as direct detector. The NbN bridges were switched from the superconducting state to the normal state by a bias current exceeding the critical current of the bridges. With this arrangement, we have measured a switching dynamic range of ∼ 13 dB at 245 GHz, demonstrating the successful operation of the multiple NbN bridges planar superconducting on/off switch.

Abstract:

The presence of spurious frequency tones emanating from a harmonically generated local oscillator (LO) signal can significantly degrade the performance of a SuperconductorInsulator-Superconductor (SIS) tunnel junction mixer [1]. The existence of harmonics is often revealed by distorted I-V curves obtained when pumping the SIS device with the LO. We have investigated this effect by using a commercially available source and correlating the photon step induced I-V curve structure with a direct frequency measurement made via a Fourier Transform Spectrometer (FTS) and a spectrum analyser. Our results suggest that intermixing frequency products are generated within the first, low frequency, stage of the LO chain, and these unwanted signals are converted to high frequency tones through subsequent stages of LO amplification and frequency up-conversion. The experimental work has been performed by using measurement facilities available at the University of Oxford and the Rutherford Appleton Laboratory (RAL), UK.

Abstract:

We present the design of planar superconducting on/off switch comprising a number of high normal resistance nano-bridges deposited across a the slot of a unilateral finline. We have simulated the performance of this device and have shown that it has a much larger dynamic range than a single nano-bridge fabricated from the same material (Niobium Nitride, NbN). The response of a single bridge device was measured either directly using a terahertz power meter or by using superconductor-insulator-superconductor (SIS) device as direct detector. In either method, we have demonstrated good agreement between simulations and measurements, and therefore confirmed the integrity of our analysis of the device performance. We have recently designed and fabricated multiple nano-bridges superconducting switches using 50 µm thick NbN film. The measurement of these devices is currently in progress and we expect to report the results in the forthcoming ISSTT conference in March.