Universität zu Köln

Fathi, Sina (2018). Development of Integrated Superconducting Balanced Mixers for THz Focal Plane Arrays. PhD thesis, Universität zu Köln.

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Abstract

This thesis work covers the development of integrated superconducting balanced mixers for heterodyne (resolution> 10^7) THz focal plane arrays. The aim was to develop on chip integrated balanced superconductor-insulator-superconductor (SIS) mixers at 800-1100 GHz and in addition at the Intermediate Frequency (IF) 4-12 GHz integrated 180° IF hybrids both for the CHAI (CCAT Heterodyne Array Instrument) receiver for the Cerro Chajnantor Atacama Telescope (CCAT) observatory. A planar 180º hybrid ring coupler, two separate slotline antennas, two separate slotline to CPW transitions to employ for the 800-1100 GHz balanced SIS mixer, based on NbTiN circuitry, were designed and all were integrated on one 6 μm silicon substrate. At the IF side of the integrated balanced mixers, two different configurations of superconducting integrated 180º IF hybrids were designed to combine the two generated IF outputs at either Σ or Δ port depending on the choice of the input port of the hybrid. These 4-12 GHz IF hybrid coupler designs were based on planar lumped elements to miniaturize the size to about 0.5×2.2 μm^2. The designs were simulated for 500 μm and 9 μm silicon substrates. The latter is studied in order to integrate it at a later stage with the RF part of the mixer on one substrate. The lithography mask and subsequently the micro-fabrication of the IF hybrids were made. The measurement of the Nb based IF hybrid on a 500 μm silicon with Vector Network Analyzer (VNA) in liquid helium is reported. The measured results are analyzed and compared to the IF hybrid with the implemented superconducting Nb in the simulation. These developments and designs will be available and applicable for future receivers of the CCAT-prime observatory. Because the original envisioned CCAT-observatory project was discontinued due to the lack of funding, it was decided that my 800-1100 GHz designs would not be further developed into working mixers for the time being and that other projects would get priority. My work continued with the the development of integrated 1.9 THz balanced Hot Electron Bolometer (HEB) mixers. This development is relevant for 1.9 THz focal plane array receivers for the airborne Stratospheric Observatory for the Far Infrared (SOFIA), for which the mixers are developed in our group. The LO power consumption of the array presently limits its usable RF bandwidth. This power consumption could be reduced by a factor of 5 by using integrated balanced mixers. The mixers are based on superconducting NbN HEBs and Au is applied as normal metal to wire the mixers. The development contained design, micro-nanofabrication, DC and cryogenic high frequency measurements. All designed planar elements consisted of an 1 octave broadband 180º RF hybrid-ring coupler, two slotline tapered antennas, two slotline to CPW transitions, two or four 120 Ω HEB bridges, two low pass filters (LPFs) and two DC/IF blocking capacitors integrated on one 3 μm silicon substrate and simulated using CST suite. A further integrated 1.9 THz HEB mixer was designed to support the two separate IF outputs at one side of a 3 μm silicon substrate with employing a CPW-CPW crossover. The micro-nanofabrication of the 1.9 THz balanced mixers was done in house using E-beam and photo-lithography techniques. Almost all DC measured devices contained HEB bridges that were sufficiently similar in normal state resistance and transition temperature. The average resistance of the devices, in the one available wafer for measurements, was about 17% too low. The transition temperature of the devices was around 7.9 K, in comparison to 8.5 K for our state of the art single pixel mixers. The two machined 1.9 THz balanced waveguide blocks for testing of the devices were machined in the institute's in house workshop. I have tested the transmission of the blocks, and thereby the workmanship, with a THz time Domain Spectrometer, showed about 2.4 dB of loss per mm waveguide length at room temperature, for a split block waveguide made of the oxygen free copper tellurium alloy (CuTe). The cryogenic RF measurements showed a very similar LO-power coupling to both mixers in the balanced device, from both input ports of the mixer. By using an in house built Quantum Cascade Laser (QCL) LO as a local oscillator and a severely attenuated Virginia Diode multiplier chain (VDI) LO as a signal source I have shown proper balanced mixer behavior, thereby validating the design. Signal suppression up to 38 dB was shown by inverting the bias voltages on one of the mixers in the balanced device, which added a 180º phase change to the IF of one of the mixers.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Fathi, Sina sina.fathi.1984@gmail.com UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-97178
Date:	9 November 2018
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Physics > Institute of Physics I
Subjects:	Physics
Uncontrolled Keywords:	Keywords Language Integrated THz Mixer, Superconducting Detector, Balanced Mixer, Hot electron Bolometer, Terahertz, THz, RF, Superconducting-Insulator-Superconducting, SIS, Integrated Superconducting 180Integrated THz Mixer, Superconducting Detector, Balanced Mixer, Hot electron Bolometer, Terahertz, THz, RF, Superconducting-Insulator-Superconducting, SIS, Integrated Superconducting hybrid coupler, Focal Plane Array Receiver English
Date of oral exam:	22 February 2019
Referee:	Name Academic Title Stutzki, Jürgen Prof. Dr. Zilges, Andreas Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/9717