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Universität zu Köln
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Mathematisch-Naturwissenschaftliche Fakultät
Fachgruppe Physik

I. Physikalisches Institut

Superconducting Devices and Mixers Group

Master thesis work in the superconducting detector group

Master thesis work  in the Superconducting Devices Group

 

1)  Cryogenic Time Domain Spectrometer Measurements of THz materials

A commercial TDS Spectrometer is expanded with a liquid helium dewar to do material measuremens at 4.2K. The set-up is still in its start up phase. Many new measurements need to be done, to which the set-up probably needs to be adapted in details. The work involves some technical work (assembling samples, filling dewars, aligning sources) but mostly  data interpretation, possibly some software development. You will learn basic lab skills including working with cryogenic liquids, optics and physics of Time Domain Spectroscopy. As a member of our group with various people very  interested in your results you will develop also your communication- and discussion skills.

 

2) Superconducting IF circuits for 0.5-10 GHz for balanced and 2SB mixers

We develop superconducting heterodyne mixers for frequencies from 300 GHz -10 THz. For the lower part of this frequency band we are currently designing balanced and side band separating(2SB) mixers. In a mixer the high frequency signal is mixed down to frequencies between 0.2-10 GHz, the intermediate frequency (IF). At the IF some additional circuitry, like 90° or 180° 3dB couplers or (Wilkinson) power combiners, is needed for balanced or 2SB  mixers. We are developing these circuits in superconducting technology to integrate them with the mixers at cryogenic temperatures. The circuits are fabricated in our in-house cleanroom. The work involves measurement ( also development of smart ways of measuring)  and analysis of existing circuits and design of new (optimzed) circuits using existing software. You will be supervised by PhD students that are developing the mixers, and so gain insight in the whole system and discuss your results in that framework.

 

Due to the (in itself very nice circumstance) that we have acquired a new, more extended, sputtering machine, which needs to be installed and taken into operation, this thesis work is on hold. Probable start would be in September 2017.

3) Development of SIS mixers with AlN tunnelbarriers for THz SIS mixers

To extend the frequency range of SIS mixers to 2 THz , superconducting thin films with a critical temperature higher than that of niobium have to be developed. To use these films in a SIS tunnel junction also new tunnel barriers fitting to these materials have to be developed. We have already developed films NbTiN, and Nb films with a Tc around 15K. A good candidate for a barrier material is AlN. We have already some experience with AlN barriers that is  promising. You will fabricate AlN barriers, on Nb to start with, with the new source, and optimize the quality of the barriers. The quality will be measured by doing DC measurements at 4.2 K ("dipstick") on Nb/AlN/Nb SIS tunnel junctions. You will learn: Basic steps in clean room fabrication techniques (sputtering, photolithography, etching) first principles of SIS device physics, DC cryogenic measurement techniques. Soft skills: team work (in clean room), discussion and communication skills.

Klaus Liebrecht price 2014

(Bild: © Patric Foud)

Marc Peter Westig received the Klaus Liebrecht dissertation price of the Faculty of Mathematics and Natural sciences of the University of Cologne for the year 2014. In his PhD thesis with the title "Quantum limited balanced superconducting 380-520 GHz mixer on a silicon membrane and mesoscopic tunnel devices for terahertz frequencies", he worked on new detector circuits for astronomical applications and on device physics aspects of superconductors out of thermal equilibrium. Key results of his thesis include the experimental proof of the first on-chip superconducting balanced mixer at 500 GHz with quantum limited performance and a new hybrid SIS junction technology, suitable for terahertz frequencies.

(fsc 2015-02-12)

Entwurf, Bau und Test eines Wilkinson Leistung-Kopplers 0.3-8 GHz für kryogene Anwendung

Durch geschicktes Aufteilen des astronomischen Messsignals (im THz-Bereich) auf zwei supraleitende Mischer und anschließende Rekombination des Zwischenfrequenz(ZF)-Signals (im GHz-Bereich) kann die Funktionalität und Leistungsfähigkeit des Systems drastisch gesteigert werden. Für die Rekombination des ZF-Signals kann ein Wilkinson-Koppler verwendet werden. Diese relativ einfache Schaltung für Mikrowellensignale kombiniert die beiden Signale mit gleicher Amplitude (-3dB=0.5) und gleicher Phase und kann für einen sehr breiten Frequenzbereich (0.3-8GHz) berechnet werden. Die Schaltung soll mit Hilfe eines Mikrowellen CAD Programms erstellt werden. Bei der Fertigung soll darauf geachtet werden, dass die Schaltung auch bei sehr tiefen Temperaturen optimal funktionieren kann.

(Betreuer: K. Jacobs / N. Honingh)

Entwurf, Bau und Test eines „-3dB 90° Hybrids“ mit 6GHz Mittenfrequenz für kryogene Anwendungen

Durch geschicktes Aufteilen des astronomischen Messsignals (im THz-Bereich) auf zwei supraleitende Mischer und anscstarthließende Rekombination des Zwischenfrequenz(ZF)-Signals (im GHz-Bereich) kann die Funktionalität und Leistungsfähigkeit des Systems drastisch gesteigert werden. Für die Rekombination des ZF-Signals kann ein „-3dB 90° Hybrid“ verwendet werden. Diese relativ einfache Schaltung für Mikrowellensignale kombiniert die beiden Signale mit gleicher Amplitude (-3dB=0.5) und einer Phasendifferenz von 90°. Die Schaltung soll mit Hilfe eines Mikrowellen CAD Programms erstellt werden. Bei der Fertigung soll darauf geachtet werden, dass die Schaltung auch bei sehr tiefen Temperaturen optimal funktionieren kann.

(Betreuer: K. Jacobs / N. Honingh)