PhD ceremony: J.W. Kooi, 16.15 uur, Academiegebouw, Broerstraat 5, Groningen
Thesis: Advanced r
eceivers for submillimeter and far infrared astronomy
Promotor(s): prof. W. Wild, prof. T.M. Klapwijk
Faculty: Mathematics and Natural Sciences
Atomic and molecular line astrophysics is demanding enhanced capabilities in terms of frequency coverage, spectral resolution, bandwidth, sensitivity, stability (quality of the data products), frequency agility, and ease of use. To help fulfill these demanding requirements, this thesis investigates advanced heterodyne receiver techniques in the submillimeter and terahertz frequency regimes. This thesis begins with a brief introduction to the field of submillimeter and terahertz astronomy. From there it proceeds to outline the submillimeter detection and operational requirements in Chapter 2. This is an important Chapter as boundary conditions are defined for the astronomical instrumentation we concern ourselves with. Once the operational requirements are established a variety of receiver configurations are introduced in Chapter 3. In particular we touch upon the concepts and implementations of the double sideband (DSB) single-ended receiver, the balanced receiver, the correlation receiver, and the sideband separating receiver. At its core, frequency down-conversion or heterodyning is accomplished with an highly non-linear mixing element. In the submillimeter, below 1000 GHz, SIS tunnel junctions are now commonly employed thanks to their superior conversion gain and sensitivity. In the terahertz regime, at frequencies beyond 1000 GHz, hot-electron bolometer mixers are at the present time the element of choice. The mixing process in both mixers is fundamentally very different. In Chapter 4 we address the physics of each device, and look in detail into a number of device related peculiarities. With the theory covered, the focus shifts in subsequent Chapters to a variety of realized receiver designs in the laboratory and on the telescope. We address in Chapter 5 an all NbTiN film quasi-optical mixer that operates in the 800 – 920 GHz atmospheric window. As a result of our work, low loss and high energy gap NbTiN superconducting films have been demonstrated, and since then been incorporated in the Heterodyne Instrument for the Far-infrared (HIFI) mixers bands 3, 4, and 5. Also in Chapter 5 we explore for the first time the use of an AlN tunnel barrier. The results were so impressive that this technology is currently being baselined at institutions like JPL, TU Delft, the University of Virginia, and the University of K¨oln. In Chapter 6, our research leads the way to addressing such important issues as the IF bandwidth and mixer gain of (NbN) hot electron bolometers. Then in Chapter 7 a recently installed (at the Caltech Submillimeter Observatory (CSO)) double sideband high-current density AlN-barrier SIS Technology development receiver (Trex) is introduced. Trex was constructed to prove many of the underlying technologies required in advanced receiver designs (Chapter 8). At the present time Trex serves as a facility heterodyne instrument for high spectral resolution wide bandwidth observations, and extended observations with the Harvard Smithsonian submillimeter array (SMA). In the first part of Chapter 8 we look in detail at the theory, characteristics, and implementation of balanced receivers, designed to cover the important 180 – 720 GHz atmospheric window(s). The second part of Chapter 8 discusses a correlation, or continuous comparison, receiver to operate between 280 – 420 GHz (covers two atmospheric windows with one SIS mixer). Finally, the last Section of Chapter 8 concerns itself with a 600 – 720 GHz (ALMA band 9) sideband separating receiver. Theory, implementation, and measurement results are presented. This is the first time a sideband separating receiver has been implemented at such a high frequency. In our treatment, instantaneous signal and IF bandwidth, sensitivity, and sideband ratio are important considerations. An important, so far un-discussed, receiver concept is the heterodyne focal plane array. Future astronomical developments will increasingly demand the mapping speed enhancement afforded by multi-pixel focal plane arrays. This thesis devotes Chapter 9 towards integrated arrays receivers with actual realized devices and measurement results. In all of the above outlined receiver designs, instrument stability is of primary importance. It affects the observed baseline quality and integration efficiency. Indeed, there are many ways in which the stability of the receiver may be compromised. The theory and actual measurement results are presented in Chapter 10.
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