Development of Coherent Detector Technologies for Sub-Millimetre Wave Astronomy Observations PDF
Preview Development of Coherent Detector Technologies for Sub-Millimetre Wave Astronomy Observations
Springer Theses Recognizing Outstanding Ph.D. Research Boon-Kok Tan Development of Coherent Detector Technologies for Sub-Millimetre Wave Astronomy Observations Springer Theses Recognizing Outstanding Ph.D. Research Aims and Scope The series “Springer Theses” brings together a selection of the very best Ph.D. theses from around the world and across the physical sciences. Nominated and endorsed by two recognized specialists, each published volume has been selected foritsscientificexcellenceandthehighimpactofitscontentsforthepertinentfield of research. For greater accessibility to non-specialists, the published versions includeanextendedintroduction,aswellasaforewordbythestudent’ssupervisor explainingthespecialrelevanceoftheworkforthefield.Asawhole,theserieswill provide a valuable resource both for newcomers to the research fields described, and for other scientists seeking detailed background information on special questions. Finally, it provides an accredited documentation of the valuable contributions made by today’s younger generation of scientists. Theses are accepted into the series by invited nomination only and must fulfill all of the following criteria (cid:129) They must be written in good English. (cid:129) ThetopicshouldfallwithintheconfinesofChemistry,Physics,EarthSciences, Engineeringandrelatedinterdisciplinary fields such asMaterials,Nanoscience, Chemical Engineering, Complex Systems and Biophysics. (cid:129) The work reported in the thesis must represent a significant scientific advance. (cid:129) Ifthethesisincludespreviouslypublishedmaterial,permissiontoreproducethis must be gained from the respective copyright holder. (cid:129) They must have been examined and passed during the 12 months prior to nomination. (cid:129) Each thesis should include a foreword by the supervisor outlining the signifi- cance of its content. 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More information about this series at http://www.springer.com/series/8790 Boon-Kok Tan Development of Coherent Detector Technologies for Sub-Millimetre Wave Astronomy Observations Doctoral Thesis accepted by University of Oxford, Oxford, UK 123 Author Supervisor Dr. Boon-KokTan Prof. GhassanYassin Department ofPhysics (Astrophysics) Department ofPhysics (Astrophysics) University of Oxford University of Oxford Oxford Oxford UK UK ISSN 2190-5053 ISSN 2190-5061 (electronic) SpringerTheses ISBN978-3-319-19362-5 ISBN978-3-319-19363-2 (eBook) DOI 10.1007/978-3-319-19363-2 LibraryofCongressControlNumber:2015948770 SpringerChamHeidelbergNewYorkDordrechtLondon ©SpringerInternationalPublishingSwitzerland2016 Thisworkissubjecttocopyright.AllrightsarereservedbythePublisher,whetherthewholeorpart of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission orinformationstorageandretrieval,electronicadaptation,computersoftware,orbysimilarordissimilar methodologynowknownorhereafterdeveloped. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publicationdoesnotimply,evenintheabsenceofaspecificstatement,thatsuchnamesareexemptfrom therelevantprotectivelawsandregulationsandthereforefreeforgeneraluse. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authorsortheeditorsgiveawarranty,expressorimplied,withrespecttothematerialcontainedhereinor foranyerrorsoromissionsthatmayhavebeenmade. Printedonacid-freepaper SpringerInternationalPublishingAGSwitzerlandispartofSpringerScience+BusinessMedia (www.springer.com) ’ Supervisor s Foreword Sub-millimetre astronomy is the branch of astronomy that makes observations at wavelengths between microwave frequencies and the far infrared. Most existing sub-millimetretelescopes,however,observeatwavelengthsbetween1mmand200 μm.Animportantareaofsciencethatsub-millimetreastronomyseekstoinvestigate isthecomplexprocessofformationofstarsfromdensemolecularclouds.Oneway of carrying out this investigation is by the coherent detection of the intensity of spectral lines emitted by the chemical elements in the star forming areas. Measurements of the spectrum at various redshifts (hence at different frequencies) allow us to study the evolution of galaxies, or in other words, the history of the universe.Formany years, theseobservationshavebeen extremelydifficult fortwo reasons.Thefirstisthatthedevelopmentofsensitivereceiversatthosewavelengths is extremely hard. For example, the small size of wavelength requires very precise machining of receiver components such as feeds and optical windows. Also, the relatively high frequency requires a detector with extremely sharp non-linearity, whichcannotbeprovidedbyconventionalsolid-statedevices.Theseconddifficulty in sub-millimetre observations is the existence of water vapour in the atmosphere that has absorption lines at these wavelengths. This restricts ground-based sub-millimetre telescopes to be installed on high and dry sites, and even then, observations are only possible within frequency bands known as the atmospheric windows. For example, the largest sub-millimetre telescope ever built ALMA (Atacama Large Millimetre/sub-millimetre Array) observes in 10 frequency bands between 30–950 GHz. The single most important technological development in sub-millimetre recei- vers in recent years is undoubtedly the employment of the superconducting tun- nel junctions as coherent detectors in sub-millimetres receivers. The SIS (Superconductor-Insulator-Superconductor) consists of two small area (*1μm2) superconducting films (*400 nm thickness) sandwiching a very thin insulator (*20 Å). When this device is biased appropriately, an incident photon may have enough energy to break a cooper pair into normal electrons that can tunnel to the opposite superconductor through the thin insulator. By measuring the tunnelling v vi Supervisor’sForeword current, the intensity of the incident beam isobtained. In most cases, however, the tunneljunctionisusedasaheterodynereceiveroranSISmixer.Here,thedeviceis illuminated by two signals: The signal from the sky at a frequency v and a Local s Oscillator(LO)signalv generatedbythereceiver.Attheoutputofthemixer,we LO obtain a signal whose frequency is equal to the difference jν (cid:2)ν j. In other s LO words, the frequency of the signal is down-converted by the mixer to a value that canbehandledbytheelectronics.Theperformanceofamixerisdeterminedbyits conversionlossanditsnoisetemperaturewhichisameasure ofthenoiseaddedto thesignalbythemixingprocess.Modernmixersoperatingatfrequenciesbelowthe superconducting gap frequency (defined as ν ¼E =h where E is the supercon- g g g ductor energy gap and h is the Planck’s constant) achieve noise temperatures that approach the limit imposed by Heisenberg uncertainty principle. All modern sub-millimetre telescopes employ heterodyne receiver based on SIS mixer includ- ing ALMA and the space telescope Herschel. The design of SIS mixers is by no means straightforward, particularly at fre- quencies approaching 700 GHz or above. First, the high energy of the photon requires quantum treatment of mixing process as the classical mixer model is no longer valid and the analysis becomes even more complicated at frequencies comparable to the superconducting gap. Also, the power incident on the receiver will need to be coupled very efficiently to a device of tiny dimension which is fabricatedinaplanarcircuit.Theworkdescribedinthisthesisaimstocontributeto the design and the analysis of SIS mixers fabricated near the superconducting gap of Niobium which is approximately 680 GHz at 4 K. It will address the issues of howtocreateahighperformancemixerdesignthatisreliableasaresultofrigorous modelling,fullyintegratedforeaseandrepeatablefabricationanddoesn’trequirea complicated mixer block. Averyimportantachievementofthisworkistheabilitytoaccuratelymodelthe full mixer chip, including the SIS device and the superconducting transmission linesdepositedonthemixerchiptocouplepowerfromthewaveguidetothetunnel junctionandtotuneoutthecapacitanceofthedevice.Thisrequiressimulationsthat combinequantummixingsoftwarebasedonTucker’stheoryofquantumdetection with commercialelectromagneticsoftware,while rigorouslyincluding theeffectof superconductivityonthewavepropagation.Thisdesignprocedurewasextensively tested experimentally and the results were truly impressive. Not only that the simulations generated by the author were able to predict the behaviour of a well fabricated mixer but perhaps more importantly it was able to predict the perfor- mance when fabrication errors were made such as an offset in the tunnel junction location. This is an extremely important feature considering the complexity of SIS mixer chip fabrication. Using these simulations the author was able to design mixers with wide RF and IF ranges. An IF bandwidth of more than 10 GHz was demonstratedandupto20GHzwasshownpossible.Thisisaverypromisingresult considering that the sensitivity of the receiver is proportional to the IF bandwidth. The SIS mixer chip employed in this work employs a finline taper that couples the power from the waveguide to a slotline. The finline transition has several Supervisor’sForeword vii advantages including wide RF bandwidth, large substrate area for planar circuits and results in a simple mixer block design that does not need an accurately machined backshort. Conventionally, SIS mixer chips were fabricated on quartz substrates of approximately 60 micron thickness at 700 GHz. The mixer chips described in this thesis use the newly developed SOI (Silicon-on-Insulator) technology where the mixercircuitsaredepositedona15micronsiliconsubstrateandtheplanarcircuits terminatedbygoldbeam leadelectrodes. Thisfabricationtechnologydevelopedat KOSMA, University of Cologne has several crucial advantages such as light loading of the waveguide, easy grounding of the chip and removing parasitic inductancefromtheIFcircuit.Also,fabricatingSISmixersusingbeamleadsonan ultra-thin SOI substrate reduces the handling process significantly, and thus makes producingalargenumberofdetectorchipsthatareidenticalinasinglefabrication runfeasible.Sinceallthemixercircuitsandthefinlinetaperusedtofeedthemixer chip are fabricated on-chip, do not rely on complicated mechanical waveguide designs to support the operation of the mixer. The mixer chip described above is mounted at the E-plane of a rectangular waveguide which is fed by an electromagnetic horn. Modern high quality SIS mixers employ corrugated horn for high beam circularity, low cross polarisation and low sidelobes. The mixer described in this work pioneers the use of multiple flare-angled horn at these frequencies, avoiding the complexity of accurately machining corrugations at the throat of the horn without compromising the per- formance. The integrity of the feed horn performance was verified directly by measuringthebeampatterns,demonstratingexcellentperformanceacross100GHz which is sufficiently wide for typical astronomical window. I believe that the work in this thesis has made important contributions to the designofSISmixersandhasdevelopedkeytechnologiesthatmakethefabrication of SIS receivers substantially easier without compromising high performance. All proposed design simulations and new technologies were verified by extensive experimental testing. This will hopefully pave the way for the fabrication of large mixer arrays and extending the operation of SIS mixer above 1 THz. Oxford, UK Ghassan Yassin August 2015 Preface Thisthesisdescribes theworkscompleted intheduration of mydoctorate study at the Department of Physics (Astrophysics) of the University of Oxford for the fulfilment of the D.Phil. degree requirements. Following from the general discus- sionsaboutthemillimetreandthesub-millimetreastronomyinChap.1,Idescribea new type of feed horn in Chap. 2 that has high performance similar to the con- ventional corrugated horn, and yet is much easier to fabricate. This is followed by severalchaptersfocusingonSISmixers,startingwithChap.3whichdescribesthe fundamentals of SIS mixer theory and the experimental techniques used to assess the performance of the newly designed mixers. Chapter 4 presents the details of newSISmixerdesigns,whichisacentralthemeinthisthesis,whileChap.5reports the measured performance of these mixers together with performance analysis. Chapter 6 describes the design and measured performances of the single-chip balanced SIS mixers at 700 GHz integrated onto an SOI (Silicon-on-Insulator) substrate, and Chap. 7 describes the design of integrated single sideband mixers such as sideband separating and balanced with sideband separating SIS mixers. In Chap. 8, I present two interesting features of the SIS mixers, namely the effect of heating in SIS tunnel junctions by the tunnelling currents, and the effect of Josephson currents on the sensitivity of SIS mixers near 700 GHz. In Chap. 9, I discuss an alternative method of generating LO power using infrared lasers, and finally in Chap. 10, I present our work on astronomical observations of 12COJ=3→2emissionlinefromtwonearbygalaxies,NGC2976andNGC3351. Afterthecompletionofthethesis,Ihavecontinuedsomeoftheworkpresented in this thesis and run several more experiments and analysis. Hence, several chaptersinthecurrentversionofthethesishavebeenupdatedtoreflecttheprogress ofthesubjects.Themainbodyofthethesishoweverremainslargelysimilartothe version submitted to the University of Oxford for the D.Phil. degree, except for these additional materials. It is therefore no longer fully compliant to the length requirement of the University. ix x Preface During this post-doctoral period,Ihavealso beenengagedinseveralothernew topics such as the development of high frequency supra-THz mixers, the con- struction of superconducting phase switches and the development of multi-pixel SIS mixer arrays. Therefore, I would like to take this opportunity to update the reader with some recent developments, especially in the area of millimetre and sub-millimetre detector technologies. Recent Developments Since the submission of the thesis, the field of millimetre and sub-millimetre astronomical instrumentations has moved on considerably. Atacama Large Millimetre/sub-millimetre Array (ALMA) is now fully operational and is per- forming routine observations for the ALMA Early Science Cycle Two science cases.Ithassincethenproducedimpressiveresultswithunprecedentedsensitivity, albeitwithonlyfourreceiverbandsfullydeployedtoits34(outoftheplanned54) 12-m antennas and 9 (out of the planned 12) 7-m compact array antenna currently installed on the Chajnantor site. Further development works are still ongoing to install the remaining antennas, and to commission new receiver bands. There is strong interest within the ALMA scientific community to extend the capability of ALMA beyond 1 THz limit as well. Apart from the ALMA observatory, several other millimetre and sub-millimetre instruments have also entered their routine observationphasenow,suchastheairborneStratosphericObservatoryforInfrared Astronomy (SOFIA) observatory, the Balloon-borne Large Aperture Sub-millimeter Telescope (BLAST) and the Atacama Pathfinder EXperiment (APEX). A new millimetre and sub-millimetre observatory, the Green Land Telescope (GLT), has also been undertaken by the Academia Sinica Institute of AstronomyandAstrophysics(ASIAA),theSmithsonianAstrophysicalObservatory (SAO), the MIT Haystack Observatory and the National Radio Astronomy Observatory(NRAO),toperformstate-of-the-artTHzobservationatthepeakofthe Greenlandicesheet,complementingthescientificcapabilityoftheALMAarrayin the southern hemisphere. Apart from the ground-based and airborne experiments, the Herschel Space Telescope has now completed its mission and produced vast amount of state-of-the-artdata and results. The millimetre andsub-millimetre instrumentation community has recently been focusing their effort on constructing new space telescopes such as SPICA (Space Infra-Red Telescope for Cosmology and Astrophysicstelescope)andtheMillimetron.FIRSPEX(Far-InfraredSpectroscopic Explorer),asmallsatellitemissionhasalsobeenproposedjointlybytheEuropean Space Agency (ESA) and the Chinese Academy of Sciences (CAS) to perform an unbiased all sky spectroscopic survey between 0.69–1.9 THz with high spectral resolution. It consists of four heterodyne detection bands, targeting at the key molecular and atomic transitions near the 1 THz frequency range, and will be stationed along the Low Earth Orbit (LEO) when launched in 2021.
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