Development of Coherent Detector Technologies for Sub-Millimetre Wave Astronomy Observations

235 Pages·2016·14.9 MB·English

by  Boon Kok Tan (auth.)

Springer Theses
Recognizing Outstanding Ph.D. Research
Boon-Kok Tan

Development of
Coherent Detector
Technologies for
Sub-Millimetre Wave
Astronomy Observations
Springer Theses
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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
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’
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.