The Far Infrared Spectroscopic Explorer (FIRSPEX) is a novel European-led astronomy mission concept developed to enable large area ultra high spectroscopic resolution surveys in the THz regime. FIRSPEX opens up a relatively unexplored spectral and spatial parameter space that will produce an enormously significant scientific legacy by focusing on the properties of the multi-phase ISM, the assembly of molecular clouds in our Galaxy and the onset of star formation; topics which are fundamental to our understanding of galaxy evolution. The mission uses a heterodyne instrument and a ~1.2 m primary antenna to scan large areas of the sky in a number of discreet spectroscopic channels from L2. The FIRSPEX bands centered at [CI] 809 GHz, [NII]1460 GHz, [CII]1900 GHz and [OI]4700 GHz have been carefully selected to target key atomic and ionic fine structure transitions difficult or impossible to access from the ground but fundamental to the study of the multi-phase ISM in the Universe. The need for state-of-the-art sensitivity dictates the use of superconducting mixers configured either as tunnel junctions or hot electron bolometers. This technology requires cooling to low temperatures, approaching 4K, in order to operate. The receivers will operate in double sideband configuration providing a total of 7 pixels on the sky. FIRSPEX will operate from L2 in both survey and pointed mode enabling velocity resolved spectroscopy of large areas of sky as well as targeted observations.
We investigate a new concept, where a single superconductor-insulator-superconductor (SIS) based mixer chip is switched between two RF frequency bands. A single broadband antenna is used to couple the RF/LO signal to two SIS mixers via a power splitter and two superconducting on/off switches, forming a switching circuit. The planar on/off switches comprise a superconducting microstrip bridging two transmission lines used to alternate the RF/LO signal between the two branches of the power splitter circuit by switching the impedance of the microstrip from the superconducting to normal state, and vice versa. An important application of this dual-band design is to enable combination of adjacent observing astronomical windows into a single receiver cartridge, freeing valuable space in the receiver cabin.
Ultra-sensitive superconducting tunnel junction heterodyne receivers used for astronomy research require relatively low levels of local oscillator (LO) power. When configured as an imaging array, however, the LO power required substantially increases and the provision and distribution of a harmonically generated LO signal to multiple pixel elements becomes a technically challenging task. Furthermore, the difficulty of generating LO power is compounded as the operational frequency is increased into the supra-THz region (<1 THz). We will present our programme of research directed towards the provision of future THz astronomy receivers, in which we have been pursuing the development of enhanced harmonic up-conversion LO technology.
The Greenland Telescope project will deploy and operate a 12m sub-millimeter telescope at the highest point of the Greenland i e sheet. The Greenland Telescope project is a joint venture between the Smithsonian As- trophysical Observatory (SAO) and the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA). In this paper we discuss the concepts, specifications, and science goals of the instruments being developed for single-dish observations with the Greenland Telescope, and the coupling optics required to couple both them and the mm-VLBI receivers to antenna. The project will outfit the ALMA North America prototype antenna for Arctic operations and deploy it to Summit Station,1 a NSF operated Arctic station at 3,100m above MSL on the Greenland I e Sheet. This site is exceptionally dry, and promises to be an excellent site for sub-millimeter astronomical observations. The main science goal of the Greenland Telescope is to carry out millimeter VLBI observations alongside other telescopes in Europe and the Americas, with the aim of resolving the event horizon of the super-massive black hole at the enter of M87. The Greenland Telescope will also be outfitted for single-dish observations from the millimeter-wave to Tera-hertz bands. In this paper we will discuss the proposed instruments that are currently in development for the Greenland Telescope - 350 GHz and 650 GHz heterodyne array receivers; 1.4 THz HEB array receivers and a W-band bolometric spectrometer. SAO is leading the development of two heterodyne array instruments for the Greenland Telescope, a 48- pixel, 325-375 GHz SIS array receiver, and a 4 pixel, 1.4 THz HEB array receiver. A key science goal for these instruments is the mapping of ortho and para H2D+ in old protostellar ores, as well as general mapping of CO and other transitions in molecular louds. An 8-pixel prototype module for the 350 GHz array is currently being built for laboratory and operational testing on the Greenland Telescope. Arizona State University are developing a 650 GHz 256 pixel SIS array receiver based on the KAPPa SIS mixer array technology and ASIAA are developing 1.4 THz HEB single pixel and array receivers. The University of Cambridge and SAO are collaborating on the development of the CAMbridge Emission Line Surveyor (CAMELS), a W-band `on- hip' spectrometer instrument with a spectral resolution of R ~ 3000. CAMELS will consist of two pairs of horn antennas, feeding super conducting niobium nitride filter banks read by tantalum based Kinetic Inductance Detectors.
The Cold-Electron Bolometer (CEB) is a sensitive millimetre-wave detector which is easy to integrate with superconducting
planar circuits. CEB detectors have other important features such as high saturation power and very fast response. We
have fabricated and tested CEB detectors integrated across the slot of a unilateral finline on a silicon substrate. Bolometers
were fabricated using two fabrication methods: e-beam direct-write trilayer technology and an advanced shadow mask
evaporation technique. The CEB performance was tested in a He<sup>3</sup> sorption cryostat at a bath temperature of 280mK. DC
I-V curves and temperature responses were measured in a current bias mode, and preliminary measurements of the optical
response were made using an IMPATT diode operating at 110GHz. These tests were conducted by coupling power directly
into the finline chip, without the use of waveguide or feedhorns. For the devices fabricated in standard direct-write technology,
the bolometer dark electrical noise equivalent power is estimated to be about 5×10<sup>-16</sup>W/√Hz, while the dark
NEP value for the shadow mask evaporation technique devices is estimated to be as low as 3×10<sup>-17</sup>W/√Hz.
We present the design of a broadband superconductor-insulator-superconductor (SIS) mixer operating near
700 GHz. A key feature of our design is the utilisation of a new type of waveguide to planar circuit transition
comprising a unilateral finline taper. This transition is markedly easier to design, simulate and fabricate than the
antipodal finline we employed previously. The finline taper and the superconducting circuitry are deposited on
a 15 μm thick silicon substrate. The employment of the very thin substrate, achieved using Silicon-On-Insulator
(SOI) technology, makes it easy to match the incoming signal to the loaded waveguide. The lightweight mixer chip
is held in the E-plane of the waveguide using gold beam leads, avoiding the need for deep grooves in the waveguide
wall. This new design yields a significantly shorter chip, free of serrations and a wider RF bandwidth. Since
tuning and all other circuits are integrated on the mixer chip, the mixer block is extremely simple, comprising
a feed horn and a waveguide section without any complicated mechanical features. We employ a new type
of smooth-walled horn which exhibits excellent beam circularity and low cross polarisation, comparable to the
conventional corrugated horn, and yet is easier to fabricate. The horn is machined by standard milling with
a drill tool shaped into the horn profile. In this paper, we describe the detailed design of the mixer chip
including electromagnetic simulations, and the mixer performance obtained with SuperMix simulations. We also
present the preliminary measurements of the smooth-walled horn radiation patterns near the mixer operating
Finlines are planar structures which allow broadband and low loss transition from waveguide to planar circuits.
Their planar structure and large substrate makes them ideal for integration with other planar circuits and
components, allowing the development of an on chip polarimeter. We have developed a method of extending the
employment of finlines to thick substrates with high dielectric constants by drilling or etching small holes into
the substrate, lowering the effective dielectric constant. We present the results of scale model measurements at
15GHz and cryogenic measurements at 90GHz which illustrate the excellent performance of finline transitions
with porous substrates and the suitability of this technique for extending the bandwidth of finline transitions.
CℓOVER is a multi-frequency experiment optimised to measure
the Cosmic Microwave Background (CMB) polarization, in
particular the B-mode component. CℓOVER comprises two
instruments observing respectively at 97 GHz and 150/225 GHz.
The focal plane of both instruments consists of an array of
corrugated feed-horns coupled to TES detectors cooled at 100
mK. The primary science goal of CℓOVER is to be sensitive to
gravitational waves down to r ~ 0.03 (at 3σ)in two years of operations.
We have fabricated TES bolometers with finline transitions for the CℓOVER project. We have measured the
optical response of CℓOVER's first prototype 97-GHz detectors and find that they have a detection efficiency
close to 100%. We have also investigated the effects of misalignment of the finline in the waveguide and of
thinning the substrate. The prototype detectors have dark NEPs as low as 1.5 x 10<sup>-17</sup>W/√Hz and satisfy
the requirement of photon-noise limited operation on CℓOVER. We describe the optical tests of CℓOVER's
prototype 97-GHz detectors and discuss their implications for the design of the science-grade detectors.
Microstrip-coupled Transition Edge Sensors (TESs) are important because they can be combined with waveguide-horn
technology to produce sensitive bolometric detectors with well-defined, single-mode beam patterns and
polarisation characteristics. They also allow superconducting RF filters to be included on the detector chips.
Our own design of TES uses a finline taper to transform between waveguide and superconducting Nb microstrip.
The microstrip transports the signal to a matched Au-Cu resistor, which is deposited on a thermally isolated SiN
membrane. The dissipated RF power causes the resistance of a Mo-Cu TES bilayer to increase, and the resulting
reduction in bias current is read out by a SQUID. We have fabricated TES bilayers with critical temperatures
of 400 to 600mK, and deduced dark NEPs as low as 3x10<sup>-17</sup>W/√Hz at 150GHz. In this paper we describe a
number of experiments that were carried out in order to investigate the electrothermal behaviour of microstrip-coupled
TESs. We show that the electrothermal behaviour of microstrip-coupled TESs can be as good as that
of free-space TESs, and therefore that they are suitable for high-performance astronomical applications.
Several technologies are now being considered for modulating the polarization in various B-mode instruments, including rotating quasioptical half-wave plates in front of the focal plane array, rotating waveguide half-wave plates and Faraday rotators. It is not at all clear that any of these techniques is feasible without heavy penalty in cost or performance. A potentially much more efficient method is to use a pseudo-correlation polarimeter in conjunction with a planar circuit phase switch.
We investigate three different devices for use as mm-wave switches, SIS tunnel junctions, capacitively coupled superconducting nanostrips and RF MEMS. The SIS tunnel junction switches operate by switching between two different bias voltages, while the nanostrip switch operates by changing the impedance of a resonant circuit by driving the nanostrip from the superconducting to normal state. In each case the RF signal sees two substantially different complex impedance states, hence could be switched from one transmission line branch to another. In MEMS this is achieved by mechanical movement of one plate of a parallel plate capacitor system. Although RF MEMS have been reported at high microwave and low mm-wave frequencies, in this work we have investigated cryogenic MEMS for operation at high mm-wave frequencies (225 GHz) using superconducting transmission lines.
We present and compare designs and simulations of the performance of phase switches based on all three switching technologies, as well as preliminary experimental results for each of the switches. Finally we also present designs of phase shift circuits that translates the on/off switching into phase modulation.
C<sub>ℓ</sub>OVER is an experiment which aims to detect the signature of gravitational waves from inflation by measuring
the B-mode polarization of the cosmic microwave background. C<sub>ℓ</sub>OVER consists of three telescopes operating
at 97, 150, and 220 GHz. The 97-GHz telescope has 160 horns in its focal plane while the 150 and 220-GHz
telescopes have 256 horns each. The horns are arranged in a hexagonal array and feed a polarimeter which
uses finline-coupled TES bolometers as detectors. To detect the two polarizations the 97-GHz telescope has 320 detectors while the 150 and 220-GHz telescopes have 512 detectors each. To achieve the required NEPs the
detectors are cooled to 100 mK for the 97 and 150-GHz polarimeters and 230 mK for the 220-GHz polarimeter.
Each detector is fabricated as a single chip to guarantee fully functioning focal planes. The detectors are
contained in linear modules made of copper which form split-block waveguides. The detector modules contain
16 or 20 detectors each for compatibility with the hexagonal arrays of horns in the telescopes' focal planes. Each
detector module contains a time-division SQUID multiplexer to read out the detectors. Further amplification of
the multiplexed signals is provided by SQUID series arrays. The first prototype detectors for C<sub>ℓ</sub>OVER operate
with a bath temperature of 230 mK and are used to validate the detector design as well as the polarimeter
technology. We describe the design of the C<sub>ℓ</sub>OVER detectors, detector blocks, and readout, and give an update
on the detector development.
In this paper we present a novel design of an antenna coupled TES
direct detector for high performance applications. In particular, the
design of the detector has been optimised to be suitable for the
measurement of the weak B-mode signal in the CMB polarization. An
important feature of this design is that it employs corrugated horn
antennas for coupling the astronomical signal to the detector. This
allows us to feed the telescope with a well collimated beam with low
sidelobes and cross polarization. The paper contains simulations
demonstrating the suitability of individual electromagnetic components
to be used in the instrument.
The Quantum Theory of Mixing developed by Tucker provides a solid framework for understanding the behaviour of SIS mixers, and subsequent developments allow the simulation of complete mixer circuits. These methods operate, however, under the assumption of small signal levels, and so neglect the non-linear behaviour of the signal path. The non-linearity of the mixer's response to applied signals is of vital importance to the calibration of SIS receiver systems. We have previously reported a procedure for calculating the full quantum behaviour of tunnel junction circuits under multiple high-level signals, allowing the accurate prediction of the saturation characteristics of SIS mixers. In this paper, we apply our procedure to both an idealized SIS mixer and one of our previously tested 700 GHz finline mixers. We find that the small signal behaviour predicted by our procedure agrees well with other simulation methods, and that the saturation properties of both of these mixers differ from that predicted by previous estimates of saturation behaviour.
We describe a procedure for modeling the optical behaviour of planar, bolometric imaging arrays. Arrays of this kind are being developed for the next generation of ground-based and space-borne, submillimetre-wave and far-infrared, astronomical telescopes. A unique feature of the scheme is that the partially coherent vector fields associated with the individual pixels are traced through the optical system simultaneously. Simultaneous tracing is achieved by propagating the second-order statistical properties of the total field. In the paper, we describe the theoretical basis of our method, and present the results of a number of illustrative simulations.
We report the successful operation of a 700 GHz SIS finline mixer employing a Nb tunnel junction and Nb transmission lines. In particular, we discuss the properties of a new mixer feed and the influence of tuning on the mixer performance. Experimental and simulation work shows that the performance of the mixer below the superconducting gap is strongly dependent on the electrical properties of the tuning stub, while at frequencies above the gap the mixer performance is dominated by both tuning and transmission line losses.