LiteBIRD is a candidate for JAXA’s strategic large mission to observe the cosmic microwave background (CMB) polarization over the full sky at large angular scales. It is planned to be launched in the 2020s with an H3 launch vehicle for three years of observations at a Sun-Earth Lagrangian point (L2). The concept design has been studied by researchers from Japan, U.S., Canada and Europe during the ISAS Phase-A1. Large scale measurements of the CMB B-mode polarization are known as the best probe to detect primordial gravitational waves. The goal of LiteBIRD is to measure the tensor-to-scalar ratio (r) with precision of r < 0:001. A 3-year full sky survey will be carried out with a low frequency (34 - 161 GHz) telescope (LFT) and a high frequency (89 - 448 GHz) telescope (HFT), which achieve a sensitivity of 2.5 μK-arcmin with an angular resolution 30 arcminutes around 100 GHz. The concept design of LiteBIRD system, payload module (PLM), cryo-structure, LFT and verification plan is described in this paper.
SAFARI is one of the focal-plane instruments for the European/ Japanese far-IR SPICA mission proposed for the ESA M5 selection. It is based on three arrays with in total 3550 TES-based bolometers with noise-equivalent powers (NEP) of 2∙10-19 W/Hz. The arrays are operated in three wavelength bands: S-band for 30-60 µm, M-band for 60-110 µm and L-band for 110-210 µm, and have high optical efficiency. SRON is developing Frequency Domain Multiplexing (FDM) for readout of large AC biased TES arrays for both the SAFARI instrument, and the XIFU instrument on the X-ray Athena mission. In FDM for SAFARI, the TES bolometers are AC biased and read out using 24 channels. Each channel contains 160 pixels of which the resonance frequencies are defined by in-house developed cryogenic lithographic LC filters. FDM is based on the amplitude modulation of a carrier signal, which also provides the AC voltage bias, with the signal detected by the TES. To overcome the dynamic range limitations of the SQUID pre-amplifier, baseband feedback (BBFB) is applied. BBFB attempts to cancel the error signal in the sum-point, at the input coil of the SQUID, by feeding back a remodulated signal to the sum-point, and therefore improving the dynamic range of the SQUID pre-amplifier.
Previously we have reported on a detailed study of the effects of electrical crosstalk using our first iteration of a prototype of the full 160 pixel FDM experiment and the successful readout of 132 pixels using our 176 pixel FDM system. After the demonstration it is important to perform more detailed measurements to consolidate the system. For instance, one of the important next steps is to expose the FDM system to an optical infrared source. The cold part of the FDM system consists of a detector chip with 176 pixels with a designed NEP of 7∙10-19 W/Hz and two matching LC filter chips, each of which contains 88 carefully placed high-Q resonators, with a total of 176 different resonance frequencies, and a single-stage SQUID. The warm electronics consist of a low-noise amplifier (LNA) and a digital board on which the generation of the bias carriers, the demodulation of the signal and remodulation of the feedback signal are performed. The optical experiment will be conducted in a Leiden Cryogenics dilution refrigerator with a cooling power of about 200µW at 100 mK. This system contains multiple optical sources. These include a conical black body radiator which can be operated in the range of 3-34K and a light pipe through which the experiment can be illuminated from outside the cryostat. Dark measurements are conducted in a Janis ADR system with a base temperature of 50mK.
In this paper we describe the experimental tests and results of the more detailed testing of our 176 pixel TES bolometer system.
GUSTO will be a NASA balloon borne terahertz observatory to be launched from Antarctica in late 2021 for a flight duration of 100-170 days. It aims at reviewing the life cycle of interstellar medium of our galaxy by simultaneously mapping the three brightest interstellar cooling lines: [OI] at 4.7 THz, [CII] at 1.9 THz, and [NII] at 1.4 THz; along the 124 degrees of the galactic plane and through a part of the Large Magellanic Cloud. It will use three arrays of 4x2 mixers based on NbN hot electron bolometers (HEBs), which are currently the most sensitive mixers for high resolution spectroscopic astronomy at these frequencies.
Here we report on the design of a novel 4.7 THz receiver for GUSTO. The receiver consists mainly of two subsystems: a 4×2 HEB quasi-optical mixer array and a 4.7 THz multi-beam LO. We describe the mixer array, which is designed as a compact monolithic unit. We show, for example, 10 potential HEB detectors with the state of the art sensitivity of 720 K measured at 2.5 THz. They have a small variation in sensitivity, being less than 3%, while also meet the LO uniformity requirements. For the multi-beam LO we demonstrate the combination of a phase grating and a single QCL at 4.7 THz, which generates 8 sub-LO beams, where the phase grating shows an efficiency of 75%. A preliminary concept for the integrated LO unit, including QCL, phase grating and beam matching optics is presented.
We give an overview of the baseline detector system for SAFARI, the prime focal-plane instrument on board the proposed space infrared observatory, SPICA. SAFARI's detectors are based on superconducting Transition Edge Sensors (TES) to provide the extreme sensitivity (dark NEP≤2×10-19 W/√Hz) needed to take advantage of SPICA's cold (<8 K) telescope. In order to read out the total of ~3500 detectors we use frequency domain multiplexing (FDM) with baseband feedback. In each multiplexing channel, a two-stage SQUID preamplifier reads out 160 detectors. We describe the detector system and discuss some of the considerations that informed its design.
SRON is developing X-ray transition edge sensor (TES) calorimeters arrays, as a backup technology for X-IFU instrument on the ATHENA space observatory. These detectors are based on a superconducting TiAu bilayer TES with critical temperature of 100 mK on a 1 μm thick SiN membrane with Au or Au/Bi absorbers. Number of devices have been fabricated and measured using a Frequency Division Multiplexing (FDM) readout system with 1-5 MHz bias frequencies. We measured IV curves, critical temperature, thermal conductance, noise and also X-ray energy resolution at number of selected bias points. So far our best calorimeter shows 3.9 eV X-ray resolution at 6 keV. Here we present a summary of our results and the latest status of development of X-ray calorimeters at SRON.
Transition Edge Sensors are ultra-sensitive superconducting detectors with applications in many areas of research, including astrophysics. The device consists of a superconducting thin film, often with additional normal metal features, held close to its transition temperature and connected to two superconducting leads of a higher transition temperature. There is currently no way to reliably assess the performance of a particular device geometry or material composition without making and testing the device. We have developed a proximity effect model based on the Usadel equations to predict the effects of device geometry and material composition on sensor performance. The model is successful in reproducing I −V curves for two devices currently under study. We use the model to suggest the optimal size and geometry for TESs, considering how small the devices can be made before their performance is compromised. In the future, device modelling prior to manufacture will reduce the need for time-consuming and expensive testing.
In this paper we present the results of our 176-pixel prototype of the FDM readout system for SAFARI, a TES-based
focal-plane instrument for the far-IR SPICA mission. We have implemented the knowledge obtained from the detailed
study on electrical crosstalk reported previously. The effect of carrier leakage is reduced by a factor two, mutual
impedance is reduced to below 1 nH and mutual inductance is removed. The pixels are connected in stages, one quarter
of the array half of the array and the full array, to resolve intermediate technical issues. A semi-automated procedure was
incorporated to find all optimal settings for all pixels. And as a final step the complete array has been connected and 132
pixels have been read out simultaneously within the frequency range of 1-3.8MHz with an average frequency separation
of 16kHz. The noise was found to be detector limited and was not affected by reading out all pixels in a FDM mode.
With this result the concept of using FDM for multiplexed bolometer read out for the SAFARI instrument has been
The SAFARI Detector Test Facility is an ultra-low background optical testbed for characterizing ultra-sensitive
prototype horn-coupled TES bolmeters for SAFARI, the grating spectrometer on board the proposed SPICA satellite.
The testbed contains internal cold and hot black-body illuminators and a light-pipe for illumination with an external
source. We have added reimaging optics to facilitate array optical measurements. The system is now being used for
optical testing of prototype detector arrays read out with frequency-domain multiplexing. We present our latest optical
measurements of prototype arrays and discuss these in terms of the instrument performance.
SRON is developing ultra-low noise Transition Edge Sensors (TESs) based on a superconducting Ti/Au bilayer on a
suspended SiN island with SiN legs for the SAFARI instrument aboard the SPICA mission. We successfully fabricated
TESs with very narrow (0.5-0.7 μm) and thin (0.25 μm) SiN legs on different sizes of SiN islands using deep reactiveion
etching process. The pixel size is 840x840 μm2 and there are variety of designs with and without optical absorbers.
For TESs without absorbers, we measured electrical NEPs as low as <1x10-19 W/√Hz with response time of 0.3 ms and
reached the phonon noise limit. Using TESs with absorbers, we quantified the darkness of our setup and confirmed a
photon noise level of 2x10-19 W/√Hz.
We have characterized the optical response of prototype detectors for SAFARI, the far-infrared imaging spectrometer for the SPICA satellite. SAFARI's three bolometer arrays will image a 2’×2’ field of view with spectral information over the wavelength range 34—210 μm. SAFARI requires extremely sensitive detectors (goal NEP ~ 0.2 aW/√Hz), with correspondingly low saturation powers (~5 fW), to take advantage of SPICA's cooled optics. We have constructed an ultra-low background optical test facility containing an internal cold black-body illuminator and have recently added an internal hot black-body source and a light-pipe for external illumination. We illustrate the performance of the test facility with results including spectral-response measurements. Based on an improved understanding of the optical throughput of the test facility we find an optical efficiency of 60% for prototype SAFARI detectors.
We report on the performance of a high sensitivity 4.7 THz heterodyne receiver based on a NbN hot electron bolometer mixer and a quantum cascade laser (QCL) as local oscillator. The receiver is developed to observe the astronomically important neutral atomic oxygen [OI] line at 4.7448 THz on a balloon based telescope. The single-line frequency control and improved beam pattern of QCL have taken advantage of a third-order distributed feedback structure. We measured a double sideband receiver noise temperature (Trec(DSB)) of 815 K, which is ~ 7 times the quantum noise limit (hν/2kB). An Allan time of 15 s at an effective noise fluctuation bandwidth of 18 MHz is demonstrated. Heterodyne performance was further supported by a measured methanol line spectrum around 4.7 THz.
We have measured the optical response of detectors designed for SAFARI, the far-infrared imaging spectrometer for the SPICA satellite. To take advantage of SPICA's cooled optics, SAFARI’s three bolometer arrays are populated with extremely sensitive (NEP~2×10-19 W/√Hz) transition edge sensors with a transition temperature close to 100 mK. The extreme sensitivity and low saturation power (~4 fW) of SAFARI’s detectors present challenges to characterizing them. We have therefore built up an ultra-low background test facility with a cryogen-free high-capacity dilution refrigerator, paying careful attention to stray-light exclusion. Our use of a pulse-tube cooler to pre-cool the dilution refrigerator required that the SAFARI Detector System Test Facility provide a high degree electrical, magnetic, and mechanical isolation for the detectors. We have carefully characterized the performance of the test facility in terms of background power loading. The test facility has been designed to be flexible and easily reconfigurable with internal illuminators that allow us to characterize the optical response of the detectors. We describe the test facility and some of the steps we took to create an ultra-low background test environment. We have measured the optical response of two detectors designed for SAFARI’s short-wave wavelength band in combination with a spherical backshort and conical feedhorn. We find an overall optical efficiency of 40% for both, compared with an ideal-case predicted optical efficiency of 66%.
The Far-Infrared Fourier transform spectrometer instrument SAFARI-SPICA which will operate with cooled optics in a low-background space environment requires ultra-sensitive detector arrays with high optical coupling efficiencies over extremely wide bandwidths. In earlier papers we described the design, fabrication and performance of ultra-low-noise Transition Edge Sensors (TESs) operated close to 100mk having dark Noise Equivalent Powers (NEPs) of order 4 × 10−19W/√Hz close to the phonon noise limit and an improvement of two orders of magnitude over TESs for ground-based applications. Here we describe the design, fabrication and testing of 388-element arrays of MoAu TESs integrated with far-infrared absorbers and optical coupling structures in a geometry appropriate for the SAFARI L-band (110 − 210 μm). The measured performance shows intrinsic response time τ ~ 11ms and saturation powers of order 10 fW, and a dark noise equivalent powers of order 7 × 10−19W/√Hz. The 100 × 100μm2 MoAu TESs have transition temperatures of order 110mK and are coupled to 320×320μm2 thin-film β-phase Ta absorbers to provide impedance matching to the incoming fields. We describe results of dark tests (i.e without optical power) to determine intrinsic pixel characteristics and their uniformity, and measurements of the optical performance of representative pixels operated with flat back-shorts coupled to pyramidal horn arrays. The measured and modeled optical efficiency is dominated by the 95Ω sheet resistance of the Ta absorbers, indicating a clear route to achieve the required performance in these ultra-sensitive detectors.
At SRON we are developing the Frequency Domain Multiplexing (FDM) for the read-out of the TES-based
detector array for the future infrared and X-ray space mission. We describe the performances of a multiplexer
designed to increase the experimental throughput in the characterisation of ultra-low noise equivalent power
(NEP) TES bolometers and high energy resolving power X-ray microcalorimeters arrays under ac and dc bias.
We discuss the results obtained using the TiAu TES bolometers array fabricated at SRON with measured dark
NEP below 5 · 10−19W/
Hz and saturation power of several fW.
The next generation of space missions targeting far-infrared wavelengths will require large-format arrays of extremely
sensitive detectors. The development of Transition Edge Sensor (TES) array technology is being developed for future
Far-Infrared (FIR) space applications such as the SAFARI instrument for SPICA where low-noise and high sensitivity is
required to achieve ambitious science goals.
In this paper we describe a modal analysis of multi-moded horn antennas feeding integrating cavities housing TES
detectors with superconducting film absorbers. In high sensitivity TES detector technology the ability to control the
electromagnetic and thermo-mechanical environment of the detector is critical. Simulating and understanding optical
behaviour of such detectors at far IR wavelengths is difficult and requires development of existing analysis tools.
The proposed modal approach offers a computationally efficient technique to describe the partial coherent response of
the full pixel in terms of optical efficiency and power leakage between pixels. Initial wok carried out as part of an ESA
technical research project on optical analysis is described and a prototype SAFARI pixel design is analyzed where the
optical coupling between the incoming field and the pixel containing horn, cavity with an air gap, and thin absorber layer
are all included in the model to allow a comprehensive optical characterization. The modal approach described is based
on the mode matching technique where the horn and cavity are described in the traditional way while a technique to
include the absorber was developed. Radiation leakage between pixels is also included making this a powerful analysis
We report a new experiment on a high-resolution heterodyne spectrometer using a 3.5 THz quantum cascade laser
(QCL) as local oscillator (LO) and a superconducting hot electron bolometer (HEB) as mixer by stabilizing both
frequency and amplitude of the QCL. The frequency locking of the QCL is demonstrated by using a methanol molecular
absorption line, a proportional-integral-derivative (PID) controller, and a direct power detector. We show that the LO
locked linewidth can be as narrow as 35 KHz. The LO power to the HEB is also stabilized by means of swing-arm
actuator placed in the beam path in combination of a second PID controller.
We report on the application of a new technique for actively stabilizing the power of a far infrared gas laser as the local
oscillator (LO) in a superconducting hot electron bolometer (HEB) heterodyne receiver system at 2.5 THz. The
technique utilizes PID feedback control of the local oscillator intensity by means of a voice-coil based swing arm
actuator placed in the beam path. The HEB itself is used as a direct detector to measure incident LO power whilst
simultaneously continuing to function as heterodyne mixer. Results presented here demonstrate a factor of 50
improvement in the measured total power and spectroscopic Allan variance time. Allan times of 30 seconds and 25
seconds respectively are shown for large and small area HEB's with a measured effective noise fluctuation bandwidth of
12 MHz. The technique is versatile and can be applied to any LO source and at any LO frequency.
We report on a twin-slot antenna coupled superconducting NbN hot electron bolometer (HEB) mixer designed for 1.6
THz. Terahertz (THz) radiation is quasi-optically coupled to the HEB with an uncoated elliptical Si lens. Measured DSB
receiver noise temperatures are 1500 K at 0.85 THz, 1200 K at 1.27 THz, 1100 K at 1.31 THz, 1100 K at 1.4 THz, and
1000 K at 1.63 THz. This value at 1.63 THz is reduced to 750 K when the hot/cold loads in vacuum are used. The
frequency dependence of the noise temperature is consistent with the measured FTS spectral response. The measured farfield
beam patterns of the lens/antenna combination show nearly collimated beams with the side lobes below -16dB by
adding a 40 μm extension to a standard Si elliptical lens design, which is understood by considering a slightly lower
dielectric constant of Si (εSi) of 11.4 instead of 11.7. The good performance of such NbN HEB mixers makes it suitable
for future high-resolution spectroscopic astronomical applications.
SPICA is an infra-red (IR) telescope with a cryogenically cooled mirror (~5K) with three instruments on board, one of
which is SAFARI that is an imaging Fourier Transform Spectrometer (FTS) with three bands covering the wavelength of
34-210 μm. We develop transition edge sensors (TES) array for short wavelength band (34-60 μm) of SAFARI. These
are based on superconducting Ti/Au bilayer as TES bolometers with a Tc of about 105 mK and thin Ta film as IR
absorbers on suspended silicon nitride (SiN) membranes. These membranes are supported by long and narrow SiN legs
that act as weak thermal links between the TES and the bath. Previously an electrical noise equivalent power (NEP) of
4×10-19 W/√Hz was achieved for a single pixel of such detectors. As an intermediate step toward a full-size SAFARI
array (43×43), we fabricated several 8×9 detector arrays. Here we describe the design and the outcome of the dark and
optical tests of several of these devices. We achieved high yield (<93%) and high uniformity in terms of critical
temperature (<5%) and normal resistance (7%) across the arrays. The measured dark NEPs are as low as 5×10-19 W/√Hz
with a response time of about 1.4 ms at preferred operating bias point. The optical coupling is implemented using
pyramidal horns array on the top and hemispherical cavity behind the chip that gives a measured total optical coupling
efficiency of 30±7%.
Future Far-IR space telescopes, such as the SAFARI instrument of the proposed JAXA/ESA SPICA
mission, will use horn antennas to couple to cavity bolometers to achieve high levels of sensitivity for
Mid-IR astronomy. In the case of the SAFARI instrument the bolometric detectors susceptibility to
currents coupling into the detector system and dissipating power within the bolometers is a particular
concern of the class of detector technology considered.1 The simulation of such structures can prove
challenging. At THz frequencies ray tracing no longer proves completely accurate for these partially
coherent large electrical structures, which also present significant computational difficulties for the
more generic EM approaches applied at longer microwave wavelengths. The Finite Difference Time
Domain method and other similar commercially viable approaches result in excessive computational
requirements, especially when a large number of modes propagate.
Work being carried out at NUI-Maynooth is utilising a mode matching approach to the simulation of
such devices. This approach is based on the already proven waveguide mode scattering code "Scatter"2
developed at NUI-Maynooth, which is a piece of mode matching code that operates by cascading a Smatrice
while conserving power at each waveguide junction. This paper outlines various approaches to
simulating such Antenna Horns and Cavities at THz frequencies, focusing primarily on the waveguide
modal Scatter approach. Recently the code has been adapted to incorporate a rectangular waveguide
basis mode set instead of the already established circular basis set.
We report the sensitivity of a superconducting NbN hot electron bolometer mixer integrated with a tight spiral antenna at
5.3 THz. Using a measurement setup with black body calibration sources and a beam splitter in vacuo, and an
antireflection coated Si lens, we obtained a double sideband receiver noise temperature of 1150 K, which is 4.5 times
hν/kB (quantum limit). Our experimental results in combination with an antenna-to-bolometer coupling simulation
suggest that HEB mixer can work well at least up to 6 THz, suitable for next generation of high-resolution spectroscopic
of the neutral atomic oxygen (OI) line at 4.7 THz.
High-resolution heterodyne spectrometers operating at above 2 THz are crucial for detecting, e.g., the HD line at 2.7
THz and oxygen OI line at 4.7 THz in astronomy. The potential receiver technology is a combination of a hot electron
bolometer (HEB) mixer and a THz quantum cascade laser (QCL) local oscillator (LO).Here we report the first highresolution
heterodyne spectroscopy measurement of a gas cell using such a HEB-QCL receiver. The receiver employs a
2.9 THz free-running QCL as local oscillator and a NbN HEB as a mixer. By using methanol (CH3OH) gas as a signal
source, we successfully recorded the methanol emission line at 2.92195 THz. Spectral lines at IF frequency at different
pressures were measured using a FFTS and well fitted with a Lorentzian profile. Our gas cell measurement is a crucial
demonstration of the QCL as LO for practical heterodyne instruments. Together with our other experimental
demonstrations, such as using a QCL at 70 K to operate a HEB mixer and the phase locking of a QCL such a receiver is
in principle ready for a next step, which is to build a real instrument for any balloon-, air-, and space-borne observatory.
In the wavelength regime between 60 and 300 microns there are a number of atomic and molecular emission lines that
are key diagnostic probes of the interstellar medium. These include transitions of [CII], [NII], [OI], HD, H2D+, OH, CO,
and H2O, some of which are among the brightest global and local far-infrared lines in the Galaxy. In Giant Molecular
Clouds (GMCs), evolved star envelopes, and planetary nebulae, these emission lines can be extended over many arc
minutes and possess complicated, often self absorbed, line profiles. High spectral resolution (R> 105) observations of
these lines at sub-arcminute angular resolution are crucial to understanding the complicated interplay between the
interstellar medium and the stars that form from it. This feedback is central to all theories of galactic evolution. Large
format heterodyne array receivers can provide the spectral resolution and spatial coverage to probe these lines over
The advent of large format (~100 pixel) spectroscopic imaging cameras in the far-infrared (FIR) will fundamentally
change the way astronomy is performed in this important wavelength regime. While the possibility of such instruments
has been discussed for more than two decades, only recently have advances in mixer and local oscillator technology,
device fabrication, micromachining, and digital signal processing made the construction of such instruments tractable.
These technologies can be implemented to construct a sensitive, flexible, heterodyne array facility instrument for
SOFIA. The instrument concept for StratoSTAR: Stratospheric Submm/THz Array Receiver includes a common user
mounting, control system, IF processor, spectrometer, and cryogenic system. The cryogenic system will be designed to
accept a frontend insert. The frontend insert and associated local oscillator system/relay optics would be provided by
individual user groups and reflect their scientific interests. Rapid technology development in this field makes SOFIA the
ideal platform to operate such a modular, continuously evolving instrument.
Transition edge sensor (TES) is the selected detector for the SAFARI FIR imaging spectrometer (focal plane arrays covering a wavelength range from 30 to 210 μm) on the Japanese SPICA telescope. Since the telescope is cooled to <7 K, the instrument sensitivity is limited by the detector noise. Therefore among all the requirements, a crucial one is the sensitivity, which should reach an NEP (Noise Equivalent Power) as low as 3E-19 W/Hz^0.5 for a base temperature of >50 mK. Also the time constant should be below 8 ms.
We fabricated and characterized low thermal conductance transition edge sensors (TES) for SAFARI instrument on SPICA. The device is based on a superconducting Ti/Au bilayer deposited on suspended SiN membrane. The critical temperature of the device is 155 mK. The low thermal conductance is realized by using narrow SiN ring-like supporting structures. All measurements were performed having the device in a light-tight box, which to a great extent eliminates the loading of the background radiation. We measured the current-voltage (IV) characteristics of the device in different bath temperatures and determine the thermal conductance (G) to be equal to 1.66 pW/K. This value corresponds to a noise equivalent power (NEP) of 1E-18 W/√Hz. The current noise and complex impedance is also measured at different bias points at 25 mK bath temperature. The measured electrical (dark) NEP is 2E-18 W/√Hz, which is about a factor of 2 higher than what we expect from the thermal conductance that comes out of the IV curves. Despite using a light-tight box, the photon noise might still be the source of this excess noise. We also measured the complex impedance of the same device at several bias points. Fitting a simple first order thermal-electrical model to the measured data, we find an effective time constant of about 65 μs and a thermal capacity of 3-4 fJ/K in the middle of the transition
The next generation of space missions targeting far-infrared bands will require large-format arrays of extremely lownoise
detectors. The development of Transition Edge Sensors (TES) array technology seems to be a viable solution for
future mm-wave to Far-Infrared (FIR) space applications where low noise and high sensitivity is required. In this paper
we concentrate on a key element for a high sensitivity TES detector array, that of the optical coupling between the
incoming electromagnetic field and the phonon system of the suspended membrane. An intermediate solution between
free space coupling and a single moded horn is where over-moded light pipes are used to concentrate energy onto multimoded
absorbers. We present a comparison of modeling techniques to analyze the optical efficiency of such light pipes
and their interaction with the front end optics and detector cavity.
In the far-infrared (FIR) / THz regime the angular (and often spectral) resolution of observing facilities is still very
restricted despite the fact that this frequency range has become of prime importance for modern astrophysics. ALMA
(Atacama Large Millimeter Array) with its superb sensitivity and angular resolution will only cover frequencies up to
about 1 THz, while the HIFI instrument for ESA'a Herschel Space Observatory will provide limited angular resolution
(10 to 30 arcsec) up to 2 THz. Observations of regions with star and planet formation require extremely high angular
resolution as well as frequency resolution in the full THz regime. In order to open these regions for high-resolution
astrophysics we present a study concept for a heterodyne space interferometer, ESPRIT (Exploratory Submm Space
Radio-Interferometric Telescope). This mission will cover the Terahertz regime inaccessible from the ground and outside
the operating range of the James Webb Space Telescope (JWST).
We describe the optimization of transition edge superconducting
(TES) detectors for use in a far-infrared (FIR) Fourier transform spectrometer (FTS) mounted on a cryogenically cooled space-borne telescope (e.g. SPICA). The required noise equivalent power (NEP) of the detectors is approximately 10-19W/√Hz in order to be lower than the photon noise from astrophysical sources in octave wide bands in the FIR. The detector time constants must be less than 10 ms in order to allow fast scanning of the FTS mechanism. The detectors consist of superconducting thermometers suspended on thin legs of thermally isolating silicon nitride and operate at a temperature of approximately 100 mK. We present the design of the detectors, a proposed focal plane layout and optical coupling scheme and measurements of thermal conductance and time constant for low NEP prototype TES bolometers.
We have studied the sensitivity of a superconducting NbN hot electron bolometer mixer integrated with a spiral antenna
at 4.3 THz. Using hot/cold blackbody loads and a beam splitter all in vacuum, we measured a double sideband receiver
noise temperature of 1300 K at the optimum local oscillator (LO) power of 330 nW, which is about 12 times the
quantum noise (hν/2kB). Our result indicates that there is no sign of degradation of the mixing process at the super-THz frequencies. Also, a measurement method is introduced where the hot/cold response of the receiver is recorded at
constant voltage bias of the mixer, while varying the LO power. We argue that this method provides an accurate
measurement of the receiver noise temperature, which is not influenced by the LO power fluctuations and the direct
detection effect. Moreover, our sensitivity data suggests that one can achieve a receiver noise temperature of 1420 K at
the frequency of [OI] line (4.7 THz), which is scaled from the sensitivity at 4.3 THz with frequency.
We have characterized a heterodyne receiver based on an NbN hot electron bolometer integrated with spiral antenna as
mixer and an optically pumped FIR ring laser at 4.3 THz as local oscillator (LO). We succeeded in measuring the
receiver output power, responding to the hot/cold load, as a function of bias voltage at optimum LO power. From the
resulted receiver noise temperature versus the bias voltage, we found a DSB receiver noise temperature of 3500 K at a
bath temperature of 4 K, which is a minimum average value. This is the highest sensitivity reported so far at frequencies
above 4 THz.
NbN hot electron bolometer (HEB) mixers are at this moment the best heterodyne detectors for frequencies above 1 THz. However, the fabrication procedure of these devices is such that the quality of the interface between the NbN superconducting film and the contact structure is not under good control. This results in a contact resistance between the NbN bolometer and the contact pad. We compare identical bolometers, with different NbN - contact pad interfaces, coupled with a spiral antenna. We find that cleaning the NbN interface and adding a thin additional superconductor prior to the gold contact deposition improves the noise temperature and the bandwidth of the HEB mixers with more than a factor of 2. We obtain a DSB noise temperature of 950 K at 2.5 THz and a Gain bandwidth of 5-6 GHz. For use in real receiver systems we design small volume (0.15x1 micron) HEB mixers with a twin slot antenna. We find that these mixers combine good sensitivity (900 K at 1.6 THz) with low LO power requirement, which is 160 - 240 nW at the Si lens of the mixer. This value is larger than expected from the isothermal technique and the known losses in the lens by a factor of 3-3.5.
Improved and reproducible heterodyne mixing (noise temperatures of 950 K at 2.5 THz) has been realized with NbN based hot-electron superconducting devices with low contact resistances. A distributed temperature numerical model of the NbN bridge, based on a local electron and a phonon temperature, has been used to understand the physical conditions during the mixing process. We find that the mixing is predominantly due to the exponential rise of the local resistivity as a function of electron temperature.
We have proposed to develop a prototype 0.5-meter far-infrared telescope and heterodyne receiver/spectrometer system for fully-automated remote operation at the summit of Dome A, the highest point on the Antarctic plateau. The unparalleled stability, exceptional dryness, low wind and extreme cold make Dome A a ground-based site without equal for astronomy at infrared and submillimeter wavelengths. HEAT, the High Elevation Antarctic Terahertz Telescope, will operate in the atmospheric windows between 150 and 400 microns, in which the most crucial astrophysical spectral diagnostics of the formation of galaxies, stars, planets, and life are found. At these wavelengths, HEAT will have high aperture efficiency and excellent atmospheric transmission most of the year. The proposed superheterodyne receiver system will be comprised of 0.8, 1.4 and 1.9 THz channels which will observe the pivotal J=7-6 line of CO, the J=2-1 line of atomic carbon, and the far-infrared fine structure lines of N+ and C+, the brightest emission lines in the entire Milky Way Galaxy. When combined with the HEAT telescope, the receiver system represents a uniquely powerful instrument for reconstructing the history of star formation in our Galaxy, with application to the distant Universe. The receiver system itself serves as a valuable testbed for heterodyne Terahertz components, using leading-edge mixer, local oscillator, low-noise amplifier, cryogenic, and digital signal processing technologies that will play essential roles in future Terahertz observatories. The proposed study will pave the way for future astronomical investigations from Dome A.
We summarize our research activities on THz Nb diffusion-cooled hot electron bolometer (HEB) mixers, carried out at Space Research Organization Netherlands (SRON) and Delft University of Technology. This paper will include our understanding on the device physics of diffusion-cooled HEB mixers, noise and IF bandwidth measurements of waveguide mixers around 0.7 THz, and in particular recent measurements of Nb quasi-optical mixers at 0.64 and 2.5 THz. The waveguide devices demonstrate a receiver noise temperature of 900 K at 0.7 THz. The quasi-optical mixers show 1200 K at 0.64 THz and 4500 K at 2.5 THz and a maximum IF bandwidth of at least 5 GHz.
We describe the preliminary design of the proposed Heterodyne Instrument for FIRST (HIFI). The instrument will have a continuous frequency coverage over the range from 480 to 1250 GHz in five bands, while a sixth band will provide coverage for 1410 - 1910 GHz and 2400 - 2700 GHz. The first five bands will use SIS mixers and varactor frequency multipliers while in the sixth band a laser photomixer local oscillator will pump HEB mixers. HIFI will have an instantaneous bandwidth of 4 GHz, analyzed in parallel by two types of spectrometers: a pair of wide-band spectrometers (WBS), and a pair of high- resolution spectrometer (HRS). The wide-band spectrometer will use acousto-optic technology with a frequency resolution of 1 MHz and a bandwidth of 4 GHz for each of the two polarizations. The HRS will provide two combinations of bandwidth and resolution: 1 GHz bandwidth at 200 kHz resolution, and at least 500 MHz at 100 kHz resolution. The HRS will be divided into 4 or 5 sub-bands, each of which can be placed anywhere within the full 4 GHz IF band. The instrument will be able to perform rapid and complete spectral line surveys with resolving powers from 103 up to 107 (300 - 0.03 km/s) and deep line observations.
Receiver B3 is a common-user facility instrument for the JCMT and was commissioned in December 1996. It includes the following features: (1) Frequency coverage of 315 to 372 GHz with optimum performance at 345 GHz. (2) Two spatially- coincident channels with orthogonal linear polarizations. (3) An IF of 4 GHz with an instantaneous bandwidth of 1.7 GHz in each channel. (4) Single side-band capability with the rejected side-band terminated on a cold load. (5) High- efficiency, frequency-independent optics. (6) Independent adjustment of the local oscillator power to the two mixers. (7) Internal ambient and cold loads for accurate receiver calibration. (8) Fully automated operation.