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.
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 μm<sup>2</sup> and there are variety of designs with and without optical absorbers.
For TESs without absorbers, we measured electrical NEPs as low as <1x10<sup>-19</sup> 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<sup>-19</sup> 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.
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 (<i>NEPs</i>) of order 4 × 10<sup>−19</sup>W/√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<sup>−19</sup>W/√Hz. The 100 × 100μm<sup>2</sup> MoAu TESs have transition temperatures of order 110mK and are coupled to 320×320μm<sup>2</sup> 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.
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<sup>-19</sup> 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%.
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<sup>−19</sup>W/
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
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 T<sub><i>c</i></sub> 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<sup>-19</sup> 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<sup>-19</sup> 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.<sup>1</sup> 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"<sup>2</sup>
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 (CH<sub>3</sub>OH) 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.
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<sup>-19</sup> 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<sup>-18</sup> 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<sup>-18</sup> 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.
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 (<i>h</i>ν/2<i>k</i><sub>B</sub>). 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.
Stability of a hot-electron bolometer (HEB) heterodyne receiver was investigated at frequencies from 0.6THz to 1.9THz. The Allan variance was measured as a function of the integration time and the Allan time was obtained for HEB mixers of different size, as well as with different types of the local oscillator: FIR laser, multiplier chain, and BWO. We have found that due to stronger dependence of the mixer gain and noise vs mixer bias voltage and current the Allan time is shorter for smaller mixers. At 1.6THz the Allan time is 3 sec for 4x0.4μm<sup>2</sup> bolometer, and 0.15-0.2 sec for 1x0.15μm<sup>2</sup> bolometer. Obtained stability apears to be the same for the FIR laser and the mulitplier chain. The Allan time for smaller bolometers increases to 0.4-0.5sec at 0.6-0.7THz LO frequencies. The influence of the IF chain on the obtained results is also analyzed.
In this paper recent developments of Hot Electron Bolometric receivers performed at Chalmers are summarized. This comprises progress on the mixers for HIFI and membrane HEB. All devices are modelled using Hot Spot model taking Andreev reflection at the interface between the normal conductor and the superconductor into
Bolometric receivers serve as direct detectors, photon counters and as heterodyne receivers in astronomical instruments. Heterodyne hot-electron bolometric mixers show record sensitivity for observation frequencies above a Terahertz. In this paper NbN phonon-cooled mixers, conversion gain, noise and stability are discussed based on device models including Andreev reflection and critical current effects. The geometry (4 μm wide, 0.4 μm long on a 35 Å thick film), critical current (as high as possible) and critical temperature (about 8.5K) of an optimum phonon-cooled bolometric receiver operated at 2 to 4 K is discussed. Smaller devices than the optimum show worse noise performance. Larger devices require too high local oscillator power.
NbN hot- electron bolometer mixers have reached the level of 10hv/k in terms of the input noise temperature with the noise bandwidth of 4-6 GHz from subMM band up to 2.5 THz. In this paper we discuss the major characteristics of this kind of receiver, i.e. the gain and the noise bandwidth, the noise temperature in a wide RF band, bias regimes and optimisation of RF coupling to the quasioptical mixer. We present the status of the development of the mixer for Band 6 Low for Herschel Telescope.