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The Athena X-ray Integral Unit (X-IFU) is the high resolution X-ray spectrometer, studied since 2015 for flying in the mid-30s on the Athena space X-ray Observatory, a versatile observatory designed to address the Hot and Energetic Universe science theme, selected in November 2013 by the Survey Science Committee. Based on a large format array of Transition Edge Sensors (TES), it aims to provide spatially resolved X-ray spectroscopy, with a spectral resolution of 2.5 eV (up to 7 keV) over an hexagonal field of view of 5 arc minutes (equivalent diameter). The X-IFU entered its System Requirement Review (SRR) in June 2022, at about the same time when ESA called for an overall X-IFU redesign (including the X-IFU cryostat and the cooling chain), due to an unanticipated cost overrun of Athena. In this paper, after illustrating the breakthrough capabilities of the X-IFU, we describe the instrument as presented at its SRR, browsing through all the subsystems and associated requirements. We then show the instrument budgets, with a particular emphasis on the anticipated budgets of some of its key performance parameters. Finally we briefly discuss on the ongoing key technology demonstration activities, the calibration and the activities foreseen in the X-IFU Instrument Science Center, and touch on communication and outreach activities, the consortium organisation, and finally on the life cycle assessment of X-IFU aiming at minimising the environmental footprint, associated with the development of the instrument. Thanks to the studies conducted so far on X-IFU, it is expected that along the design-to-cost exercise requested by ESA, the X-IFU will maintain flagship capabilities in spatially resolved high resolution X-ray spectroscopy, enabling most of the original X-IFU related scientific objectives of the Athena mission to be retained. (abridged)., Comment: 48 pages, 29 figures, Accepted for publication in Experimental Astronomy with minor editing

The SPICA-SAFARI instrument requires extremely sensitive transition edge sensor (TES) arrays with a noise equivalent power of $$2 \times 10^{ - 19} \;{\text{W/}}\sqrt {\text{Hz}}$$2×10-19W/Hz and a readout system with an output noise that is dominated by the detector noise. It is essential to ensure the frequency domain multiplexing (FDM) readout system in SAFARI meets the noise requirement. The FDM system in SAFARI consists essentially of LC filters, a superconducting quantum interference device, a room-temperature low-noise amplifier (LNA), and a demultiplexer. Here we present a noise study of the LNA from a laboratory amplifier chain. We found the equivalent current and voltage noise of the LNA to be $$5.4\;{\text{pA/}}\sqrt {\text{Hz}}$$5.4pA/Hz and $$315\;{\text{pV/}}\sqrt {\text{Hz}}$$315pV/Hz, respectively, which are low enough to read out SAFARI’s TES arrays.

We are developing X-ray microcalorimeters as a backup option for the baseline detectors in the X-IFU instrument on board the ATHENA space mission led by ESA and to be launched in the early 2030s.5$\times$5 mixed arrays with TiAu transition-edge sensor (TES), which have different high aspect ratios and thus high resistances, have been designed and fabricated to meet the energy resolution requirement of the X-IFU instrument. Such arrays can also be used to optimise the performance of the Frequency Domain Multiplexing (FDM) readout and lead to the final steps for the fabrication of a large detector array. In this work we present the experimental results from tens of the devices with an aspect ratio (length-to-width) ranging from 1-to-1 up to 6-to-1, measured in a single-pixel mode with a FDM readout system developed at SRON/VTT. We observed a nominal energy resolution of about 2.5 eV at 5.9 keV at bias frequencies ranging from 1 to 5 MHz. These detectors are proving to be the best TES microcalorimeters ever reported in Europe, being able to meet not only the requirements of the X-IFU instrument, but also those of other future challenging X-ray space missions, fundamental physics experiments, plasma characterization and material analysis.

We have characterized and mapped the electrical cross talk (ECT) of a frequency division multiplexing (FDM) system with a transition edge sensor (TES) bolometer array, which is intended for space applications. By adding a small modulation at 120 Hz to the AC bias voltage of one bolometer and measuring the cross talk response in the current noise spectra of the others simultaneously, we have for the first time mapped the ECT level of 61 pixels with a nominal frequency spacing of 32 kHz in a 61 × 61 matrix and a carrier frequency ranging from 1 MHz to 4 MHz. We find that about 94% of the pixels show an ECT level of less than 0.4%. Only the adjacent pixels reach this level, and the ECT for the rest of the pixels is less than 0.1%. We also observe higher ECT levels, up to 10%, between some of the pixels, which have bundled long, parallel coplanar wires connecting TES bolometers to inductor-capacitor filters. In this case, the high mutual inductances dominate. To mitigate this source of ECT, the coplanar wires should be replaced by microstrip wires in the array. Our study suggests that an FDM system can have a relatively low ECT level, e.g., around 0.4% if the frequency spacing is 30 kHz. Our results successfully demonstrate a low electrical cross talk for a space FDM technology.

We demonstrate multiplexing readout of 60 transition edge sensor (TES) bolometers operating at 90 mK using a frequency division multiplexing readout chain with bias frequencies ranging from 1 to 3.5 MHz and with a typical frequency spacing of 32 kHz. The readout chain starts with a two-stage SQUID amplifier and has a noise level of 9.5 pA/ √{ Hz } . We compare current-voltage curves and noise spectra of TESs measured in a single-pixel mode and in a multiplexing mode. We also map the noise equivalent power (NEP) and the saturation power of the bolometers in both modes, where there are 43 pixels that do not show more than 10% difference in NEP and 5% in saturation power when measured in single pixel and multiplex modes. We have read out a TES with an NEP of 0.45 aW/ √{ Hz } in the multiplexing-mode, which demonstrates the capability of reading out ultra-low noise TES bolometer arrays for space applications.

The SAFARI instrument is a spectrometer on board SPICA, using ~4000 TES bolometers with an unprecedented sensitivity. We are currently optimizing an FDM readout prototype for SAFARI, which is capable of reading out 160 pixels with the bias frequency range 1-4 MHz and 16 kHz spacing. We present our latest results with emphasis on the noise and cross-talk performance of this system. The measured readout noise is discussed in various configurations. We also present a method for mapping out the electrical cross-talk between the pixels in the array and elaborate on the measured data and analysis.

We are developing the frequency-domain multiplexing (FDM) readout for the SAFARI far-infrared spectrometer on board the SPICA space observatory. Each readout channel comprises a set of ~ 160 TESs and LC filters and is read out with a SQUID preamplifier. Baseband feedback is applied to overcome the dynamic range limitations of the SQUID. We have carried out extensive characterization of a test readout system coupled to a 176-pixel TES bolometer array in order to understand and optimize the system. We present our latest measurements of this 176-pixel FDM demonstrator, focusing on the noise contribution from the SQUID.

Transition-edge sensors (TESs) are used as very sensitive thermometers in microcalorimeters aimed at detection of different wavelengths. In particular, for soft X-ray astrophysics, science goals require very high-resolution microcalorimeters which can be achieved with TESs coupled to suitable absorbers. For many applications, there is also need for a high number of pixels which typically requires multiplexing in the readout stage. Frequency-domain multiplexing (FDM) is a common scheme and is the baseline proposed for the ATHENA mission. FDM requires biasing the TES in AC at MHz frequencies. Recently, there has been reported degradation in performances under AC with respect to DC bias. In order to assess the performances of TESs to be used with FDM, it is thus of great interest to compare the performances of the same device both under AC bias and DC bias. This requires two different measurement set-ups with different processes for making the characterization. We report in this work the preliminary results of a single-pixel characterization performed on a TiAu TES under AC and afterwards under DC bias in different facilities. Extraction of dynamical parameters and noise performances are compared in both cases as a first stage for further AC/DC comparison of these devices., This work is partly funded by European Space Agency (ESA) and coordinated with other European efforts under ESA CTP contract ITT AO/1-7947/14/NL/BW. It has also received funding from the European Union’s Horizon 2020 Programme under the AHEAD (Activities for the High-Energy Astrophysics Domain) project with grant agreement number 654215. CSIC work is financed by the Spanish Ministerio de Ciencia, Innovación y Universidades-MICINN (projects ESP2016-76683-C3-2-R and RTI2018-096686-B-C22). Personnel from ICMAB acknowledge financial support from MINECO, through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV-2015-0496).

Superconducting transition-edge sensors (TESs) are highly sensitive detectors. Based on the outstanding performance on spectral resolution, the X-ray integral field unit (X-IFU) instrument on-board athena will be equipped with a large array of TES-based microcalorimeters. For optimal performance in terms of the energy resolution, it is essential to limit undesirable nonlinearity effects in the TES detector. Weak-link behavior induced on the TES by superconducting leads is such a nonlinearity effect. We designed and fabricated smart test structures to study the effect of the superconducting leads on the intrinsic transition curve of our TiAu-based TES bilayer. We measured and analyzed the resistance versus temperature transition curves of the test structures. We found relations of long-distance proximity effects with TES length and different lead materials. Based on these results, we can redesign and further optimize our TES-based X-ray detectors.

Dilution and adiabatic demagnetization refrigerators based on pulse tube cryocoolers are nowadays used in many low temperature physics experiments, such as atomic force and scanning tunneling microscopy, quantum computing, radiation detectors, and many others. A pulse tube refrigerator greatly simplifies the laboratory activities being a cryogen-free system. The major disadvantage of a pulse tube cooler is the high level of mechanical vibrations at the warm and cold interfaces that could substantially affect the performance of very sensitive cryogenic instruments. In this paper, we describe the performance of a very simple mechanical attenuation system used to eliminate the pulse-tube-induced low frequency noise of the superconducting transition-edge sensors under development for the instruments of the next generation of infra-red and X-ray space observatories.

The next generation of Far-infrared and X-ray space observatories will require detector arrays with thousands of transition edge sensor (TES) pixel. It is extremely important to have a tool that is able to characterize all the pixels and that can give a clear picture of the performance of the devices. In particular, we refer to those aspects that can affect the global energy resolution of the array: logarithmic resistance sensitivity with respect to temperature and current (α and β parameters, respectively), uniformity of the TESs and the correct understanding of the detector thermal model. Complex impedance measurement of a TES is the only technique that can give all this information at once, but it has been established only for a single pixel under DC bias. We have developed a complex impedance measurement method for TESs that are AC biased since we are using a MHz frequency domain multiplexing (FDM) system to readout an array. The FDM readout demands for some modifications to the complex-impedance technique and extra considerations, e.g. how to modulate a small fraction of the bias carrier frequencies in order to get a proper excitation current through the TESs and how to perform an accurate demodulation and recombination of the output signals. Also, it requires careful calibration to remove the presence of parasitic impedances in the entire readout system. We perform a complete set of AC impedance measurements for different X-ray TES microcalorimeters based on superconducting TiAu bilayers with or without normal metal Au bar structures. We discuss the statistical analysis of the residual between impedance data and fitting model to determine the proper calorimeter thermal model for our detectors. Extracted parameters are used to improve our understanding of the differences and capabilities among the detectors and additionally the quality of the array. Moreover, we use the results to compare the calculated noise spectra with the measured data.

We are developing a transition edge sensor (TES) microcalorimeter array based on a Ti/Au superconducting bilayer, as a backup option for the X-IFU instrument on the Athena X-ray observatory. The array is read out by a frequency-division multiplexing readout system using a 1–5 MHz frequency band. Extensive research collaborations between NASA/Goddard and SRON have led to new design rules for microcalorimeters such as low resistivity of the superconductor bilayer, moderately high ohmic resistance of the TES by changing the aspect ratio and no extra metal strips. We have improved our detector fabrication process according to these design principles and produced TES arrays. Although single-pixel characterizations of these arrays are ongoing, the best energy resolution of 2.0 eV for 5.9 keV X-ray has been observed with a 120 × 20 μm2 TES with a normal resistance of 150 mΩ, biased at 2.2 MHz frequency. This shows that our Ti/Au TES array has a potential to fulfill the detector requirements of the X-IFU instrument.

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.

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.

Ultra-low-noise transition edge sensors (TES) with noise equivalent power lower than 2 $$\times $$ 10 $$^{-19}$$ W/Hz $$^{1/2 }$$ have been fabricated by SRON, which meet the sensitivity requirements for the far-infrared SAFARI instrument on space infrared telescope for cosmology and astrophysics. Our TES detector is based on a titanium/gold (Ti/Au) thermistor on a silicon nitride (SiN) island. The island is thermally linked with SiN legs to a silicon support structure at the bath temperature. The SiN legs are very thin (250 nm), narrow (500 nm), and long (above 300 $${\upmu } $$ m); these dimensions are needed in leg-isolated bolometers to achieve the required level of sensitivity. In this paper, we describe the latest fabrication process for our TES bolometers with improved sensitivity.

Controlling the critical temperature ( $$T_{C}$$ ) of Ti/Au bilayers is vital in the development of practical TES detectors. Previously empirical studies have been done on aging effects in Ti/Au and other superconducting bilayers but no link with theory has been made. Here we attempt to explain the change in $$T_{C}$$ with a diffusion mechanism. The change in $$T_{C}$$ has been measured for a set of Ti/Au bilayer samples that have been given a variety of bake-out treatments, where we found a trend that can be partly explained by an inter-diffusion mechanism. With an empirical model based on diffusion a safe zone can be defined as a region of bake-out treatments, where the $$T_{C}$$ is not affected beyond the requirements. This will shine light on the bake-out and the storage condition boundaries of these detectors.

SRON is developing TES detectors based on a superconducting Ti/Au bilayer on a suspended SiN membrane for the short-wavelength band of the SAFARI instrument on SPICA mission. We have recently replaced the wet KOH etching of the Si substrate by deep reactive ion etching. The new process enables us to fabricate the detectors on the substrate and release the membrane at the very last step. Therefore the production of SAFARI large arrays (43 $$\,{\times }\,$$ 43) on thin SiN membrane (250 nm) is feasible. It also makes it possible to realize narrow supporting SiN legs of $$\le $$ 1 $$\upmu $$ m, which are needed to meet SAFARI NEP requirements. Here we report the current–voltage characteristics, noise performance and impedance measurement of these devices. The measured results are then compared with the distributed leg model that takes into account the thermal fluctuation noise due to the SiN legs. We measured a dark NEP of $$\sim $$ 0.7 aW/ $$\sqrt{\hbox {Hz}}$$ , which is 1.6 times higher than the theoretically expected phonon noise.

Recently we have reported on the effects of magnetic field on our low noise (NEP = 4 $$\times 10^{-19}$$ W/ $$\surd $$ Hz) [1] TiAu TES bolometers that are being developed at SRON for the SAFARI FIR Imaging Spectrometer on SPICA telescope that will be operated in three different wavelength bands: S-band for 30–60 $$\upmu \hbox {m}$$ , M-band for 60–110 $$\upmu \hbox {m}$$ and L-band for 110–210 $$\upmu \hbox {m}$$ . The arrays for the S- and M- band will be based on TiAu TES bolometer arrays, developed by SRON. The L-band array will be based on a MoAu TES bolometer developed by University of Cambridge. We have investigated the effect of the magnetic field on the current, responsivity, speed and critical current for both the TiAu and MoAu TES bolometers in our high accuracy magnetic field set-up. A clear difference in weak link behavior is observed between the two types of TES bolometers in both strength of the effect and period of the oscillations.

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., 9 pages, 8 figures. Copyright 2016 Society of Photo Optical Instrumentation Engineers. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited

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 -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.

Athena is a European X-ray observatory, scheduled for launch in $$\sim $$ 2028. Athena will employ a high-resolution imaging spectrometer called X-ray integral field unit (X-IFU), consisting of an array of $$\sim $$ 4000 transition edge sensor (TES) microcalorimeter pixels. For the readout of X-IFU, we are developing frequency domain multiplexing, which is the baseline readout system. In this paper, we report on the performance of a TES X-ray calorimeter array fabricated at Goddard Space Flight Center (GSFC) at MHz frequencies for the baseline of X-IFU detector. During single-pixel AC bias characterization, we measured X-ray energy resolutions (at 6 keV) of about 2.9 eV at both 2.3 and 3.7 MHz. Furthermore, in the multiplexing mode, we measured X-ray energy resolutions of about 2.9 eV at 1.3 and 1.7 MHz.

The next generation of far infrared space observatories will require extremely sensitive detectors that can be realized only by combining extremely low intrinsic noise with high optical efficiency. We have measured the broad-band optical response of ultra-sensitive TES bolometers (NEP$\approx2\rm\ aW/\sqrt Hz$) in the 30--60-$\mu\rm m$ band where radiation is coupled to the detectors with a few-moded conical feedhorn and a hemispherical backshort. We show that these detectors have an optical efficiency of 60% (the ratio of the power detected by the TES bolometer to the total power propagating through the feedhorn). We find that the measured optical efficiency can be understood in terms of the modes propagating through the feedhorn with the aid of a spatial mode-filtering technique., Comment: 22 pages, 11 figures. The following article has been accepted by Review of Scientific Instruments. After it is published, it will be found at http://scitation.aip.org/content/aip/journal/rsi

We report the experimental evidence of the ac Josephson effect in a transition edge sensor (TES) operating in a frequency domain multiplexer and biased by ac voltage at MHz frequencies. The effect is observed by measuring the non-linear impedance of the sensor. The TES is treated as a weakly linked superconducting system and within the resistively shunted junction model framework. We provide a full theoretical explanation of the results by finding the analytic solution of the non-inertial Langevian equation of the system and calculating the non-linear response of the detector to a large ac bias current in the presence of noise., Comment: 4 pages, 4 figures

Transition edge sensors (TES) are the chosen detector technology for the SAFARI imaging spectrometer on the SPICA telescope. The TES are required to have an NEP of 2–3 × 10?19 W/? Hz to take full advantage of the cooled mirror. SRON has developed TiAu TES bolometers for the short wavelength band (30–60 ?m). The TES are on SiN membranes, in which long and narrow legs act as thermal links between the TES and the bath. We present a distributed model that accounts for the heat conductance and the heat capacity in the long legs that provides a guideline for designing low noise detectors. We report our latest results that include a measured dark NEP of 4.2×10?19 W/? Hz and a saturation power of about 10 fW.

A large number of low temperature detectors may be read out via few wires using frequency division multiplexing. In this scheme, each detector is biased by a sine wave with a unique carrier frequency. Numerous carriers can be combined in a single pair of wires. However, the combined signals occasionally result in large peak amplitudes in the bias or the readout. This stresses the capabilities of the electronics. We developed an efficient algorithm to optimize the phases and frequencies of the carriers in order to obtain a low crest factor. We also find that more regularly spaced carrier frequencies result in a lower crest factor, compared to irregularly spaced carriers.

We demonstrate a quasi-optical NbN hot electron bolometer (HEB) mixer using a twin-slot antenna on a Si lens to couple terahertz radiation. The mixer shows a receiver noise temperature of 1150 K at 2.5 THz, which is expected based on a model that includes quantum noise. The measured direct response is understood by taking into account the main beam efficiency and the parasitic reactance due to the geometric change between bolometer and transmission line. The measured beam of the mixer is nearly collimated and has a Gaussian beam efficiency of 90% with side-lobes below -16 dB.

This paper discusses the current distribution in thin-film devices, especially in a hot-electron bolometer (HEB) mixer at terahertz frequencies, and the consequences of different current distributions on the device impedance. We first present an approximate analytical model from which we derive a proposed rule of thumb for deciding when the film is thin enough to support a current distribution that is uniform in the transverse direction. We then verify this rule by performing electromagnetic simulations. Our conclusion is that the current distribution in thin films with a relatively high DC resistivity and small film thickness with respect to the skin depth is essentially uniform up to 8 THz. These results are crucial, e.g., for understanding radiation coupling between an HEB and an antenna and indispensable when analyzing detectors and receivers based on bolometric properties of thin films.

We measured the noise equivalent power (NEP) of an ultra-sensitive membrane-isolated transition-edge sensor (TES) designed for the SpicA FAR Infrared instrument (SAFARI) along with several Johnson noise Thermometry Devices (JTDs) simultaneously in a “dark” experimental setup. Our goal was to characterize the dark power PD present in our experimental setup and its effect on the measured NEP. By employing resistive attenuators, filters, lossy coaxial connections, and a light-tight box, we reduced PD to 0.25 fW in our setup. For the TES, G was measured to be (325±25) fW/K at TC=78 mK giving an upper-limit expected NEP of (3.5±0.25)×10-19 W/Hz1/2. We measured an actual NEP on the TES equal to (6.5±1.5)×10-19 W/Hz1/2, which is approximately double the expected value and likely due to residual electrical dark power in our setup.

Results of study optical performance of single pixel transition edge sensor (TES) bolometer presented at wavelength band 30-60 μm are presented. FIR radiation is coupled into a multimode horn with entrance aperture of 450 μm, length 4.5 mm and exit aperture of 45 μm , which feeds a metal integrating cavity containing the detector. The radiation band is defined by a pair of lowpass and highpass mesh filters in front of the horn. Here we present measurements of optical noise equivalent power (NEP), optical efficiency, dynamic range and time constant. The results show that measured TES detectors are close to meeting the requirement of the “Band S” of SAFARI FTS imaging instrument on the SPICA mission.

5 páginas, 6 figuras, 1 tabla.-- et al., We fabricated and characterized a low thermal conductance (G) transition edge sensor (TES). The device is based on a superconducting Ti/Au bilayer deposited on a 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 1×10-18 W/√Hz. The current noise and complex impedance are also measured at different bias points at 25 mK bath temperature. The measured electrical (dark) NEP is 2×10-18 W/√Hz, which is about a factor of 2 higher than what is expected from the thermal conductance that comes out of the IV curves analysis. 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 62 μs and a thermal capacity of 3 fJ/K., This work was supported in part by ESA TRP under Contract ITT-1-5922/08/NL/EM TES spectrometer.

Complex impedance measurements are widely used to characterize superconducting transition edge sensors (TESs) in bolometers and microcalorimeters. Typically, models are fit to impedance data to find parameters which pertain to the performance of these detectors. After the parameters are determined, the models are then used to compute the response and noise of these devices. In this paper, we present general relationships between the measured impedance, the thermal response to power in the TES, and noise. We describe a method for measuring αI and βI of the superconducting phase transition, which does not require model fitting. We find bolometer response is determined from the impedance provided that the absorber is strongly coupled to the TES electron system. We also demonstrate how to calculate upper and lower limits on the noise directly from the impedance data without modeling. Additionally, the relations can be used to check the validity of the models and to understand what information can and cannot be obtained from measurements of impedance, response, and noise.

We highlight the progress on NbN hot electron bolometer (HEB) mixers achieved through fruitful collaboration between SRON Netherlands Institute for Space Research and Delft University of Technology, the Netherlands. This includes the best receiver noise temperatures of 700 K at 1.63 THz using a twin-slot antenna mixer and 1050 K at 2.84 THz using a spiral antenna coupled HEB mixer. The mixers are based on thin NbN films on Si and fabricated with a new contact-process and-structure. By reducing their areas HEB mixers have shown an LO power requirement as low as 30 nW. Those small HEB mixers have demonstrated equivalent sensitivity as those with large areas provided the direct detection effect due to broadband radiation is removed. To manifest that a HEB based heterodyne receiver can in practice be used at arbitrary frequencies above 2 THz, we demonstrate a 2.8 THz receiver using a THz quantum cascade laser (QCL) as local oscillator.

TES-based bolometer and microcalorimeter arrays with thousands of pixels are under development for several space-based and ground-based applications. A linear detector response and low levels of cross talk facilitate the calibration of the instruments. In an effort to improve the properties of TES-based detectors, fixing the TES resistance in a resistance-locked loop (RLL) under optical loading has recently been proposed. Earlier theoretical work on this mode of operation has shown that the detector speed, linearity and dynamic range should improve with respect to voltage biased operation. This paper presents an experimental demonstration of multiplexed readout in this mode of operation in a TES-based detector array with noise equivalent power values (NEP) of $3.5\cdot 10^{-19} $W/$\sqrt{\mathrm{Hz}}$. The measured noise and dynamic properties of the detector in the RLL will be compared with the earlier modelling work. Furthermore, the practical implementation routes for future FDM systems for the readout of bolometer and microcalorimeter arrays will be discussed., Comment: 4 pages, 4 figures, ASC14

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. © (2014) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

We have measured the optical response of prototype detectors for SAFARI, the far-infrared imaging spectrometer for the SPICA satellite. SAFARI’s three bolometer arrays, coupled with a Fourier transform spectrometer, will provide images of a 2′×2′ field of view with spectral information over the wavelength range 34–210 μm. Each horn-coupled bolometer consists of a transition edge sensor (TES), with a transition temperature close to 100 mK, and a thin-film Ta absorber on a thermally-isolated silicon nitride membrane. SAFARI requires extremely sensitive detectors (NEP∼2×10−19 W/ $\sqrt{\mathrm{Hz}}$ ), with correspondingly low saturation powers (∼5 fW), to take advantage of SPICA’s cooled optics. To meet the challenge of testing such sensitive detectors we have constructed an ultra-low background test facility based on a cryogen-free high-capacity dilution refrigerator, paying careful attention to stray-light exclusion, shielding, and vibration isolation. For optical measurements the system contains internal cold (3–30 K) and hot (∼300 K) black-body calibration sources, as well as a light pipe for external illumination. We discuss our measurements of high optical efficiency in prototype SAFARI detectors and describe recent improvements to the test facility that will enable us to test the full SAFARI focal-plane arrays.

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 $5\cdot 10^{-19}W/Hz^{1/2}$ and saturation power of several fW, Comment: 7 pages, 6 figures, Proc. SPIE 8453, High Energy, Optical, and Infrared Detectors for Astronomy V, 845333 (25 September 2012). arXiv admin note: text overlap with arXiv:1604.02593

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 (

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 tool.

Transition-edge-sensor (TES) bolometers are the chosen detector technology for the SAFARI Imaging Spectrometer on the SPICA telescope. For this mission, SRON is developing bolometers, each consisting of a TiAu TES that is weakly coupled to the thermal bath through thin legs of silicon nitride. In order to understand and optimize the bolometer and to verify our detector models, we characterize the devices using a series of complex impedance measurements. We apply equivalent bolometer analysis (EBA) in combination with model fitting to these data. From this analysis, we obtain important parameters that give us confidence in our understating of the response and noise of these sensitive detectors.

At the Netherlands Institute for Space Research (SRON) we are developing Transition Edge Sensors (TES) bolometers for the SAFARI Imaging Spectrometer on the SPICA telescope. With the current design of the SPICA telescope a background magnetic field of about a Gauss with fluctuations of a couple percent is expected. The effect of this external magnetic field on the bolometer needs to be well understood in order to design effective shielding. We present the results of an investigation of magnetic field on the performance of TES bolometers. For this experiment we used a Ti/Au TES bolometer which has a measured NEP of 5.0 × 10?19 W/Hz1/2. The TES is exposed to a parallel and perpendicular magnetic field of varying strength and the baseline and responsivity are determined. We see that the bolometers are sensitive to the perpendicular field but are relative insensitive to the parallel magnetic field. Fluctuations with a period of 30 mG are present as function of perpendicular magnetic field

We have constructed a test facility for characterizing the focal plane arrays of SAFARI, the far-infrared imaging spectrometer for the SPICA satellite. SAFARI's three bolometer arrays are populated with extremely sensitive (NEP similar to 2 x 10(-19) W/root Hz) transition edge sensors with a transition temperature close to 100 mK. The extreme sensitivity and low saturation power (similar to 4 fW) of SAFARI's detectors present challenges to characterizing them. In optimizing the SAFARI Detector System Test Facility we have paid careful attention to stray-light exclusion as well as electrical, magnetic, and mechanical isolation. We present measurements verifying the facility's performance and analyze them in terms of a two-fluid model of the TES current on the transition to investigate the background power level. We have measured a detector NEP of (5.1 +/- 0.4) x 10(-19) WHz(-1/2), showing that the facility is ready to test the SAFARI prototype arrays and is approaching the performance needed for testing the flight arrays.

SRON is developing an electronic read-out system for an array of transition edge sensors (TES) based on the techniques of frequency domain multiplexing (FDM) and base-band feedback (BBFB). The astronomical applications of our system are the read-out of soft X-ray micro-calorimeters in a potential instrument on the European X-ray mission-under-study Athena and far-IR bolometers for the SAFARI instrument on the Japanese mission SPICA. In this paper we demonstrate the simultaneous locking of up to 51 BBFB loops. While locked, the in-band read-out noise of the loops is shown to reach the \(\mbox{10~pA/}\sqrt{\mathrm{Hz}}\) level required for these missions.

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.

Frequency domain multiplexing (FDM) is one of the candidates for the SQUID based readout of TES-based imaging microcalorimeter arrays for applications such as IXO, and imaging bolometer arrays for the SAFARI instrument on the Japanese SPICA space telescope. Usage of the TES as modulating element by applying an AC bias is an essential part of FDM. Within this framework the small signal characteristics of array pixels under both AC and DC bias have been measured. We will present a small signal model of the TES under AC bias, and apply the results to the measurements on an X-ray microcalorimeter.

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 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

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.

We report the measured sensitivities of a superconducting NbN hot electron bolometer (HEB) heterodyne receiver at 5.25 THz. Terahertz (THz) radiation is quasioptically coupled to a HEB mixer with a lens and a spiral antenna. 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 (DSB) receiver noise temperature (T^(DSB)_(rec)) of 1150 K, which is nine times hν/2k, where h is the Planck constant, ν the frequency, and k the Boltzmann constant. In addition, the measured far field beam patterns of the integrated lens antenna show nearly collimated beams from 2.5 to 5.3 THz that allow reliable measurement of (T^(DSB)_(rec)) using the vacuum setup. Our experimental results in combination with an antenna-to-bolometer coupling simulation suggest that the HEB mixer can work well at least up to 6 THz, making it suitable for next generation of high-resolution spectroscopic space telescopes and, in particular, for the detection of the neutral atomic oxygen line at 4.7 THz.

We have measured the noise temperature of a single, sensitive superconducting NbN hot electron bolometer (HEB) mixer in a frequency range from 1.6 to 5.3 THz, using a setup with all the key components in vacuum. By analyzing the measured receiver noise temperature using a quantum noise (QN) model for HEB mixers, we confirm the effect of QN. The QN is found to be responsible for about half of the receiver noise at the highest frequency in our measurements. The beta-factor (the quantum efficiency of the HEB) obtained experimentally agrees reasonably well with the calculated value.

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.