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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.

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.

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.

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.

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$\le2\times10^{-19}\rm\ W/\sqrt Hz$) needed to take advantage of SPICA's cold (, 7 pages, 3 figures, Proc. SPIE 10708, Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy IX, 107080K (9 July 2018); (fixed typo in abstract)

Large Transition Edge Sensor (TES) arrays, required for the SAFARI imaging spectrometer on a planned SPICA space telescope, will be read out by Frequency Division Multiplexing (FDM). All the pixels are connected with densely packed and equally long superconducting wires. Inductive coupling between wire pairs forms critical performance issues. Mutual inductance is one of the origins of electronic crosstalk among pixels. We find that available literature is not complete in providing suitable calculation methods for the required modelling. In this paper, we describe a relatively fast inductance calculation method for long, arbitrary wires, which can be useful for analyzing crosstalk between pixels and can also be important for guiding wiring layout design. The mutual inductance between two wire loops is calculated using Neumann's formula by a double summation of analytical expressions implemented in Python. Self-inductance for a multi-segment wire can be calculated by the summation of self-inductance of each wire segment and the mutual inductance between all segments. Combining this method with other modelling techniques, we have simulated the inductances of an entire 160 pixel FDM test setup.

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.

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

SRON is developing the Frequency Domain Multiplexing (FDM) readout and the ultra low NEP TES bolometers array for the infrared spectrometer SAFARI on board of the Japanese space mission SPICA. The FDM prototype of the instrument requires critical and complex optimizations. For single pixel characterization under AC bias we are developing a simple FDM system working in the frequency range from 1 to 5 MHz, based on the open loop read-out of a linearized two-stage SQUID amplifier and high Q lithographic LC resonators. We describe the details of the experimental set-up required to achieve low power loading (< 1 fW) and low noise (NEP $\sim 10^{-19} W/Hz^{1/2}$) in the TES bolometers. We conclude the paper by comparing the performance of a $4 \cdot 10^{-19} W/Hz^{1/2}$ TES bolometer measured under DC and AC bias., 6 pages, 5 figures

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.

SRON is developing Frequency Domain Multiplexing (FDM) for the read-out of transition edge sensor (TES) soft x-ray microcalorimeters for the XMS instrument of the International X-ray Observatory and far-infrared bolometers for the SAFARI instrument on the Japanese mission SPICA. In FDM the TESs are AC voltage biased at frequencies from 0.5 to 6 MHz in a superconducting LC resonant circuit and the signal is read-out by low noise and high dynamic range SQUIDs amplifiers. The TES works as an amplitude modulator. We report on several AC bias experiments performed on different detectors. In particular, we discuss the results on the characterization of Goddard Space Flight Center x-ray pixels and SRON bolometers. The paper focuses on the analysis of different read-out configurations developed to optimize the noise and the impedance matching between the detectors and the SQUID amplifier. A novel feedback network electronics has been developed to keep the SQUID in flux locked loop, when coupled to superconducting high Q circuits, and to optimally tune the resonant bias circuit. The achieved detector performances are discussed in view of the instrument requirement for the two space missions.

We have developed a microscopic model of the transition edge sensor (TES). In order to describe the TES as a laterally proximized S-S'-S weak link we used the Usadel equations. We developed a fully self-consistent approach, taking into account the appropriate boundary conditions at the contacts with the large gap superconducting leads. We have obtained an analytical solution of the Usadel equations for the simplest TES geometry. For a more general situation we discuss numerical solutions. We present the explicit form of the lateral profile of the order parameter for a typical TES sensor as a function of temperature, size and bias current. We calculate current-phase relations and the critical Josephson current, and discuss the important differences between modeling of the TES as either S-S'-S or S-N-S link.

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.

In previous attempts to model the behavior of the charge output of superconducting tunnel junctions, we have postulated the existence of traps which locally confine quasiparticles. Recently, we gathered more insight in the nature of these traps and developed a theory of inelastic transitions incorporating intra-gap localized states at magnetic impurities.

In a superconductor with magnetic impurities, scattering of conduction electrons results in the formation of localized states inside the superconducting gap. We show that inelastic electronic transitions involving quasiparticle scattering into and out of the localized states result in significant changes in the non equilibrium properties of the superconductor. The individual processes are: quasiparticle trapping into discrete states, enhanced recombination with localized quasiparticles, and pair breaking and de-trapping of localized quasiparticles by sub-gap phonons. We find that all these processes give rise to clearly distinguishable temperature dependences of the kinetic parameters indicating a possible role of magnetic scattering on the response of a photon detector.

The DROID (Distributed Read-Out Imaging Detector) is being developed to overcome the limitation in sensitive area with the use of single STJ’s (Superconducting Tunnel Junctions). The DROID configuration allows the reconstruction of the position of the photon absorption and therefore it can replace a number of single STJ’s in a detector array. The position resolution dictates how many STJ the DROID can replace. We present direct measurements of the position resolution in DROIDs, using a 10 µm spot of visible light which illuminates the DROID locally and which is scanned along the absorber length. The DROIDs used for the measurements have 100 nm thick Ta absorber strips with Ta/Al/AlO x /Al/Ta STJ’s on either end. The STJ’s are square with the same size as the absorber width and the base Ta layer are adjacent to the absorber. The position resolution is measured for absorber length ranging from 200 to 400 µm and 30 µm width.

X-ray imaging is an essential tool for a wide range of disciplines. Whilst the majority of applications rely on analog integrating or on photon counting formats, the future development of this field lies in the exploitation of spatially resolved spectroscopy and in particular, using materials which can operate with near Fano limited energy resolution at room temperature. This communication focuses on a new prototype 16×16 cadmium zinc telluride (CdZnTe) pixel array fabricated by eV Products [ http://www.evproducts.com/ [1] ], which, assembled with an ASIC made by Ideas ASA [ http://www.ideas.no/products/ASICs/pdf/XA1.6.pdf [2] ], provides both imaging and spectroscopic capabilities at the individual pixel level. We report detailed results of hard X-ray measurements carried out at the X1 beam line of the HASYLAB synchrotron radiation facility in Hamburg, Germany [ http://www-hasylab.desy.de/facility/ [3] ].

Superconducting Tunnel Junctions (STJs) have been extensively investigated as photon detectors covering the range from near-infrared to X-ray energies. A 6×6 array of Ta junctions has already been used in an optical spectro-photometer. With this camera, the European Space Agency has performed multiple astronomical observations of optical sources using the William Herschel 4.2 m telescope at La Palma. Following the success of this programe, we are now completing the development of a second generation camera. The goals of this programe were to increase the field of view of the instrument from 4”×4” to 11”×9”, to optimize IR rejection filters (possibly extending the ‘red’ response to ∼1 μm), and to increase the electronics readout speed. For these purposes, we have developed a new STJ array consisting of 10×12 Ta/Al devices as well as an improved readout system. In this paper, we review the instrument's architecture and describe the performance of the new detector array.

For the read out of the Transition Edge Sensors (TES) bolometer arrays of the SAFARI instrument on the Japanese background-limited far-IR SPICA mission SRON is developing a Frequency Domain Multiplexing (FDM) read-out system. The next step after the successful demonstration of the read out of 38 TES bolometers using FDM was to demonstrate the FDM readout of the required 160 TES bolometers. Of the 160 LC filter and TES bolometer chains 151 have been connected and after cooldown 148 of the resonances could be identified. Although initial operation and locking of the pixels went smoothly the experiment revealed several complications. In this paper we describe the 160 pixel FDM set-up, show the results and discuss the issues faced during operation of the 160 pixel FDM experiment.

Operation of TES-based bolometers in a resistance locked loop (RLL), i.e. keeping the operating resistance constant by means of feedback on the bias voltage, provides a number of attractive properties for applications. In combination with frequency domain multiplexing, the technique reduces electrical cross talk, and provides a more detector load independent behaviour with respect to operation under standard voltage bias. This paper shows a quantitative estimation of the large signal properties under the RLL, and shows a comparison with the situation under voltage bias. Furthermore, an unorthodox, low-bandwidth mode of operation will be discussed, from the perspective of the electro-thermal stability in the RLL.

SRON is developing the electronic read-out for arrays of transition edge sensors using frequency domain multiplexing in combination with base-band feedback. The astronomical applications of this 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 read-out of 38 bolometer pixels at a 1 $$-$$ 2 aW/ $$\surd $$ Hz dark NEP level. The stability of the read-out is assessed over 400 s. time spans. Although some 1/f noise is present, there are several bolometers for which 1/f-free read-out can be demonstrated.

To overcome the limited field of view that can be achieved with single Superconducting Tunneling Junctions (STJ) arrays, Distributed Read Out Imaging Devices (DROIDs) are being developed. DROIDs consist of a superconducting absorber strip with proximized STJs on either end. The ratio of the two signals from the STJs provides information on the absorption position and the sum signal is a measure for the energy of the absorbed photon. To produce a large field of view with the least number of connection wires possible, the size of the DROID is an important parameter. A set of devices with different lengths, ranging from 200 to 700 μm, has been tested at optical wavelengths. The widths of the DROIDs are 30 μm with 30×30 μm2 STJs Ta/Al STJs on either side. With 30 nm layer thickness of Al the trapping of quasiparticles in the STJ is not optimal, but the devices can comfortably be operated at 300 mK. All devices have been processed on a single wafer and are located on the same chip. Thus the STJs are all identical and any variation in response can be attributed to a difference in geometry. The position resolution is found to be degraded for shorter absorbers due to cross-talk between the two STJs. The charge output of the different devices decreases with length due to reduced tunnel probability and losses in the absorber.

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

SRON is developing an electronic system for the multiplexed read-out of an array of transition edge sensors (TES) by combining the techniques of frequency domain multiplexing (FDM) with base-band feedback (BBFB). The astronomical applications are the read-out of soft X-ray microcalorimeters and the far-infrared bolometers for the SAFARI instrument on the Japanese mission SPICA. In this paper we derive the requirements for the read-out system regarding noise and dynamic range in the context of the SAFARI instrument, and demonstrate that the current experimental prototype is capable of simultaneously locking 57 channels and complies with these requirements.

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.

We present results of the analysis of electrical noise in a transition edge sensor (TES) as a resistively shunted conducting junction (the RSJ model). We derive an expression for the spectral density of voltage fluctuations taking into account weak superconductivity of a TES. We analyse and discuss differences in voltage noise in a TES compared to the familiar situation for the Josephson junction. The spectral density of voltage noise in the RSJ model has a unique analytical structure and cannot be reduced to the expression for the nonlinear Johnson noise near equilibrium

We have developed a model for the resistive transition in a transition edge sensor (TES) based on the model of a resistively shunted junction, taking into account phase-slips of a superconducting system across the barriers of the tilted washing board potential. We obtained analytical expressions for the resistance of the TES, R(T,I), and its partial logarithmic derivatives α I and β I as functions of temperature and current. We have shown that all the major parameters describing the resistive state of the TES are determined by the dependence on temperature of the Josephson critical current, rather than by intrinsic properties of the S-N transition. The complex impedance of a pristine TES exhibits two-pole behaviour due to its own intrinsic reactance.

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 present a model to describe the responsivity of distributed read-out imaging devices following photon absorption in the absorber or in the base or top film of the superconducting tunnel junctions at either end of the absorber. The model describes the processes most relevant for photon detection, taking into account diffusion of quasiparticles across the absorber and imperfect confinement in the superconducting tunnel junctions via exchange of quasiparticles between absorber and the junction. It incorporates diffusion mismatch between superconducting tunnel junction and absorber, possible asymmetry between the two junctions and asymmetry between base and top electrodes within each junction. We have conducted dedicated experiments in which different experimental conditions were varied in order to test the model. A good agreement was found between the experimental results and model predictions.

Within the SCAM project of the European Space Agency the next step in the development of a cryogenic optical photon counting imaging spectrometer would be to increase the field of view using DROIDs (Distributed Read‐Out Imaging Detector). We present the results of the first system test using an array of 60 360×33.5 μm2 DROIDs in a 3×20 format for optical photon detection. This is an increase in area by a factor of 5.5 compared to the successful S‐Cam 3 detector. The responsivity of the DROID array tested is too low for actual use on the telescope. However the spatial resolution of ∼35 μm is just above the size of a virtual pixel and imaging capabilities of the array can be demonstrated. With increasing responsivity this will improve, yielding a DROID array which can be used as an astronomical optical photon counting imaging spectrometer.

We describe the next generation of our Distributed Read‐Out Imaging X‐ray Detectors based on superconducting tunnel junctions (STJs). Various shapes of the readout STJs and sizes of absorbers are compared with a view to optimizing the sensitive area and fill factor of future arrays while maintaining good energy resolution. Also different absorber configurations will be explored: the traditional integrated absorber is compared with an absorber deposited after definition of the read‐out structures. In support of this work, the feasibility of Re as an absorber is demonstrated with the detection of single UV photons in a Re/Al STJ. In addition, our tunnel barriers have successfully been made more transparent without loss of their low leakage properties. Finally, Nb contacts which prevent quasiparticles from escaping the STJs are identified as a source of device to device variability in the performance.

The DROID (Distributed Read‐Out Imaging Detector) is being developed to overcome the limitation in sensitive area with the use of single STJ’s (Superconducting Tunnel Junctions). The DROID configuration allows the reconstruction of the position of the photon absorption and therefore it can replace a number of single STJ’s in a detector array. We present a 2D model which describes the response of DROIDs with partial trapping in the STJs. The model describes diffusion of quasiparticles (qps) and imperfect confinement via exchange of qps between the absorber and STJ. It incorporates possible diffusion mismatch between absorber and STJ, possible asymmetry between the STJs as well as between the base and top electrodes of the STJs, and photon absorption in the absorber or base or top film of the STJ. Dedicated experiments have been conducted to test the different aspects of the model. We find a good agreement between the model and experimental results.

We present direct measurements of spatial and spectral resolution of cryogenic distributed readout imaging detectors (DROIDs). The spatial and spectral resolutions have been experimentally determined by scanning a 10μm spot of monochromatic visible light across the detector. The influences of the photon energy, bias voltage, and absorber length and width on the spatial and spectral resolutions have been examined. The confinement of quasiparticles in the readout sensors (superconducting tunnel junctions) as well as the detector’s signal amplitude can be optimized by tuning the bias voltage, thereby improving both the spatial and spectral resolutions. Changing the length of the absorber affects the spatial and spectral resolutions in opposite manner, making it an important parameter to optimize the DROID for the application at hand. The results have been used to test expressions for photon energy, position, and spatial and spectral resolutions which have been derived by using an existing one-dimensional mod...

The recent introduction of coplanar grid techniques has led to resurgence in interest in developing large volume compound semiconductor detectors that have a reasonable γ-ray response but also good spectroscopic resolution. We report the results of a series of hard X- and γ-ray measurements on a large 15×15 mm 2 , 10 mm thick CdZnTe coplanar grid detector. The measurements were carried out at the HASYLAB and ESRF synchrotron radiation facilities using highly monochromatic pencil beams across the energy range 20–800 keV. Additional full-area measurements were carried out using radioactive sources. All measurements were carried out at room temperature. The measured energy resolution under full-area illumination was 8 and 12 keV FWHM at 59.95 and 662 keV, respectively. Under pencil beam illumination, the measured resolutions were essentially the same. The detector energy response function was found to be linear over the energy range 20–1400 keV with an average rms nonlinearity of 1.4%, consistent with statistics. The spatial uniformity of the detector was evaluated at 30, 60 and 180 keV by raster scanning a 20×20 μm 2 monoenergetic X-ray beam across the active area. Apart from a few localized areas, the detector response was found to uniform at the few percent level, consistent with statistics. At 180 keV, the nonuniformity in the energy response due to the grids was estimated to be at the 0.7% level.

To overcome the limited field of view, which can be achieved with single superconducting tunneling junction (STJ) arrays, distributed read-out imaging devices (DROIDs) are being developed. DROIDs consist of a superconducting absorber strip with proximized STJs on either end. The ratio of the two signals from the STJs provides information on the absorption position, and the sum signal is a measure for the energy of the absorbed photon. In our devices, the absorber is an epitaxial Ta strip that extends underneath the Ta/Al read-out STJs. Thus, the bottom electrode of the STJs is an integral part of the absorber. Due to the proximity effect, the STJs have a lower energy gap than the absorber, causing trapping of quasiparticles (QPs) in the STJs. The trapping will change with thicker Al layers because the energy gap of the devices will decrease. A series of 50×200 μm2 and 20×200 μm2 absorbers (including 50×50 μm2 STJs) and different Al trapping layer thicknesses, ranging from 65 to 130 nm, have been tested. The devices have been illuminated with 6 keV 55Fe photons. The position resolution is found to improve with increasing Al thickness. It is found that the current model needs to be adapted for DROIDs to account for different injection of QPs into the STJ and extra losses to the absorber.

Superconducting Tunnel Junctions (STJs) have been extensively investigated as photon detectors covering the range from near-infrared to X-ray energies. A 10×12 array of Tantalum/Aluminium junctions has been integrated into the S-Cam3 camera for ground based astronomy. With this camera, the European Space Agency has performed multiple astronomical observations of optical sources using the William Herschel 4.2m telescope at La Palma and the Agency's 1-m Optical Ground Station telescope at Tenerife. Compared to its predecessor, this new instrument features a 10"×12" field-of-view, an optimized IR rejection reducing baseline noise and increasing optical light throughput and ultra-stable operations. In this paper, we review the instrument's architecture and describe the system's performance and in particular the energy resolution and count-rate capabilities of the detector arrays. Finally, we shall present first astronomical images taken during the Optical Ground Station's 2005 and 2006 campaigns which demonstrate the system's timing, photometric and spectroscopic capabilities.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

S-Cam 3 is the third generation of a cryogenic camera, based on superconducting tunnel junctions (STJs), for ground-based optical astronomy, deployed at the 4.2 m William Herschel Telescope (WHT) at La Palma (Spain). It exploits a 10×12 pixel array of Ta/Al STJs, covering a field of view of ∼9″×11″ on the sky. The wavelength band extends from 330–750 nm, with a wavelength resolving power of ∼10 at 500 nm. The detectors are operated at ∼285 mK, achieved with a double stage 4He–3He sorption cooler. Each pixel has its own electronic readout chain at room temperature, with a JFET-based charge sensitive preamplifier. The instrument has undergone extensive testing and calibration, followed by the first observation campaign at La Palma in July 2004. This campaign has focused on point sources with time variability, exploiting the instrument's unique combination of spectrophotometry with high time resolution.

In the process to develop the transition edge sensor (TES) bolometer technology for the SAFARI FIR Imaging Spectrometer on the SPICA telescope and for future FIR space telescopes, we have recently demonstrated a TiAu TES with a Ta thin film absorber on a thin SiN membrane, which has a dark NEP as low as 4.2 × 10-19 W/√Hz. It is known that superconductors are sensitive to magnetic fields. Therefore, it is important to know how these extremely sensitive TES detectors are affected by external magnetic fields. This is desirable from an application point of view since the design of the SAFARI instrument demands such knowledge and interesting from a fundamental point of view. In this paper, we will describe a detailed experimental study of the effects of magnetic fields, both parallel and perpendicular to the TiAu plane, on the performance of a TES. We investigated the effect on the current, responsivity, speed, and TES parameters of α (dependence of temperature) and β (dependence of current), around the operating points. In order to achieve this highly sensitive measurement, the temperature stability has been improved and a new coil assembly has been designed. The latter allows accurately applying a parallel magnetic field.