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The electromagnetic spectrum in the terahertz frequency region is of significant importance for understanding the formation and evolution of galaxies and stars throughout the history of the universe and the process of planet formation. Within the star forming clouds the constituent atoms and molecules are excited to produce characteristic emission and absorption lines, many of which happen at the terahertz frequencies. Thus, detecting the spectral signatures as unique fingerprints of molecules and atoms require terahertz spectrometers, which need to be operated in a space observatory because of the water vapor absorption in the earth atmosphere. However, current terahertz spectrometers face several challenges that limit their performances and applications, including a low resolution, limited bandwidth, large volume, and complexity. In this paper, we address the last two issues by demonstrating a concept of a compact terahertz spectrometer using metasurface. We start by modelling, designing, and fabricating a metasurface, aiming to optimize its performance within a band from 1.7 to 2.5 THz. Next, we make use of an array of quantum cascade lasers that operate at slightly different frequencies around 2.1 THz to validate the performance of the spectrometer. Finally, we apply the spectrum inversion method to analyse the measured data to confirm a resolution R of at least 273. Our results demonstrated a miniaturized terahertz spectrometer concept successfully.

With applications in cosmology, infrared astronomy and CMB survey, frequency-division multiplexing (FDM) proved to be a viable readout for transition-edge sensors (TES). We investigate the occurrence of out-of-band resonances (OBR) which could constrain the bandwidth of the FDM readout of TES bolometers. The study includes SPICE modeling of the entire setup including the cryogenic harness, LC filters, Superconducting Quantum Interference Device (SQUID) and room-temperature amplifier. Simulation results show that the long harness (for flight model) could cause multiple reflections that generate repetitive spikes in the spectrum. Peaks of the OBR are mainly due to the parasitic capacitances at the input of SQUID. Implementing a low-pass RC circuit (snubber) at the input of the SQUID dampened the OBR. As a result, the first peak only appears around 20 MHz which is a safe margin for the 1 MHz-3.8 MHz FDM in use in the prototype readout. Using a spectrum analyzer and broadband LNAs,we also measured the OBR for the prototype FDM readout in the lab up to 500 MHz. The measurement was conducted at temperatures of 50 mK and 4 K and for various biasing of the DC SQUID. It turns out that OBR are more intense at 50 mK and are caused by the harness impedance mismatch rather than the SQUID. Simulation codes and supporting materials are available at https://github.com/githubamin/LT-Spice-Simulation-of-FDM-readout., Comment: Keywords: Out of Band Resonances(OBR), FDM, TES, Bolometer, SQUID, Infrared Astronomy. 7 pages, 7 Figures. Accepted for publication in Journal of Low Temperature Physics

In the early 2030s, ESAs new X-ray observatory, Athena, is scheduled to be launched. It will carry two main instruments, one of which is the X-ray Integral Field Unit (X-IFU), an X-ray imaging spectrometer, which will consist of an array of several thousand transition-edge sensors (TESs) with a proposed energy resolution of 2.5 eV for photon energies up to 7 keV. At SRON we develop the backup TES array based on Ti/Au bilayers with a transition temperature just below 100 mK. In this contribution we will give a broad overview of the properties and capabilities of these state-of-the-art detectors. Over the years we have fabricated and studied a large number of detectors with various geometries, providing us with a good understanding of how to precisely control the properties of our detectors. We are able to accurately vary the most important detector properties, such as the normal resistance, thermal conductance and critical temperature. This allows us to finely tune our detectors to meet the demands of various applications. The detectors have demonstrated excellent energy resolutions of below 1.8 eV for 5.9 keV X-rays. By tuning the properties of the devices, they can be optimally matched to various read-out schemes using both AC and DC biasing. The next step is to increase the size of our TES arrays from our current kilo-pixel arrays towards the full-sized array for X-IFU., Comment: Submitted as a conference proceeding for SPIE Astronomical Telescopes + Instrumentation 2022

We present a characterization of the sensitivity of TES X-ray micro-calorimeters to environmental conditions under frequency-domain multiplexing (FDM) readout. In the FDM scheme, each TES in a readout chain is in series with a LC band-pass filter and AC biased with an independent carrier at MHz range. Using TES arrays, cold readout circuitry and warm electronics fabricated at SRON and SQUIDs produced at VTT Finland, we characterize the sensitivity of the detectors to bias voltage, bath temperature and magnetic field. We compare our results with the requirements for the Athena X-IFU instrument, showing the compliance of the measured sensitivities. We find in particular that FDM is intrinsically insensitive to the magnetic field because of TES design and AC readout., Comment: This paper has been accepted for publication in Journal of Low Temperature Physics

Transition edge sensor (TES) microcalorimeters and bolometers are used for a variety of applications. The sensors are based on the steep temperature-dependent resistance of the normal-to-superconducting transition, and are thus intrinsically sensitive to magnetic fields. Conventionally the detectors are shielded from stray magnetic fields using external magnetic shields. However, in particular for applications with strict limits on the available space and mass of an instrument, external magnetic shields might not be enough to obtain the required shielding factors or field homogeneity. Additionally, these shields are only effective for magnetic fields generated external to the TES array, and are ineffective to mitigate the impact of internally generated magnetic fields. Here we present an alternative shielding method based on a superconducting groundplane deposited directly on the backside of the silicon nitride membrane on which the TESs are located. We demonstrate that this local shielding for external magnetic fields has a shielding factor of at the least ~ 75, and is also effective at reducing internal self-induced magnetic fields, as demonstrated by measurements and simulation of the eddy current losses in our AC biased detectors. Measurements of 5.9 keV X-ray photons show that our shielded detectors have a high resilience to external magnetic fields, showing no degradation of the energy resolution or shifts of the energy scale calibration for fields of several microTesla, values higher than expected in typical real-world applications., Comment: 11 pages, 8 figures

We have measured and characterized the thermal crosstalk in two different arrays of transition-edge sensor (TES) X-ray micro-calorimeters with frequency-domain multiplexing (FDM) readout. The TES arrays are fabricated at SRON and are a 8$\times$8 and a 32$\times$32 "kilo-pixel" uniform array. The amount of crosstalk is evaluated as the ratio between the averaged crosstalk signal and the X-ray pulse amplitudes. The crosstalk ratios (CR) for our detectors are compliant with the requirements for future X-ray space missions, such as Athena X-IFU (CR$< 10^{-3}$ for first-neighbour pixels): we measured a nearest-neighbour thermal crosstalk ratio at a level of $10^{-4}$, with a highest crosstalk ratio of $4\times 10^{-4}$ for the kilo-pixel array (worst case, center of array) and $1\times 10^{-4}$ for the 8$\times$8 array, with a margin of improvement achievable by optimizing the Cu metallization and the width of the Si supporting structures (muntins) in the backside of the TES array chip. Based on the measured crosstalk ratios, we have estimated the impact on the spectral resolution by means of noise equivalent power (NEP) considerations and a Monte Carlo simulation, finding an average degradation in quadrature of less than 40~meV, compliant with the < 0.2~eV requirement for Athena X-IFU.

We are developing a kilo-pixels Ti/Au TES array as a backup option for Athena X-IFU. Here we report on single-pixel performance of a 32$\times$32 array operated in a Frequency Division Multiplexing (FDM) readout system, with bias frequencies in the range 1-5 MHz. We have tested the pixels response at several photon energies, by means of a $^{55}$Fe radioactive source (emitting Mn-K$\alpha$ at 5.9 keV) and a Modulated X-ray Source (MXS, providing Cr-K$\alpha$ at 5.4 keV and Cu-K$\alpha$ at 8.0 keV). First, we report the procedure used to perform the detector energy scale calibration, usually achieving a calibration accuracy better than $\sim$ 0.5 eV in the 5.4 - 8.9 keV energy range. Then, we present the measured energy resolution at the different energies (best single pixel performance: $\Delta$E$_{FWHM}$ = 2.40 $\pm$ 0.09 eV @ 5.4 keV; 2.53 $\pm$ 0.10 eV @ 5.9 keV; 2.78 $\pm$ 0.16 eV @ 8.0 keV), investigating also the performance dependency from the pixel bias frequency and the count rate. Thanks to long background measurements ($\sim$ 1 day), we finally detected also the Al-K$\alpha$ line at 1.5 keV, generated by fluorescence inside the experimental setup. We analyzed this line to obtain a first assessment of the single-pixel performance also at low energy ($\Delta$E$_{FWHM}$ = 1.91 eV $\pm$ 0.21 eV @ 1.5 keV), and to evaluate the linearity of the detector response in a large energy band (1.5 - 8.9 keV)., Comment: Accepted for publication in IEEE Transactions on Applied Superconductivity for ASC2020 special issue

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 (<8 K) telescope. In order to read out the total of ~3500 detectors we use frequency domain multiplexing (FDM) with baseband feedback. In each multiplexing channel, a two-stage SQUID preamplifier reads out 160 detectors. We describe the detector system and discuss some of the considerations that informed its design., Comment: 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)

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., Comment: 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

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 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., Comment: 6 pages, 5 figures

We characterised a TES-based X-ray microcalorimeter in an adiabatic demagnetisation refrigerator (ADR) using synchrotron radiation. The detector response and energy resolution was measured at the beam-line in the PTB radiometry laboratory at the electron storage ring BESSY II in the range from 200 to 1800 eV. We present and discuss the results of the energy resolution measurements as a function of energy, beam intensity and detector working point. The measured energy resolution ranges between 1.5 to 2.1 eV in the investigated energy range and is weakly dependent on the detector set point. A first analysis shows a count-rate capability, without considerable loss of performance, of about 500 counts per second., Comment: 6 pages, 5 figures, Proceedings of the 12th International Workshop on Low Temperature Detectors (LTD12)

It has been recently demonstrated that superconducting transition edge sensors (TESs) behave as weak-links due to longitudinally induced superconductivity from the leads with higher Tc. In this work we study the implication of this behaviour for TES-based bolometers and microcalorimeter under ac bias. The TESs are read-out at frequencies between 1 and 5 MHz by a Frequency Domain Multiplexer based on a linearised two-stage SQUID amplifier and high- Q lithographically made superconducting LC resonators. In particular, we focus on SRON TiAu TES bolometers with a measured dark Noise Equivalent Power (NEP) of $3.2 \cdot 10^{-19} W/Hz^{1/2}$ developed for the short wavelength band for the instrument SAFARI on the SPICA telescope., Comment: 6 pages, 5 figure

We are developing transition-edge sensor (TES)-based microcalorimeters for the X-ray Integral Field Unit (XIFU) of the future European X-Ray Observatory Athena. The microcalorimeters are based on TiAu TESs coupled to 250{\mu}m squared, AuBi absorbers. We designed and fabricated devices with different contact geometries between the absorber and the TES to optimise the detector performance and with different wiring topology to mitigate the self-magnetic field. The design is tailored to optimise the performance under Frequency Domain Multiplexing. In this paper we review the main design feature of the pixels array and we report on the performance of the 18 channels, 2-5MHz frequency domain multiplexer that will be used to characterised the detector array., Comment: 9 pages, 8 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

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