Superconducting bolometers for millimeter and submillimeter wave astronomy

Bolometers are simple and robust incoherent continuum detectors which nevertheless can reach sensitivities close to the fundamental noise limit. This thesis describes the theory, design, fabrication and testing of the superconducting bolometers, developed in collaboration between the Max Planck Institute for Radio Astronomy (MPIfR), Bonn and the Institute for Photonic Technology (IPHT), Jena, Germany. The voltage biased superconducting bolometer (VSB) offers various advantages over the traditional semiconducting bolometer; it is faster, more sensitive, has a higher dynamic range, allows complete microlithographic fabrication and can be multiplexed with Superconducting Quantum Interference Devices (SQUIDs). The low noise SQUID amplifiers operate at bolometer temperatures and have very low power dissipation. The low impedance characteristics of VSBs and SQUIDs makes them less sensitive to microphonic pickup and it is possible to achieve very low noise equivalent power (NEP) levels. The fabrication of bolometers with integrated SQUIDs and the multiplexing electronics will allow the production of bolometer arrays with several hundred or more pixels. The superconducting thermistor, deposited on the low stress silicon nitride membrane, is a bilayer of gold-palladium and molybdenum and is designed for a transition temperature of 450 mK. Bolometers for the 1.2 mm atmospheric window were designed, built and tested. Different test arrays with seven bolometers were fabricated to study the properties of the thermistor and the silicon nitride membrane. The thermal conductance (G) of the bolometer is tuned by structuring the silicon nitride membrane into spider-like geometries. The bolometers are divided into three different categories, High-G, Medium-G and Low-G, depending on their thermal conductance. The silicon nitride membrane is continuous for the High-G and it is structured into a spider-like geometry for Medium-G and Low-G bolometers. The thermal conductance of Low-G bolometers is too low for operating with a 300 K background, because under this condition, the bolometer will be driven into the normal conducting state by the radiation alone. The thermal conductance of Medium-G bolometers is appropriate for the operation with a 300 K background and for the experimental purposes the silicon nitride membrane of the Medium-G bolometer is structured into 8-legs, 16-legs and 32-legs spider geometries. The incident radiation is absorbed by crossed dipoles made from gold-palladium (Au-Pd) alloy with a surface resistance of 10 Ohms. The base temperature of 300 mK is provided by a liquid 4He cryostat with integrated 3He stage. The time constant of the bolometer is derived by measuring the modulated signal of a blackbody using a lock-in amplifier. The noise is measured as a timeseries and analyzed using National Instruments’ LabVIEW package. A bolometer model has been developed to understand the physics of the bolometer. Using the COSMOS finite element analysis (FEA) package, the thermal conductance is obtained for the bolometers of different geometries. The ideal performance of the bolometer is derived from VSB theory and the results from the bolometer model are compared with experimental results. FEA simulations showed that the deposition of a gold (Au) ring around the absorbing area could increase the sensitivity of the bolometer. Therefore, a new Medium-G layout was fabricated, with a gold ring around the absorbing center patch of the silicon nitride membrane. For the Medium-G bolometer without the gold ring, the measured optical noise equivalent power (NEP) is 1.9 × 10−16 W/√Hz and the time constant is in the range between 0.2 and 0.38 ms. For the Medium-G bolometer with gold ring, the measured NEP is 1.7 × 10−16 W/√Hz and the time constant is in the range between 1.4 and 2 ms. The gold ring increases the heat capacity, and this is a way to increase the time constant of the bolometer. This will be useful for time domain multiplexed arrays. The performance of Medium-G bolometers is close to the 300 K background limit in the 1.2 mm atmospheric window

An Investigation of the Longitudinal Proximity Effect in Superconducting and Normal Metal TES

As the TES volume and (effective) Tc become very small - for volume > the TES Tc, connected at opposite ends of TES approaches zero, superconductivity is induced parallel to the current flow, or longitudinally, and results in a much higher effective TES Te. Here we present effective Te measurements of Mo/Au TES bounded by Nb leads as a function of L which ranges between 4 and 36 micrometer. We observe that the effective Te is suppressed for current density of order 10(exp -6) A/sq micrometers. We also explore the possibility of using a normal metal TES

Development of Superconducting Transition Edge Sensors Based on Electron-Phonon Decoupling

We have successfully fabricated a superconducting transition edge sensor (TES), bolometer that centers on the use of electron-phonon decoupling (EPD) for thermal isolation. We have selected a design approach that separates the two functions of far-infrared and THz radiative power absorption and temperature measurement, allowing separate optimization of the performance of each element. We have integrated molybdenum/gold (Mo/Au) bilayer TES and ion assisted thermally evaporated (IAE) bismuth (Bi) films as radiation absorber coupled to a low-loss microstripline from niobium (Nb) ground plane to a twin-slot antenna structure. The thermal conductance (G) and the time constant for the different geometry device have been measured. For one such device, the measured G is 1.16 x 10(exp -10) W/K (plus or minus 0.61 x 10(exp- 10) W/K) at 60 mK, which corresponds to noise equivalent power (NEP) = 1.65 X 10(exp -18)W/vHz and time constant of approximately 5 microseconds

Ultrasensitive Superconducting Transition Edge Sensors Based On Electron-Phonon Decoupling

We have successfully fabricated the superconducting transition edge sensor (TES), bolometer technology that centers on the use of electron-phonon decoupling (EPD) to thermally isolate the bolometer. Along with material characterization for large format antenna coupled bolometer arrays, we present the initial test results of bolometer based on EPD designed for THz detection. We have selected a design approach that separates the two functions of photon absorption and temperature measurement, allowing separate optimization of the performance of each element. We have integrated Molybdenum/Gold (Mo/Au) bilayer TES and ion assisted thermally evaporated (IAE) Bismuth (Bi) films as radiation absorber coupled to a low-loss microstripline from Niobium (Nb) ground plane to a twin-slot antenna structure. The thermal conductance and the time constant of these devices have been measured, and are consistent with our calculations. The device exhibits a single time constant at 0.1 K of approx.160 IlS, which is compatible with readout by a high-bandwidth single SQUID or a time domain SQUID multiplexer. The effects of thermal conductance and electrothermal feedback are major determinants of the time constant, but the electronic heat capacity also plays a major role. The NEP achieved in the device described above is 2.5x10(exp -17)W(gamma)Hz. Our plan is to demonstrate a reduction of the volume in the superconducting element to 5 microns x 5 microns in films of half the thickness at Tc = 60mK. By calculation, this new geometry corresponds to an NEP reduction of two orders of magnitude to 2.5x10(exp -19)W/(gamma)Hz, with a time constant of ~130/ls

LABOCA commissioned on APEX Testing of Multi-conjugate AO Demonstrator Astroparticle Physics in Europe Weighing Ultracompact Dwarf GalaxiesTelescopes and Instrumentation A New Era in Submillimetre Continuum Astronomy has Begun: LABOCA Starts Operation on

LABOCA silicon wafer. Each small square is a bolometer. 1 �Max-Planck-Institut für Radioastronomie

5,120 Superconducting Bolometers for the PIPER Balloon-Borne CMB Polarization Experiment

We are constructing the Primordial Inflation Polarization Explorer (PIPER) to measure the polarization of the cosmic microwave background (CMB) and search for the imprint of gravity waves produced during an inflationary epoch in the early universe. The signal is faint and lies behind confusing foregrounds, both astrophysical and cosmological, and so many detectors are required to complete the measurement in a limited time. We will use four of our matured 1,280 pixel, high-filling-factor backshort-under-grid bolometer arrays for efficient operation at the PIPER CMB wavelengths. All four arrays observe at a common wavelength set by passband filters in the optical path. PIPER will fly four times to observe at wavelengths of 1500, 1100, 850, and 500 microns in order to separate CMB from foreground emission. The arrays employ leg-isolated superconducting transition edge sensor bolometers operated at 145 mK; tuned resonant backshorts for efficient optical coupling; and a second-generation superconducting quantum interference device multiplexer readout. We describe the design, development, and performance of PIPER bolometer array technology to achieve background-limited sensitivity for a cryogenic balloon-borne telescope

How does a simulated soccer match affect regional differences in biceps femoris muscle architecture?

Soccer is played by thousands of athletes across the globe and its participation increases the overall risk of injury, in particular, hamstring strain injuries (HSI). Biceps femoris (BF) has been shown to be involved the in 5 out of 6 HSI cases and risk factors including fatigue and short BF fascicle length (FL) have been identified. Furthermore, previous studies suggest that different muscle regions may undergo different strains during dynamic tasks, which could contribute to injury risk. The primary aim of this study was to evaluate the effects of a soccer match on regional differences in the BF muscle architecture. A secondary aim was to assess the reliability of the extended field of view (EFOV) 2D ultrasound imaging to measure muscle architecture parameters. Muscle architecture was assessed, using ultrasound, in 9 amateur soccer players and 5 physically active men, before and after a 45 minutes soccer specific fatigue protocol (SAFT)or 20 mintues of rest, respectively. Significant muscle architecture changes were found after SAFT, however, these were smaller than the minimal detectable change associated with the scanning method. No correlations were found between force reductions and muscle architecture changes. Good reliability was found for FL measurements but poor reliability was found for pennation angle and muscle thickness. Muscle architecture changes after 45 minutes of a football match may not be a mechanism to explain the increased HSI rates. Furthermore, when using EFOV ultrasound, care must be taken when interpreting statistically significant results, since these can be below the minimal detectable change or not reliable for all the parameters