Here we report on the optical design and on the spectral-spatial characterisation of a small 16 pixel camera. The prototype uses TES detectors with NEPs ~10-16 W/Hz0.5 which have been fabricated with near identical optical coupling structures to mimic their much lower NEP counterparts (~10-19 W/Hz0.5). This modification, which is achieved through changing only the pixel thermal conductance, G, has allowed us to perform spectral/spatial cryogenic testing using a 100mK ADR to view room temperature thermal sources. The measurements show a flat spectral response across the waveband and minimal side lobe structure in the antenna patterns down to 30dB.

The motivation for this work is to develop submillimeter wave and far-infrared imaging technology in which each detector in a focal plane is intrinsically capable of yielding detailed spectroscopic information.A first step towards this is the development of the CAMELS instrument, which will eventually be used to survey nearby galaxies at 3 mm with a spectral resolution, R = δλ/λ, of about 3000.The CAMELS instrument is based on the Microwave Kinetic Inductance Detector (MKID), operating at 100 mK, combined with an integrated filter bank design to provide 512 spectral channels between 103.0 and 114.7 GHz.In this work, we present the ongoing optical measurements of the CAMELS detectors, including dark tests and planned line source and gas cell tests.

We describe the geometry, architecture, dark-and optical performance of ultra-low-noise transition edge sensors as THz detectors for the SAFARI instrument. The TESs are fabricated from superconducting Mo/Au bilayers coupled to impedance-matched superconducting beta-phase Ta thin-film absorbers. The detectors have phonon-limited dark noise equivalent powers of order 0.5 - 1.0 aW/root Hz and saturation powers of order 20 - 40 fW. The low temperature test configuration incorporating micro-machined backshorts is also described, and construction and typical performance characteristics for the optical load are shown. We report preliminary measurements of the optical performance of these TESs for two SAFARI bands; L-band at 110 - 210 mu m and S-band 34 - 60 mu m.

The aim of CAmbridge Emission Line Surveyor (CAMELS) is to provide an operational demonstration of an Integrated Filter Bank Spectrometer (IFBS) for mm-wave astronomy. The prototype will observe from 103-114.7 GHz, providing of order 500 channels with a spectral resolution of 3000. In this paper we discuss the design of the instrument and ongoing work towards its realisation. Fabrication of a first set of devices to verify the key technologies has recently been completed. We will present results from a measurement campaign to characterise resonator performance and describe our planned optical tests.

In order to achieve low Noise-Equivalent-Powers (NEP < 1 aW Hz$^{-1/2}$) Transition Edge Sensors (TES) require high levels of thermal isolation between the superconducting bilayer and the heat bath. We propose that short micro-machined acoustic interferometers can be used for low-noise thermal isolation, avoiding many of the difficulties inherent in conventional, long- legged TES designs. In this paper, we present a detailed elastic wave model of interferometric phononic legs, and demonstrate the successful fabrication of TESs that are thermally isolated by interferometric structures.

We have carried out a detailed programme to explore the superconducting characteristics of reactive DC-magnetron sputtered NbN. The basic principle is to ignite a plasma using argon, and then to introduce a small additional nitrogen flow to achieve the nitridation of a Nb target. Subsequent sputtering leads to the deposition of NbN onto the host substrate. The characteristics of a sputtered film depend on a number of parameters: argon pressure, nitrogen flow rate and time-evolution profile, substrate material, etc. Crucially, the hysteresis in the target voltage as a function of the nitrogen flow can be used to provide a highly effective monitor of nitrogen consumption during the reactive process. By studying these dependencies we have been able to achieve highly reproducible film characteristics on sapphire, silicon dioxide on silicon, and silicon nitride on silicon. Intrinsic film stress was minimised by optimising the argon pressure, giving NbN films having Tc = 14.65 K. In the paper, we report characteristics such as deposition rate, Residual Resistance Ratio (RRR), film resistivity, transition temperature, and stress, as a function of deposition conditions. In order to enhance our understanding of the microwave properties of the films, we fabricated a wide range of microstrip NbN resonators (half wavelength, quarter wavelength, ring resonators). In the paper, we provide an illustrative result from this work showing a 2.1097 GHz resonator having a Q of 15,962 at 3.3 K.

We have fabricated Transition Edge Sensors (TESs) whose thermal characteristics are completely characterised by few-mode ballistic phonon exchange with the heat bath. These TESs have exceptionally small amorphous SiNx support legs: 0.2 μm thick, 0.7–1.0 μm wide, and 1.0 to 4.0 μm long. We show, using classical elastic wave theory, that it is only necessary to know the geometry and bulk elastic constants of the material to calculate the thermal conductance and fluctuation noise. Our devices operate in the few-mode regime, between 5 and 7 modes per leg, and have noise equivalent powers (NEPs) of 1.2 aW Hz−1∕2. The NEP is dominated by the thermal fluctuation noise in the legs, which itself is dominated by phonon shot-noise. Thus, TESs have been demonstrated whose thermal characteristics are fully accounted for by an elastic noise-wave model. Our current devices, and second-generation devices based on patterned phononic filters, can be used to produce optically compact, mechanically robust, highly sensitive TES imaging arrays, circumventing many of the problems inherent in conventional long-legged designs.

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 (<93%) and high uniformity in terms of critical temperature (<5%) and normal resistance (7%) across the arrays. The measured dark NEPs are as low as 5×10-19 W/√Hz with a response time of about 1.4 ms at preferred operating bias point. The optical coupling is implemented using pyramidal horns array on the top and hemispherical cavity behind the chip that gives a measured total optical coupling efficiency of 30±7%.

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.

We describe a process route to fabricate arrays of Ultra-low-Noise MoAu Transition Edge Sensors (TESs). The low thermal conductance required for space applications is achieved using 200 nm-thick Silicon Nitride (SiN x ) patterned to form long-thin legs with widths of 2.1 μm. Using bilayers formed on SiN x islands from films with 40 nm-thick Mo and Au thicknesses in the range 30 to 280 nm deposited by dc-sputtering in ultra-high vacuum we can obtain tunable transition temperatures in the range 700 to 70 mK. The sensors use large-area absorbers fabricated from high resistivity, thin-film β-phase Ta to provide impedance-matching to incident radiation. The absorbers are patterned to reduce the heat capacity associated with the nitride support structure and include Au thermalizing features to assist the heat flow into the TES. Arrays of 400 detectors at the pixel spacing required for the long-wavelength band of the far-infrared instrument SAFARI are now being fabricated. Device yields approaching 99% are achieved.

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.

We report initial measurements on our first MoAu transition edge sensors (TESs). The TESs formed from a bilayer of 40 nm of Mo and 106 nm of Au showed transition temperatures of about 320 mK, higher than identical TESs with a MoCu bilayer which is consistent with a reduced electron transmission coefficient between the bilayer films. We report measurements of thermal conductance in the 200 nm thick silicon nitride SiN (x) support structures at this temperature, TES dynamic behaviour and current noise measurements.

The next generation of ultra-low-noise cryogenic detectors for space science applications require continued exploration of materials characteristics at low temperatures. The low noise and good energy sensitivity of current Transition Edge Sensors (TESs) permits measurements of thermal parameters of mesoscopic systems with unprecedented precision. We describe a radiometric technique for differential measurements of materials characteristics at low temperatures (below about 3K). The technique relies on the very broadband thermal radiation that couples between impedance-matched resistors that terminate a Nb superconducting microstrip and the power exchanged is measured using a TES. The capability of the TES to deliver fast, time-resolved thermometry further expands the parameter space: for example to investigate time-dependent heat capacity. Thermal properties of isolated structures can be measured in geometries that eliminate the need for complicating additional components such as the electrical wires of the thermometer itself. Differential measurements allow easy monitoring of temperature drifts in the cryogenic environment. The technique is rapid to use and easily calibrated. Preliminary results will be discussed.

A study of ultra-low-noise MoCu transition edge sensors (TESs) has been performed in the context of realizing the highly sensitive far infrared imaging arrays needed for the next generation of space telescopes. More than 50 TESs, on four different chips, cut out of two different wafers were characterized. The TESs were in the form of 16-element arrays and were read out using superconducting quantum interference device (SQUID) time division multiplexing. The devices were fabricated on 200-nm-thick silicon nitride membranes, with leg widths and lengths covering the ranges of 1–4 μm and 160–960 μm, respectively. The apparent critical temperatures varied over 110–127 mK, but it is shown that much of the variation was due to differential loading by stray light, amounting to 2 ± 2 fW across the array. The measured thermal conductances to the heat bath spanned the range 0.12–1.1 pW/K, with the lowest values being typical of those needed for ultra-low-noise operation. We also studied the inherent variation in the conductances of 15 nominally identical TESs on the same chip and found a value of ±10%, which is higher than that seen on our high-conductance devices designed for ground-based operation. We measured and modeled the electrical input impedance of a subset of these TESs, and studied their step responses. The models, based on previously determined material parameters, are in excellent agreement with the measurements. Dark noise spectra were recorded and compared with the same electrothermal models using the same parameters as the dynamical simulations. The measured noise is reasonably well described by the sum of the contributions from phonon noise in the legs, Johnson noise in the bilayer, and SQUID readout noise. Dark noise equivalent powers as low as 4.2 × 10−19 W/Hz were measured. The NEP was higher than the theoretical limit by a factor of about 1.6.

The next generation of space missions targeting far-infrared bands will require large-format arrays of extremely low-noise 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 multi-moded absorbers. We present a comparison of modelling techniques to analyse the optical efficiency of such light pipes and their interaction with the front end optics and detector cavity.

Planar waveguide E-field probes have been used in a number of millimeter wave instruments although usually for single polarization detection in rectangular waveguide. The coupling efficiency of these types of probes is found to be high over the full waveguide bandwidth. Because of the advantages in integration, scalability and mass of planar OMTs compared to external waveguide OMTs, a planar OMT coupled to a corrugated horn is a good option for future CMB polarisation missions such as B-Pol. We estimate the performance of planar OMTs where the signals from the probes are combined after detection (power combination) and before detection (field combination) and show that under ideal circumstances the two methods give equivalent performance over a 20% bandwidth. However, field combination suppresses coupling to higher order waveguide modes and therefore enables wider bandwidth operation (up to 40%). We present a design of an ortho-mode transducer (OMT) to separate two orthogonal linearly polarized signals traveling in a circular waveguide in the frequency band from 190-250 GHz band using 4 probes in circular waveguide. In this design, the power in each linear polarisation is divided between two opposite probes. A field combiner (hybrid coupler) combines the signals from the probes on opposite sides of the waveguide before detection using planar microstrip structures. We use Ansoft/HFSS 3-D electromagnetic simulation software and the Agilent Advanced Design System (ADS) to evaluate the performance of the design and to determine the optimal probe geometry and feed impedance. The main questions have been investigated for this technology are coupling efficiency and cross-polarization response, achievable bandwidth, and scalability.

The next generation of space missions targeting far-infrared bands will require large-format arrays of extremely low-noise 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 multi-moded 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.

Electromagnetic drop-on-demand direct ceramic inkjet printing (EM/DCIJP) was employed to fabricate dense yttria-stabilized zirconia (YSZ) electrolyte layers on a porous NiO–YSZ anode support from ceramic suspensions. Printing parameters including pressure, nozzle opening time and droplet overlapping were studied in order to optimize the surface quality of the YSZ coating. It was found that moderate overlapping and multiple coatings produce the desired membrane quality. A single fuel cell with a NiO–YSZ/YSZ (∼6μm)/LSM+YSZ/LSM architecture was successfully prepared. The cell was tested using humidified hydrogen as the fuel and ambient air as the oxidant. The cell provided a power density of 170mWcm−2 at 800°C. Scanning electron microscopy (SEM) revealed a highly coherent dense YSZ electrolyte layer with no open porosity. These results suggest that the EM/DCIJP inkjet printing technique can be successfully implemented to fabricate electrolyte coatings for SOFC thinner than 10μm and comparable in quality to those fabricated by more conventional ceramic processing methods.