R. A. Hijmering

Efficiency of quasiparticle creation in proximized superconducting photon detectors

In previous work using thin superconducting films as photon detectors it has been assumed implicitly that the quasiparticle yield in proximized superconducting bilayers should be the same as for a pure superconducting layer with the same energy gap. The reasoning is that, following the energy down conversion cascade, the resultant quasiparticles will all finish up at the edge of the density of states, which has the same energy throughout the whole structure regardless or whether it is pure or proximized. In this paper we show that, although the energy gap is the same, the actual density of quasiparticle states may vary considerably across a proximized structure, with a secondary peak at the energy of the higher gap material. Our calculations indicate that this peak can give rise to the generation of excess subgap phonons through which a larger portion of the original photon energy is lost from the quasiparticle system. The associated lower quasiparticle yield effectively reduces the responsivity of the pr...

Accurate time-resolved optical photospectroscopy with superconducting tunnel junction arrays

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 Agencys 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 instruments architecture and describe the systems 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 Stations 2005 and 2006 campaigns which demonstrate the systems timing, photometric and spectroscopic capabilities.

First results of a cryogenic optical photon-counting imaging spectrometer using a DROID array

Context. We present the first system test in which we demonstrate the concept of using an array of Distributed Read Out Imaging Devices (DROIDs) for optical photon detection. Aims. After the successful S-Cam 3 detector, the next step in the development of a cryogenic optical photon counting imaging spectrometer under the S-Cam project is to increase the field of view using DROIDs. With this modification the field of view of the camera has been increased by a factor of five in a given area while keeping the number of readout channels the same. Methods. The test has been performed using the flexible S-Cam 3 system and exchanging the 10 × 12 Superconducting Tunnel Junction array for a 3 × 20 DROID array. The extra data reduction needed with DROIDs is performed offline. Results. We show that, although the responsivity (number of tunnelled quasiparticles per unit of absorbed photon energy, e − /eV) of the current array is too low for direct astronomical applications, the imaging quality is already good enough for pattern detection and will improve further with increasing responsivity. Conclusions. The obtained knowledge can be used to optimise the system for the use of DROIDs.

Experimental and theoretical response of distributed read-out imaging devices with imperfect charge confinement

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.

First Results On The Imaging Capabilities Of A DROID Array In The UV/Visible

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 60 360×33.5u2009μm2 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.

Towards An Imaging Soft X‐ray Spectrometer Based On Superconducting Tunnel Junction Detectors

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.

Model On DROID Response With Imperfect Trapping Tested On Experimental Results

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.

Position dependent spatial and spectral resolution measurement of distributed readout superconducting imaging detectors

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...

Superconducting tunnel junction detectors for soft x-ray astrophysics

The requirement on energy resolution for detectors in future X-ray satelite missions such as XEUS (X-ray Evolving Universe Spectroscopy mission) is <2eV in the soft x-ray range of 50-2000 eV, with a detection efficiency >80%. In addition, the requirements for field of view and angular resolution demand a detector array of typically 150x150 micron sized pixels in a 30x30 pixel format. DROIDs (Distributed Read Out Imaging Devices), consisting of a superconducting absorber strip with superconducting tunnel junctions (STJs) as read-out devices on either end, can fulfill these requirements. The amplitudes of the two signals from the STJs provide information on the absorption position and the energy of the incoming photon in the absorber. In this paper we present the development status of Ta/Al 1-D DROIDs, as well as the the short term development program that should result in a full size XEUS array.

Imaging spectroscopy with Ta/Al DROIDs: performance for different geometries

In this paper we present the preliminary results from experiments with Distributed Read Out Imaging Devices (DROIDs) in the optical and in the X-ray regime. For the optical results DROIDs of different lengths ranging from 200 to 700 μm have been used with an STJ lay-up of Ta/Al/AlOx/Al/Ta with thicknesses of 100/30/1/30/100 nm. With this data the behavior with different absorber length has been investigated to determine an optimal absorber size for a DROID array to be used in the optical wavelength regime. The optimum absorber size has been found to be 30x400 um. The X-ray data has been obtained with a similar device structure but with 60 nm aluminium trapping layers to increase the trapping of quasiparticles in the STJs. In this paper we only present the data obtained with the standard DROID size of 400μm. With this device an extensive set of measurements have been performed which involves; a scan in photon energy ranging from 50eV to 1900 eV, a scan in temperature and a scan in bias voltage. We report here only results from the preliminary analysis of the data obtained with readout electronics comprising the normal preamplifier and subsequent shaping stage. For the final analyzes the pulses resulting from the STJs have been digitized and are ready to be analyzed. The pulses have been used to estimate the decay time of the STJs which appear to be very short. This is probably caused by the poor trapping of quasiparticles. Detailed results on this process will be presented however at a later date.