With increasing array size, it is increasingly important to control stray radiation inside the detector chips themselves. We demonstrate this effect with focal plane arrays of absorber coupled Lumped Element microwave Kinetic Inductance Detectors (LEKIDs) and lens-antenna coupled distributed quarter wavelength Microwave Kinetic Inductance Detectors (MKIDs). In these arrays the response from a point source at the pixel position is at a similar level to the stray response integrated over the entire chip area. For the antenna coupled arrays, we show that this effect can be suppressed by incorporating an on-chip stray light absorber. A similar method should be possible with the LEKID array, especially when they are lens coupled.
The main advantage of Microwave Kinetic Inductance Detector arrays (MKID) is their multiplexing capability, which allows for building cameras with a large number of pixels and good sensitivity, particularly suitable to perform large blank galaxy surveys. However, to have as many pixels as possible it is necessary to arrange detectors close in readout frequency. Consequently KIDs overlap in frequency and are coupled to each other producing crosstalk. Because crosstalk can be only minimised by improving the array design, in this work we aim to correct for this effect a posteriori. We analysed a MKID array consisting of 880 KIDs with readout frequencies at 4-8 GHz. We measured the beam patterns for every detector in the array and described the response of each detector by using a two-dimensional Gaussian fit. Then, we identified detectors affected by crosstalk above -30 dB level from the maximum and removed the signal of the crosstalking detectors. Moreover, we modelled the crosstalk level for each KID as a function of the readout frequency separation starting from the assumption that the transmission of a KID is a Lorenztian function in power. We were able to describe the general crosstalk level of the array and the crosstalk of each KID within 5 dB, so enabling the design of future arrays with the crosstalk as a design criterion. In this work, we demonstrate that it is possible to process MKID images a posteriori to decrease the crosstalk effect, subtracting the response of each coupled KID from the original map.
Here we summarize the initial results from a complex field radiation pattern measurement of a kinetic inductance
detector instrument. These detectors are phase insensitive and have thus been limited to scalar, or amplitude-only, beam
measurements. Vector beam scans, of both amplitude and phase, double the information received in comparison to scalar
beam scans. Scalar beam measurements require multiple scans at varying distances along the optical path of the receiver
to fully constrain the divergence angle of the optical system and locate the primary focus. Vector scans provide this
information with a single scan, reducing the total measurement time required for new systems and also limiting the
influence of system instabilities. The vector scan can be taken at any point along the optical axis of the system including
the near-field, which makes beam measurements possible for large systems at high frequencies where these
measurements may be inconceivable to be tested in-situ. Therefore, the methodology presented here should enable
common heterodyne analysis for direct detector instruments. In principle, this coherent measurement strategy allows
phase dependent analysis to be performed on any direct-detect receiver instrument.
Microwave Kinetic Inductance Detectors (MKIDs) are becoming a very promising candidate for next generation imaging
instruments for the far infrared. A MKID consists of a superconducting resonator coupled to a feed-line used for the
readout. In the devices presented here radiation coupling is achieved by coupling the MKID directly to planar antenna.
The antenna is placed in the focus of an elliptical lens to increase the filling factor and to match efficiently to fore-optics.
In this paper we present the design and the optical performance of MKIDs optimized for operation at 350 GHz. We have
measured a device consisting of 14 pixels, characterized the coupling efficiency, antenna-lens frequency response and
beam pattern and compared these to theoretical simulations. The optical efficiency has been measured by means of a
black body radiator mounted in an ADR cryostat, through the variation of the black body temperature a variable
illumination of each pixel (from 0.1 fW to 2 pW) is achieved. The frequency response and beam pattern have been
directly measured in a He3 cryostat directly via the cryostat window and without the use of intermediate optics.
SPACEKIDS, a European Union FP-7 project, has recently been completed. It has focused on developing kinetic
inductance detector (KID) arrays and demonstrating their suitability for space applications at far infrared and
submillimetre wavelengths. KID arrays have been developed for both low-background (typical of astrophysical
applications) and high-background (typical of Earth-observation applications), based on performance specifications
derived from the science requirements of representative potential future missions. KID pixel and array designs have
been developed, together with readout electronics necessary to read out large numbers of pixels. Two laboratory
demonstrator systems have been built and used for comprehensive evaluation of large-format array characteristics and
performance in environments representative of both astronomy and Earth observing applications. We present an
overview of the SPACEKIDS project and a summary of its main results and conclusions.
In the next decades millimeter and sub-mm astronomy requires large format imaging arrays and broad-band spectrometers to complement the high spatial and spectral resolution of the Atacama Large Millimeter/submillimeter Array. The desired sensors for these instruments should have a background limited sensitivity and a high optical efficiency and enable arrays thousands of pixels in size. Hybrid microwave kinetic inductance detectors consisting of NbTiN and Al have shown to satisfy these requirements. We present the second generation hybrid NbTiN-Al MKIDs, which are photon noise limited in both phase and amplitude readout for loading levels P850GHz < 10 fW. Thanks to the increased responsivity, the photon noise level achieved in phase allows us to simultaneously read out approximately 8000 pixels using state-of-the-art electronics. In addition, the choice of superconducting materials and the use of a Si lens in combination with a planar antenna gives these resonators the flexibility to operate within the frequency range 0:09 < v < 1:1 THz. Given these specifications, hybrid NbTiN-Al MKIDs will enable astronomically usable kilopixel arrays for sub-mm imaging and moderate resolution spectroscopy.
A cryogenic iris mechanism is under development as part of the ground calibration source for the SAFARI instrument.
The iris mechanism is a variable aperture used as an optical shutter to fine-tune and modulate the absolute power output
of the calibration source. It has 4 stainless steel blades that create a near-circular aperture in every position. The
operating temperature is 4.5 Kelvin to provide a negligible background to the SAFARI detectors, and ‘hot spots’ above
9K should be prevented. Cryogenic testing proved that the iris works at 4K. It can be used in a broad range of cryogenic
optical instruments where optical throughput needs to be controlled.
Challenges in the design include the low cooling power available (5mW) and low friction at cryogenic temperatures. The
actuator is an ‘arc-type’ rotary voice-coil motor. The use of flexural pivots creates a mono-stable mechanism with a
resonance frequency at 26Hz. Accurate and fast position control with disturbance rejection is managed by a PID servo
loop using a hall-sensor as input. At 4 Kelvin, the frequency is limited to 4Hz to avoid excess dissipation and heating.
In this paper, the design and performance of the iris are discussed. The design was optimized using a thermal, magnetic
and mechanical model made with COMSOL Finite Element Analysis software. The dynamical and state-space modeling
of the mechanism and the concept of the electrical control are presented. The performance of the iris show good
agreement to the analytical and COMSOL modeling.
In the context of the SAFARI instrument (SpicA FAR-infrared Instrument) SRON is developing a test environment to
verify the SAFARI performance. The characterization of the detector focal plane will be performed with a backilluminated
pinhole over a reimaged SAFARI focal plane by an XYZ scanning mechanism that consists of three linear
stages stacked together. In order to reduce background radiation that can couple into the high sensitivity cryogenic
detectors (goal NEP of 2•10-19 W/√Hz and saturation power of few femtoWatts) the scanner is mounted inside the
cryostat in the 4K environment. The required readout accuracy is 3 μm and reproducibility of 1 μm along the total travel
of 32 mm. The stage will be operated in “on the fly” mode to prevent vibrations of the scanner mechanism and will
move with a constant speed varying from 60 μm/s to 400 μm/s. In order to meet the requirements of large stroke, low
dissipation (low friction) and high accuracy a DC motor plus spindle stage solution has been chosen.
In this paper we will present the stage design and stage characterization, describing also the measurements setup. The
room temperature performance has been measured with a 3D measuring machine cross calibrated with a laser
interferometer and a 2-axis tilt sensor. The low temperature verification has been performed in a wet 4K cryostat using a
laser interferometer for measuring the linear displacements and a theodolite for measuring the angular displacements.
The angular displacements can be calibrated with a precision of 4 arcsec and the position could be determined with high
accuracy. The presence of friction caused higher values of torque than predicted and consequently higher dissipation.
The thermal model of the stage has also been verified at 4K.
Distant, dusty and extremely luminous galaxies form a key component of the high redshift universe, tracing the period of intense cosmic activity that ultimately gave rise to the present-day universe. These highly luminous galaxies, first detected in the ground-based submillimeter region, are however optically very faint, which hampers identification of the optical counterpart and the measurement of a redshift. We are developing a new direct-detection submm spectrograph DESHIMA. By taking advantage of the rapidly advancing technology of superconducting microresonators, DESHIMA will revolutionize the appearance and capabilities of a submm spectrograph. There will no longer be large grating optics; instead DESHIMA will be equipped with a single chip, onto which the entire system of a dispersive filterbank and MKID sensor array is integrated. This chip will host 5000-10000 MKID sensors to instantaneously cover the entire submillimeter wave band (320-950 GHz) with a resolution of f/Δf = 1000, further multiplied by 6-9 spatial pixels. With the broader bandwidth and higher detector sensitivity, DESHIMA will be very efficient compared to ALMA in picking up THz lines from submm galaxies with unknown redshifts. The expected outcome of this project is; 1) a record of the properties and evolution of distant luminous galaxies, 2) a powerful and compact multi-purpose spectrometer suitable for future ground base telescopes as well as satellite missions, and 3) the emergence of a new branch of observational astronomy based on flexible on-chip submillimeter optics.
We have measured the optical response of detectors designed for SAFARI, the far-infrared imaging spectrometer for the SPICA satellite. To take advantage of SPICA's cooled optics, SAFARI’s three bolometer arrays are populated with extremely sensitive (NEP~2×10-19 W/√Hz) transition edge sensors with a transition temperature close to 100 mK. The extreme sensitivity and low saturation power (~4 fW) of SAFARI’s detectors present challenges to characterizing them. We have therefore built up an ultra-low background test facility with a cryogen-free high-capacity dilution refrigerator, paying careful attention to stray-light exclusion. Our use of a pulse-tube cooler to pre-cool the dilution refrigerator required that the SAFARI Detector System Test Facility provide a high degree electrical, magnetic, and mechanical isolation for the detectors. We have carefully characterized the performance of the test facility in terms of background power loading. The test facility has been designed to be flexible and easily reconfigurable with internal illuminators that allow us to characterize the optical response of the detectors. We describe the test facility and some of the steps we took to create an ultra-low background test environment. We have measured the optical response of two detectors designed for SAFARI’s short-wave wavelength band in combination with a spherical backshort and conical feedhorn. We find an overall optical efficiency of 40% for both, compared with an ideal-case predicted optical efficiency of 66%.
SAFARI is a far-infrared camera to be launched in 2021 onboard the SPICA satellite. SAFARI offers imaging
spectroscopy and imaging photometry in the wavelength range of 34 to 210 μm with detector NEP of 2•10-19 W/√Hz.
A cryogenic test facility for SAFARI on-ground calibration and characterization is being developed. The main design
driver is the required low background of a few attoWatts per pixel. This prohibits optical access to room temperature and
hence all test equipment needs to be inside the cryostat at 4.5K. The instrument parameters to be verified are interfaces
with the SPICA satellite, sensitivity, alignment, image quality, spectral response, frequency calibration, and point spread
function. The instrument sensitivity is calibrated by a calibration source providing a spatially homogeneous signal at the
attoWatt level. This low light intensity is achieved by geometrical dilution of a 150K source to an integrating sphere. The
beam quality and point spread function is measured by a pinhole/mask plate wheel, back-illuminated by a second
integrating sphere. This sphere is fed by a stable wide-band source, providing spectral lines via a cryogenic etalon.
Kinetic Inductance Detectors (KIDs) with frequency domain read-out are intrinsically very suitable to use as
building blocks for very large arrays. KIDs therefore are an attractive detector option for the SAFARI instrument on
SPICA, Millimetron and also for large scale ground based imaging arrays. To study the properties of large KID
arrays we have fabricated 400 pixels array made from 40 nm thick Al films on high resistivity Si substrates. The
array is tested in a dry dilution refrigerator at 100 mK. We present the device design and experimental results. We
also present a new design of the array with lithographic air bridges over the coplanar waveguide feedline. The air
bridges are designed to suppress the slot line mode in the feedline and that will improve the pixel to pixel
reproducibility of large arrays.
The technique which combines high resolution spectroscopy with imaging capability is a powerful tool to extract
fundamental information in X-ray Astrophysics and Cosmology. TES (Transition Edge Sensors)-based
microcalorimeters match at best the requirements for doing fine spectroscopy and imaging of both bright (high count
rate) and faint (poor signal-to-noise ratio) sources. For this reason they are considered among the most promising
detectors for the next high energy space missions and are being developed for use on the focal plane of the IXO
(International X-ray Observatory) mission. In order to achieve the required signal-to-noise ratio for faint or diffuse
sources it is necessary to reduce the particle-induced background by almost two orders of magnitude. This reduction can
only be achieved by adopting an active anticoincidence technique. In this paper, we will present a novel anticoincidence
detector based on a TES sensor developed for the IXO mission. The pulse duration and the large area of the IXO TESarray
(XMS X-ray Microcalorimeter Spectrometer) require a proper design of the anticoincidence detector. It has to
cover the full XMS area, yet delivering a fast response. We have therefore chosen to develop it in a four-pixel design.
Experimental results from the large-area pixel prototypes will be discussed, also including design considerations.
The evidence of excess noise in the power spectrum of many natural systems that span over the mHz to the THz, such as
biological system, superconductors at dendritic regime, Barkhausen noise of magnetic system and plasma emission from
nanometric transistors, was observed and related to a class of statistical models of correlated processes. Intrinsic or
induced fluctuations of the elementary processes taking place in transport phenomena couple each other giving rise to
time-amplitude correlated avalanches. TES sensors for X-ray microcalorimeters have shown a clear evidence that this
excess noise has typical spectral behavior spanning from 100 Hz to 10 kHz. We present an analysis of the excess noise
using this statistical avalanche model of TES operating on Si substrate and suspended SiN membrane.
A program for developing TES microcalorimeters for contributions to future Italian X-ray astronomy missions is under
course. Its main scientific goals are the spectroscopic study of extreme astrophysical objects, characterized by very large
energy release over short time scale, in particular gamma-ray bursts and transient compact objects, and the study of the
early and close-by Universe by using gamma-ray bursts as cosmological beacons. Presently, the energy resolution of our
detector has been improved to about 6 eV at 6 keV, with rise-time of about 10 μs and fall time of few hundreds of μs.
We are developing and studying the suitable absorbers for high count rate performances.