An enhanced version of the ”Polarimeter für bolometer Kameras” (PolKa) has been installed on the APEX telescope (Atacama Pathfinder EXperiment) in October 2009, to work in combination with LABOCA (the Large APEX Bolometer Camera). This polarimeter was included in the design of LABOCA’s optics from the beginning and it is now going through a commissioning and science verification phase. The combination of PolKa, LABOCA and APEX provides superior capabilities in mapping the polarization of the continuum at submillimeter wavelengths. We present here some preliminary results of the last commissioning run.
Cryogenic bolometers are among the most sensitive devices for the detection of electromagnetic radiation in the submillimeter
wavelength range. Such radiation is of interest for astronomical observations as well as for security checks.
We describe how we fabricate an array of these bolometers. Standard contact lithography is sufficient for these relatively
coarse features. To increase the sensitivity, it is imperative to weaken the thermal link between the thermistors (the
sensing devices) and the temperature bath. This is achieved by placing them on a silicon nitride membrane that is
structured so that the thermistors are placed on a platform which is held only by a few beams. The fabrication process
does not require sophisticated lithographic techniques, but special care to achieve the desired yield of 100 % intact
bolometers in one array. We discuss bolometer basics and requirements for our applications, critical fabrication issues,
and show results of complete systems built for a radio telescope and for security cameras.
An enhanced version of the "Polarimeter f¨ur bolometer Kameras" (PolKa) has been installed on the APEX
telescope (Atacama Pathfinder EXperiment) in October 2009, to work in combination with LABOCA (the
Large APEX Bolometer Camera). This polarimeter was included in the design of LABOCA's optics from the
beginning and it is now going through a commissioning phase. Preliminary tests on sky have confirmed that the
combination of PolKa, LABOCA and APEX provides unprecedented capabilities in mapping the polarization of
the continuum emission at submillimeter wavelengths.
We present the experimental results and a bolometer model of the voltage-biased superconducting bolometer
on the low stress silicon nitride (Si<sub>3</sub>N<sub>4</sub>) membrane, developed in collaboration between the Max-Planck-Institut
fur Radioastronomie (MPIfR), Bonn and the Institute for Photonic Technology (IPHT), Jena, Germany. 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. The thermal conductance of the bolometer is tuned by structuring the
silicon nitride membrane into spider-like geometries. The incident radiation is absorbed by crossed dipoles
made from gold-palladium alloy with a surface resistance of 10 Ω/square. Using the COSMOS finite element analysis
package, the thermal conductance is obtained for the bolometers of different geometries. FEA simulations showed
that the deposition of a gold ring around the absorbing area could increase the sensitivity of the bolometer.
Therefore, a gold ring is deposited around the center absorbing patch of the silicon nitride membrane. For the
bolometer with a gold ring, the measured NEP is 1.7 X (see manuscript for formula)
Hz and the time constant is in the range
between 1.4 and 2 ms.
A new facility instrument, the Large APEX Bolometer Camera (LABOCA), developed by the Max-Planck-Institut f&diaeru;r Radioastronomie (MPIfR, Bonn, Germany), has been commissioned in May 2007 for operation on the Atacama Pathfinder Experiment telescope (APEX), a 12 m submillimeter radio telescope located at 5100 m altitude on Llano de Chajnantor in northern Chile. For mapping, this 295-bolometer camera for the 870 micron atmospheric window operates in total power mode without wobbling the secondary mirror. One LABOCA beam is 19 arcsec FWHM and the field of view of the complete array covers 100 square arcmin. Combined with the high efficiency of APEX and the excellent atmospheric transmission at the site, LABOCA offers unprecedented capability in large scale mapping of submillimeter continuum emission. Details of design and operation are
Terahertz (THz) cameras are expected to be a powerful tool for future security applications. If such a technology shall
be useful for typical security scenarios (e.g. airport check-in) it has to meet some minimum standards. A THz camera
should record images with video rate from a safe distance
(stand-off). Although active cameras are conceivable, a
passive system has the benefit of concealed operation. Additionally, from an ethic perspective, the lack of exposure to a
radiation source is a considerable advantage in public acceptance.
Taking all these requirements into account, only cooled detectors are able to achieve the needed sensitivity. A big leap
forward in the detector performance and scalability was driven by the astrophysics community. Superconducting
bolometers and midsized arrays of them have been developed and are in routine use. Although devices with many pixels
are foreseeable nowadays a device with an additional scanning optic is the straightest way to an imaging system with a
useful resolution. We demonstrate the capabilities of a concept for a passive Terahertz video camera based on
superconducting technology. The actual prototype utilizes a small Cassegrain telescope with a gyrating secondary
mirror to record 2 kilopixel THz images with 1 second frame rate.
The <b>S</b>uper<b>CO</b>nducting <b>T</b>erahertz <b>I</b>mager (SCOTI) is a small Cassegrain-type telescope with a scanning secondary
mirror designed for a frequency of 0.34 THz. It can map objects at a distance of 5 meter using a small array of
superconducting bolometers. The resolution at the object area is about 1 cm. Using SCOTI purely passive images of
interesting objects can be taken, thus opening a wide field of applications.
Ever since the first proposal of the voltage-biased transition-edge bolometer the astrophysics community desired bolometer arrays with as many pixels as possible. With respect to the technical problem due to the need of lots of readout SQUID sensors only with multiplexing it is possible to go beyond a few hundred pixel. A technology which allows the manufacture of detector and readout on one chip would simplify this task substantially. Here we demonstrate the fabrication of a transition edge sensor based on a thermistor out of a molybdenum / gold-palladium bilayer. The alloy of gold-palladium (Au-Pd), which allows the tuning of molybdenum's critical temperature by one order of magnitude, is taken from our foundry process for SQUID manufacturing. Au-Pd can further be used for shunt resistances, absorber patterns and bond pads, and, therefore, it is a good choice for a combined technology. The thermistor is placed on a moderately patterned silicon nitride membrane in the shape of an 8-legged spider. The radiation band of interest is coupled via a conical feed horn to a simple grid of dipole-like antenna patterns. This removes the need for the poorly reproducible high-resistance absorption films for the matching of the free space impedance. The simple detector technology is compatible with the SQUID manufacturing. Hence, some of the SQUID layers can be merged with the corresponding detector layer, i.e. the thermistor wiring and the SQUID washer are made in a single niobium layer. The concept of feed horn coupling eases the design requirements, consequently the SQUID can be placed close to the detector, thereby allowing a simpler wiring to be used and in theory a better performance to be obtained.
We present the experimental results of voltage-biased superconducting bolometers (VSB) on silicon nitride
(Si<sub>3</sub>N<sub>4</sub>) membranes with niobium wiring developed in collaboration between the Institut fur Physikalische
Hochtechnologie (IPHT), Jena, Germany and the Max-Planck-Institut fur Radioastronomie (MPIfR), Bonn,
Germany. The bolometer current is measured with the superconducting quantum interference device (SQUID),
and as expected, the current responsivity is proportional to the inverse of the bias voltage. The experiments
were performed with bilayer gold-palladium molybdenum thermistor at 300 mK <sup>3</sup>He cooled cryostat and the
desired transition temperature of T<sub>c</sub> = 450 mK is achieved. The strong negative electro-thermal feedback of
the VSB maintains the constant bolometer temperature and reduces the response time from 4 ms to 100 μs. We
have tested thermistors of various size and shape on a continuous membrane and achieved a noise equivalent
power (NEP) of 3.5 × 10<sup>-16</sup> W/√Hz. The measured NEP is relatively high due to the comparatively high
background and high thermal conductance of the unstructured silicon nitride (Si<sub>3</sub>N<sub>4</sub>) membrane. We have
fabricated 8-leg spider structured membranes in three different geometries and the relation between the
geometry and the thermal conductance (<i>G</i>) is studied. Using the COSMOS finite element analysis tool,
we have modeled the TES bolometers to determine the thermal conductance for different geometries and
calculated the various parameters. Due to the demands of large number pixel bolometer camera we plan to
implement multiplex readout with integrated SQUIDs in our design.
APEX, the Atacama Pathfinder Experiment, has been successfully commissioned and is in operation now. This novel submillimeter telescope is located at 5107 m altitude on Llano de Chajnantor in the Chilean High Andes, on what is considered one of the world's outstanding sites for submillimeter astronomy. The primary reflector with 12 m diameter has been carefully adjusted by means of holography. Its surface smoothness of 17-18 μm makes APEX suitable for observations up to 200 μm, through all atmospheric submm windows accessible from the ground.
With ESO and Onsala Space Observatory as partners, the Max-Planck-Institut for Radioastronomie (MPIfR) is building a submillimeter telescope of 12 m diameter (APEX), to be placed on the ALMA site (Chajnantor) in Chile. The telescope will be a modified copy of that ALMA prototype antenna, which has been designed by Vertex. First light is foreseen for 2003. As a result of the excellent atmospheric conditions of the site, APEX will offer unique opportunities for submm astronomy in the southern hemisphere. Many kinds of astronomical reseach projects benefit from large format bolometer arrays, especially the search for early galaxies and QSOs at very high redshifts. Designed for this purpose, LABOCA, the large bolometer camera, will operate at a wavelength of 870 μm and is planned to be operational soon after first light of APEX.
This paper reviews transmission anomalies of freestanding frequency surfaces (FSS), which are commonly used as filter and guiding structures in the far IR region. The effect is characterized by a sharp breakdown of the transmission factor in the passband and was first revealed by measurements. The paper demonstrates the numerical confirmation of these effects by computer aided calculations based on the method of moments combined with the Floquet theorem. The scattering characteristics are examined for different sets of parameters like the geometry of the structure, frequency, angle and polarization of the incident wave. The modeling with electric and magnetic currents allows the characterization of both patch and slot arrays.
Continuum radiometers based on bolometers have a long tradition at the Max-Planck-Institut fur Radioastronomie (MPIfR) in Bonn, Germany. Arrays of bolometers have been under development since the early 90s. A small 7-element system, operating at 300 mK, saw first light in 1992 at the IRAM 30 m- telescope and has been used successfully by numerous observers at that facility since then. While this array had a conventional 'composite' design, it was obvious that larger arrays, especially for higher frequencies, could take advantage of microfabrication technology. The recent MPIfR bolometer arrays employ a hybrid approach. They combine a single-mode horn array with a planar bolometer array on a single crystal Silicon wafer with Silicon-Nitride membranes. With efficient absorbing structures, the bolometers couple to the single mode of the radiation field collected by the horns, without needing integrating cavities. Readout is provided by NTD-Germanium thermistors that are attached to the absorbers. This paper covers the history of this development, the general aspects of the bolometer arrays, including the coupling to the telescope, and the status of work in progress.
The far-infrared filters for the satellite subexperiment ISOPHOT have to fulfil a number of specific requirements. This is a report about the developed filter concept which was used to match all specifications. In a wide wavelength region this concept is capable of defining broad or narrow bandpasses in a flexible way. The filters which have been constructed are robust and survive multiple cryogenic cycles. They combine high passband transmission with extreme stopband rejection. Details are given for some representative ISOPHOT FIR-filters.
The Infrared Space Observatory (ISO) photometer is one of four instruments on board the European Space Agency ISO satellite scheduled for launch in September 1995. It covers the wavelength range 2.5 to 240 μm with wide and narrow spectral bands. Diffraction-limited observations as well as wide-beam measurements of faint extended sources are possible. Polarimetric observations can be made over the whole wavelength range. The minimal detectable flux is ~10 mJy. The astronomical areas to be addressed range from solar system objects to cosmology.
ISOPHOT is one of four instruments onboard the ESA Infrared Space Observatory scheduled for launch in September 1995. It covers the wavelength range 2.5 micrometers to 240 micrometers with wide and marrow spectral bands. Diffraction limited observations as well as wide beam measurements of faint extended sources are possible. Polarimetric observation can be made over the whole wavelength range. The minimal detectable flux is approximately 10 mJy. The astronomical areas to be addressed range from solar system objects to cosmology.