Erik Heinz, Torsten May, Detlef Born, Gabriel Zieger, Katja Peiselt, Vyacheslav Zakosarenko, Torsten Krause, André Krüger, Marco Schulz, Frank Bauer, Hans-Georg Meyer
Since 2007 we are developing passive submillimeter-wave video cameras for personal security screening. In
contradiction to established portal-based millimeter-wave scanning techniques, these are suitable for stand-off
or stealth operation. The cameras operate in the 350GHz band and use arrays of superconducting transition-edge
sensors (TES), reflector optics, and opto-mechanical scanners. Whereas the basic principle of these devices
remains unchanged, there has been a continuous development of the technical details, as the detector array, the
scanning scheme, and the readout, as well as system integration and performance. The latest prototype of this
camera development features a linear array of 128 detectors and a linear scanner capable of 25Hz frame rate.
Using different types of reflector optics, a field of view of 1×2m2 and a spatial resolution of 1–2 cm is provided
at object distances of about 5–25m. We present the concept of this camera and give details on system design
and performance. Demonstration videos show its capability for hidden threat detection and illustrate possible
application scenarios.
Erik Heinz, Torsten May, Detlef Born, Gabriel Zieger, Katja Peiselt, Anika Brömel, Solveig Anders, Vyacheslav Zakosarenko, Torsten Krause, André Krüger, Marco Schulz, Hans-Georg Meyer
Passive submillimeter wave imaging is a concept that has been in the focus of interest as a promising technology for security applications for a number of years. It utilizes the unique optical properties of submillimeter waves and promises an alternative to millimeter-wave and X-ray backscattering portals for personal security screening in particular. Possible application scenarios demand sensitive, fast, and fleixible high-quality imaging techniques. Considering the low radiometric contrast of indoor scenes in the submillimeter range, this objective calls for an extremely high detector sensitivity that can only be achieved using cooled detectors. Our approach to this task is a series of passives standoff video cameras for the 350 GHz band that represent an evolving concept and a continuous development since 2007. The cameras utilize arrays of superconducting transition-edge sensors (TES), i.e. cryogenic microbolometers, as radiation detectors. The TES are operate at temperatures below 1K, cooled by a closed-cycle cooling system, and coupled to superconducting readout electronics. By this means, background limited photometry (BLIP) mode is achieved providing the maximum possible signal to noise ratio. At video rates, this leads to a pixel NETD well below 1K. The imaging system is completed by reflector optics based on free-form mirrors. For object distances of 3–10m, a field of view up to 2m height and a diffraction-limited spatial resolution in the order of 1–2cm is provided. Opto-mechanical scanning systems are part of the optical setup and capable frame rates up to 25 frames per second. Both spiraliform and linear scanning schemes have been developed.
Amrutha Gopal, Torsten May, Pushkar Singh, Sven Herzer, Wolfgang Ziegler, Gerhard Paulus, Albrecht Schmidt, Andreas Reinhard, Ulrich Dillner, Hans-Georg Meyer, Anupam Karmakar, Dirk Broemmel, Paul Gibbon
Here we report a laser plasma-driven source of T-rays with the highest pulse energy ever recorded in a laboratory. T-rays are emitted from the rear surface of a solid target in the non-collinear direction at incident laser intensities ~ 1019 W/cm2. Pulse energy measurements reported T-ray pulses with peak energies no less than 700 μJ. Temporal measurements using a single-shot electro-optic method showed the presence of sub-picosecond T-ray pulses with 570 fs duration, thus rendering the peak-power of the source higher even than that of state-of-the-art synchrotrons. A conversion efficiency of higher than 10−3 and an average power of 7 mW makes it the most efficient compact and powerful THz source known today. Spectral analysis revealed the presences of frequencies ranging from 0.1 − 133 THz, while most of the energy is localised in the low frequency region. The dependence of T-ray yield on incident laser energy is linear and shows no signs of saturation. The spatial distribution of the recorded T-rays indicates that most of the T-rays are emitted in the non-collinear direction from the rear-surface of a solid target and the contribution in the forward direction is very small. 2D particle-in-cell simulations show the presence of transient current at the target rear surface.
Erik Heinz, Torsten May, Detlef Born, Gabriel Zieger, Anika Brömel, Solveig Anders, Vyacheslav Zakosarenko, Torsten Krause, André Krüger, Marco Schulz, Frank Bauer, Hans-Georg Meyer
Passive submillimeter-wave imaging is a concept that has been in the focus of interest as a promising technology for security applications for a number of years. It utilizes the unique optical properties of submillimeter waves and promises an alternative to millimeter-wave and X-ray backscattering portals for personal security screening in particular. Possible application scenarios demand sensitive, fast, and flexible high-quality imaging techniques. Considering the low radiometric contrast of indoor scenes in the submillimeter range, this objective calls for an extremely high detector sensitivity that can only be achieved using cooled detectors. Our approach to this task is a series of passive standoff video cameras for the 350 GHz band that represent an evolving concept and a continuous development since 2007. The cameras utilize arrays of superconducting transition-edge sensors (TES), i. e. cryogenic microbolometers, as radiation detectors. The TES are operated at temperatures below 1 K, cooled by a closed-cycle cooling system, and coupled to superconducting readout electronics. By this means, background limited photometry (BLIP) mode is achieved providing the maximum possible signal to noise ratio. At video rates, this leads to a pixel NETD well below 1K. The imaging system is completed by reflector optics based on free-form mirrors. For object distances of 3–10 m, a field of view up to 2m height and a diffraction-limited spatial resolution in the order of 1–2 cm is provided. Opto-mechanical scanning systems are part of the optical setup and capable frame rates up to 25 frames per second. Both spiraliform and linear scanning schemes have been developed. Several electronic and software components are used for system control, signal amplification, and data processing. Our objective is the design of an application-ready and user-friendly imaging system. For application in real world security screening scenarios, it can be extended using image processing and automated threat detection software.
Torsten May, Julia Toussaint, Roman Grüner, Marco Schubert, Hans-Georg Meyer, Benjamin Dietzek, Jürgen Popp, Matthias Hofherr, Konstantin Il'in, Dagmar Henrich, Matthias Arndt, Michael Siegel
Raman scattering spectroscopy allows the direct and fast study of molecules by analysis of their vibrational normal
modes. However, for certain materials the scattered signal is superimposed by fluorescence, which - if present -
overwhelms the intrinsically weak Raman signal by orders of magnitude. An approved method to resolve the
instantaneous Raman signal of interest from the delayed fluorescence background is time-correlated single-photon
counting (TCSPC). For that, a single-photon detector with fast dynamics is required.
The, so-called, superconducting nanowire single-photon detector (SNSPD) is a promising candidate for TCSPC. We
have developed an optical instrument using such a SNSPD for the TCSPC method. The detector is made from a 5 nm
thick NbN film, patterned by electron-beam lithography in a meander line with a width of 100 nm and a filling-factor of
50 %, covering an active area of 4 × 4 μm2. As a proof of concept we have shown that it is possible to resolve low power
optical signals (λ between 520 and 630 nm) with a timing jitter of about 35 ps. Based on our experimental results we will
discuss perspectives and limits of SNSPD application for spectroscopy.
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.
Erik Heinz, Torsten May, Detlef Born, Gabriel Zieger, Guenter Thorwirth, Solveig Anders, Viatcheslav Zakosarenko, Torsten Krause, André Krüger, Marco Schulz, Hans-Georg Meyer, Marco Schubert, Michael Starkloff
Against a background of newly emerged security threats, the well-established idea of utilizing submillimeter-wave radiation for personal security screening applications has recently evolved into a promising technology. Possible application scenarios demand sensitive, fast, flexible and high-quality imaging techniques. At present, best results are obtained by passive imaging using cryogenic microbolometers as radiation detectors. Building upon the concept of a passive submillimeter-wave stand-off video camera introduced previously, we present the evolution of this concept into a practical application-ready imaging device. This has been achieved using a variety of measures such as optimizing the detector parameters, improving the scanning mechanism, increasing the sampling speed, and enhancing the image generation software. The camera concept is based on a Cassegrain-type mirror optics, an optomechanical scanner, an array of 20 superconducting transition-edge sensors operated at a temperature of 450 to 650 mK, and a closed-cycle cryogen-free cooling system. The main figures of the system include: a frequency band of 350±40 GHz, an object distance of 7 to 10 m, a circular field of view of 1.05 m diameter, and a spatial resolution in the image center of 2 cm at 8.5 m distance, a noise equivalent temperature difference of 0.1 to 0.4 K, and a maximum frame rate of 10 Hz.
Erik Heinz, Torsten May, Detlef Born, Gabriel Zieger, Solveig Anders, Viatcheslav Zakosarenko, Marco Schubert, Torsten Krause, André Krüger, Marco Schulz, Hans-Georg Meyer
Against a background of newly emerged security threats the well-established idea of utilizing submillimeter-wave
radiation for personal security screening applications has recently evolved into a promising technology.
Possible application scenarios demand sensitive, fast, flexible and high-quality imaging techniques. At present,
best results are obtained by passive imaging using cryogenic microbolometers as radiation detectors. Building
upon the concept of a passive submillimeter-wave stand-off video camera introduced previously, we present
the evolution of this concept in a practical application-ready imaging device. This has been achieved using a
variety of measures such as optimizing the detector parameters, improving the scanning mechanism, increasing
the sampling speed, and enhancing the camera software. The image generation algorithm has been improved and
an automatic sensor calibration technique has been implemented taking advantage of redundancy in the sensor
data. The concept is based on a Cassegrain-type mirror optics, an opto-mechanical scanner providing spiraliform
scanning traces, and an array of 20 superconducting transition-edge sensors (TES) operated at a temperature of
450-650 mK. The TES are cooled by a closed-cycle cooling system and read out by superconducting quantum
interference devices (SQUIDs). The frequency band of operation centers around 350 GHz. The camera can
operate at an object distance of 7-10 m. At 9m distance it covers a field of view of 110 cm diameter, achieves a
spatial resolution of 2 cm and a pixel NETD (noise equivalent temperature difference) of 0.1-0.4 K. The maximum
frame rate is 10 frames per second.
Erik Heinz, Detlef Born, Gabriel Zieger, Torsten May, Torsten Krause, Andre Krüger, Marco Schulz, Solveig Anders, Viatcheslav Zakosarenko, Hans-Georg Meyer, Michael Starkloff, Mario Rößler, Guenter Thorwirth, Ulf Krause
As reported before,1, 2 Safe VISITOR (Safe VISible, Infrared and Terahertz Object recognition) is a German
project to build a passive security camera which visualizes sub-mm wavelengths using cooled bolometer arrays.
This camera could be used for a variety of application scenarios, such as airport screenings or to protect military
camps. In all cases, a practical instrument requires ease of use, in particular a flexible installation and a
straightforward usage by the security personnel.
Here we present a new generation of Safe VISITOR designed to meet these requirements. The main condition
for an effective operation is a high frame rate of the imager. Safe VISITOR is able to record videos up to 10 Hz,
using a small array of superconducting bolometers in combination with an opto-mechanical scanner. The required
cooling of the detector array is provided by a commercial pulse tube cooler with a second, self-contained cooling
stage. The cooling cycle is completely automated; after 10 hours of initial cooling from room temperature the
system can operate quasi-continuously.
For imaging, a 50 cm diameter optics is used which is able to provide an object resolution of approximately
1.5 cm at 8 m distance. For a flexible installation, the object distance can be tuned manually between 7 and
10 m. Additionally, video streams from two commercial cameras are fused with the sub-mm stream: a CCD for
visible light and a microbolometer for far infrared (14 μm). This combines the ability of identification of the
person under test with the unprecedented temperature resolution at infrared and the almost perfect transmission
at sub-mm. To assist a security official, all image data are displayed in various graphic renditions by a unified
system software.
Security solutions with the purpose to detect hidden objects underneath the clothing of persons are desired in many
environments. With the variety of application scenarios criteria like flexibility and mobility become more important. So,
many developments trend to focus on cameras, which can image scenes from a distance. This new generation of tools
will have the advantage of hidden operation, which is believed by experts to add to the security because of its
unpredictability.
Such stand-off cameras do have some divergent requirements compared to mm-wave portal scanners. They will benefit
from shorter wavelengths because of the higher optical resolution. In contrast to that, the needed transmission properties
might become impractical at higher frequencies. A commonly accepted compromise is the use of wavelengths around
0.5mm. However, for stand-off cameras without oversized optical apertures, a resolution around 1cm is a practical limit.
For our security camera "Safe VISITOR" (Safe VISible, Infrared and Terhaertz Object recognition) we have chosen to
combine images from three different camera modules: a CCD for visible light, a microbolometer for long infrared
(14μm) and a superconducting bolometer for 870μm. This combines the highest optical resolution (visible), the
unprecedented temperature resolution at infrared and the almost perfect transmission at terahertz. We have built a first
prototype and tested it in a field trial. We will present experimental results and try to assess the false error rate of our
system.
KEYWORDS: Bolometers, Silicon, Gold, Finite element methods, Signal to noise ratio, Superconductors, Resistance, Vestigial sideband modulation, Sensors, Temperature metrology
We present the experimental results and a bolometer model of the voltage-biased superconducting bolometer
on the low stress silicon nitride (Si3N4) 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.
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 SuperCOnducting Terahertz Imager (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.
We present the experimental results of voltage-biased superconducting bolometers (VSB) on silicon nitride
(Si3N4) 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 3He cooled cryostat and the
desired transition temperature of Tc = 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 × 10-16 W/√Hz. The measured NEP is relatively high due to the comparatively high
background and high thermal conductance of the unstructured silicon nitride (Si3N4) membrane. We have
fabricated 8-leg spider structured membranes in three different geometries and the relation between the
geometry and the thermal conductance (G) 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.
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.
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.
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