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Cluster Report Photonics in the Capital Region Berlin-Brandenburg

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5.1.3.3 Terahertz

5.1.3.3 Terahertz Technology Short waves create entirely new insights Heinz-Wilhelm Hübers, Kai Kolwitz In the electromagnetic spectrum, the terahertz (THz) wave range is located between millimeter waves and infrared radiation. Large hopes and expectations are pinned on them - largely due to their specific properties. Materials such as paper, plastics or ceramics are transparent for terahertz radiation, however, neither are metal nor water. One can use it to illuminate objects in a way similar to X-rays, but without the damaging effects of the latter. And with its extremely short wavelength, it allows higher resolution in many areas than previously used technologies. Thus, many application areas are imaginable for the radiation, for example in nondestructive materials testing as well as in process control. Terahertz radiation also penetrates clothing and can reveal hidden objects underneath, making it attractive for security applications. Biomedicine and communication could also become areas in which the radiation could be applied. Berlin-Brandenburg is very well positioned in this field. In national as well as international comparisons, it is characterized by a unique concentration of expertise in basic and applied THz research and development. Besides the basic research at the universities of Berlin, the Max-Born-Institute or the Helmholtz-Center Berlin (HZB), the synchrotron radiation sources with dedicated THz beam lines at HZB and the Physikalisch-Technische Bundesanstalt (PTB) in particular attract visitors from all around the world; this also not least because the synchrotron BESSY II of the HZB in Berlin-Adlershof succeeded in 2003 to produce coherent radiation in the terahertz range and measure its properties with special antennas for the first time worldwide ever. Many approaches to the development of THz components In order to transfer terahertz technology into new fields of practical and commercial use, it is currently necessary to develop all essential components. Here Berlin-Brandenburg research institutions are active in various fields. The Ferdinand-Braun-Institute (FBH) is currently systematically intensifying its THz activities; the emphasis lies on electronic components in the frequency range up to 0.5 THz. The technological basis for this is a transferred-substrate process with indium-phosphide heterojunction bipolar transistors for manufacturing integrated circuits. This is complemented by a station for on-wafer measurements up to 500 THz, and the corresponding design activities. The Leibniz-Institute for Innovative Microelectronics (IHP) focuses on research and development of silicon-germanium heterojunction bipolar transistors (HBTs) operating near a frequency of 0.5 THz. These can be used, for example, for high frequency circuitry in wireless and broadband communications. Additionally, IHP and FBH are also working on an integration of indium phosphide transistors in CMOS semiconductor devices. The Paul-Drude-Institute (PDI) develops quantum cascade lasers (QCLs) in the THz range (2.5 to 5 THz) based on the material system GaAs/(Al,Ga)As. These QCLs can be used as narrow-band radiation sources that are of interest in spectroscopic and imaging applications. At the PDI, laser structures are designed and optimized and then manufactured via molecular beam epitaxy. Then laser strips are being processed and characterized by Fourier transform spectroscopy. Thereby compact integrated front-end modules for radar and communication systems can be implemented. Terahertz spectrometer for mobile use Fast optoelectronic THz emitters and detectors, so-called photo mixers, based on indium phosphide technology, are the main subject of the Heinrich-Hertz-Institute of the Fraunhofer Gesellschaft (HHI). This technology, which has matured in telecommunications, enables the manufacture of very robust and cost-effective THz components covering the frequency range from 100 GHz to 4 THz. For example, they could be used for in-line process control. Already in 2009 researchers at the HHI succeeded in developing such a photo mixer – that converts, back and forth, light and electric power into THz waves - for 1.5µ fibers. Thereby, they created the prerequisite for realizing all components of a terahertz spectrometer in fiber technology and enabled the construction of these kinds of spectrometers as handheld devices. In a collaboration the Physikalisch-Technische Bundesanstalt (PTB) and the company Sensor- und Lasertechnik headquartered in Neuenhagen/Brandenburg are working on pyrometric THz detectors that do not require cooling. In addition, they are working on detectors based on TES bolometers, superconducting radiation sensors that operate at low temperatures below 4.2 degrees Kelvin. The aim of the TES-bolometer research is to develop a highly linear THz receiver for quantitative Fourier transform spectroscopy. Microstructured thin metal films as THz absorbers for TES bolometer (left: wafer, right: detail) © PTB 64

5.1.3.3 Terahertz Technology The German Center for Aerospace (DLR) in Berlin is also working on the development of superconducting THz detectors. They focus on highly sensitive nanostructured bolometers, which are used for remote sensing in astronomy and planetary science. The technology is also used in the detection of short THz pulses. Systems from Berlin in the stratosphere Once individual components are ready for production, their integration into complete systems follows. With the HHI, the Federal Institute for Materials Research and Testing (BAM) and the DLR there are key players represented in the region, that are leaders in Germany in the development of systems for applications in civil security, in non-destructive testing and in the aerospace industry. The THz-components from the HHI are robust fiber-coupled modules. They allow flexible implementation of coherent THz systems according to customer specifications. The consistent use of components from optical communication technology allows the HHI to implement extremely compact and portable THz spectrometers. ring “Metrology Light Source” of the PTB in Berlin-Adlershof is the first ring worldwide developed for the generation of coherent radiation in the THz range. The PTB performs, sometimes as part of cooperations, work on radiometry and photometry on the dedicated THz beam line. Cooperation is promoted Berlin is a popular starting node for international research in terahertz waves. And even within the region itself cooperation leads to success in national and international competition. One example is a collaboration of the Berlin company eagleyard with DLR, the FBH, the Humboldt University and the PDI, which aims to develop compact THz quantum cascade lasers for spectroscopic applications. The world's most compact THz laser system has emerged from this cooperation. THz quantum cascade laser system for spectroscopy and imaging © DLR, PDI Fiber-coupled THz module © Fraunhofer HHI Naturally, BAM focusses on material testing in their THz activities. They are developing methods for the nondestructive testing of polymers, ceramics and high performance materials using terahertz technology. In contrast, DLR develops THz systems for applications in astronomy, planetary research and safety. Examples are a heterodyne spectrometer for SOFIA, the Stratospheric Observatory for Infrared Astronomy in a Boeing 747, and THz scanners with which objects hidden on bodies can be detected at greater distances. Compact THz systems for imaging and spectroscopy based on quantum cascade lasers are also being developed. Since the THz companies are still young and small, exchange and collaborations with research institutions are essential for economic success and they are actively promoted. The aim is to further explore the market potential of THz technology and to establish the region as a leading world player in this field. Contact: Prof. Dr. H.-W. Hübers Deutsches Zentrum für Luft- und Raumfahrt e.V. Phone: + 49 (0) 30 / 670 55596 Email: Heinz-Wilhelm.Huebers@dlr.de Calibration and standards Since there is still pioneering work to do in many areas of terahertz technologies, standardization is also a big issue. The Physikalisch-Technische Bundesanstalt (PTB) in Berlin is working in this area. They are currently building up their THz radiometry. The aim is to characterize THz detectors suitable for calibration, and to adjust their calibration of THz radiation output sensitivity to the international system of units - in the frequency range from 1 to 5 THz. The technical prerequisites are in place: the electron storage 65

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