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01 Oct 2008
Terahertz radiation holds great promise for enhanced security systems, industrial inspection and sophisticated spectroscopy. Marie Freebody speaks to Hartmut Roskos to find out about the progress that has been made so far and the key challenges that remain.
Hartmut Roskos is professor of physics at Johann Wolfgang Goethe-University, Germany. Earlier this year, OC Oerlikon awarded his research group a five-year endowed Associate Professorship for Terahertz Photonics. The group's recent achievements include the development of terahertz emission diagnostics for the carrier envelope phase of few-cycle light pulses, the realization of very fast 3D terahertz scanners, and the development of multi-pixel terahertz detectors based entirely on silicon CMOS transistor technology.
Can you summarize how terahertz radiation is produced?
Terahertz radiation spans 100 GHz to 10 THz, which lies between the traditional realm of electronics on the low-frequency side and that of infrared optics on the high-frequency side. This means that there are both electronic and optoelectronic approaches to generating terahertz radiation. The electronic approach uses semiconductor or vacuum electronic devices such as oscillators and multipliers, while optical methods employ photomixers and lasers. All these approaches suffer from limited output power: electronic devices must generally be small, and with terahertz lasers, it is difficult to achieve the required population inversion. A way out of the power problem is to employ powerful large-scale generators such as free-electron lasers and synchrotrons.
Why is terahertz radiation an important area of research?
Terahertz continues to be important for spectroscopy applications in fundamental materials research, astrophysics and so on, and is becoming valuable for the contactless identification of substances. A key feature of terahertz radiation is that it can penetrate a number of materials such as cardboard, paper, clothes, many plastics and some types of glass. This allows us to look through covers and into boxes, packages and luggage, which is useful for security purposes and for quality control. Terahertz radiation can be measured such that the electric field radiation and phase is detected, which is useful for depth profilometry and the generation of 3D images. With information technology gradually expanding to ever higher frequencies, the time will come when terahertz frequencies will be interesting for data communication and processing.
What are the main applications and when do you expect them to occur?
A market for terahertz systems in spectroscopic applications already exists and electronic imagers that reach up to 300 GHz are available for security applications. In the coming years, I expect to see a variety of more sophisticated imaging systems bundled into packages with other sensors such as video cameras working in the visible and near-infrared, X-ray imagers and mass spectrometers. Security applications act as a technology driver because the public and industry are willing to invest. Other commercial applications, such as industrial quality and process control will profit enormously from any technological advances, and if costs come down, will offer ample market opportunities. Biomedical applications remain a possibility. I see potential for radar-like applications of terahertz radiation in robotics. However, beam power and system costs remain key issues.
What have been the most important technological advances?
The invention of the terahertz quantum cascade laser in 2002 has been a key milestone, but the laser is limited by an operating temperature below 200 K. In time-domain terahertz spectroscopy, the detection bandwidth has been extended to many tens of terahertz. For basic research, the availability of large-scale terahertz facilities provides improved research capabilities. There is also significant development in multipixel detectors in which room-temperature microbolometer arrays developed for the infrared were found to work well in the upper terahertz frequency range. In the sub-1 THz range, transistor-based detectors have helped to develop room-temperature real-time terahertz cameras.
What key challenges remain?
The most important challenges are developing higher beam power and fast room-temperature multipixel detectors. It is also crucial to decrease the cost of the technology. The use of near-infrared fibre lasers in photomixer-based systems has helped to bring the cost of such systems down below €50,000. However, terahertz devices need to be linked to mainstream technologies and exploit integration in order to slash costs. Specific challenges include using phase information for 3D imaging.
What is the next big breakthrough?
The current revival of vacuum electronics promises compact high-power sources of terahertz radiation such as diamond-film-based backward-wave oscillators. I also expect major impact from silicon-based devices in the future. We are seeing promising reports of a 410 GHz CMOS oscillator and a 650 GHz room-temperature multipixel CMOS detector, which we have developed together with colleagues. The terahertz frequency range is also a good testing ground for photonic bandgap materials and left-handed metamaterials.
• This article originally appeared in the October 2008 issue of Optics & Laser Europe magazine.
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