Just a few years ago sensors operating at terahertz wavelengths were almost unheard of. Today, the technology is being used to improve the safety of the Space Shuttle, and could soon be used in airport-security scanners. Nadya Anscombe talks to René Beigang, head of the Ultrafast Photonics and Terahertz Physics Group at the Technical University of Kaiserslautern in Germany, about how terahertz radiation is being used to create a wide range of new test and measurement systems
What is so special about terahertz radiation?
Many materials used in everyday life are highly transparent in the terahertz (THz) region, while others absorb the radiation in very characteristic ways. We can therefore use THz radiation to see through materials such as clothes or plastic film and to analyse other materials using spectroscopy. In solids, weak non-covalent interactions between molecules can be observed, which can provide information about their crystalline structure; and metals are opaque at THz frequencies, so they can also be easily identified.
“Terahertz radiation is ideal for detecting non-metal objects hidden under clothing”
It is a non-ionizing radiation, which means that it is safe to use on humans. For example, at airports, it is ideal for detecting non-metal objects hidden under clothing. And, in the pharmaceutical industry, it could be used to perform chemical analysis of samples through containers such as blister packaging. No other region of the electromagnetic spectrum has these properties.
Why is there currently so much interest in this region of the spectrum?
The THz part of the spectrum is located between microwave and infrared radiation (100 GHz – 10 Hz) and has, until recently, not been easily accessible due to the a of efficient sources and detectors. Research in THz radiation is now benefiting from advances in other areas of physics, such as the development of lasers that can reliably produce femtosecond-long light pulsesand advances in non-linear optics — both of which can used to generate THz radiation. Now that good sources and detectors are available, companies and research groups have made impressive progress in developing products and applications utilizing this region of the spectrum.
What has been the most important technical advance in this area in the last few years?
The development of THz time-domain spectroscopy (TDS) — a powerful tool for material identification and chemical analysis — would not have been possible without advances in femtosecond lasers. The generation and detection scheme is sensitive to the effect of a material on both the amplitude and the phase of THz radiation. In this respect, the technique can provide more information than conventional Fourier-transform spectroscopy, which is only sensitive to the amplitude.
Are THz measurements being used today in industrial applications?
Yes, and the number is growing every day. But the industries that could make the most use of the technique — aerospace, defence and pharmaceutical — are notoriously cautious about adopting new technologies, making progress slow. Several companies have already launched passive THz cameras for use in airport-security systems and some are also developing active systems. For example, the UK companies ThruVision and QinetiQ have systems on the market.
Many people believe that THz technology has the potential to replace X-rays and metal detectors at airports, but I have my doubts. Security screening is a very challenging application because of scattering from clothing and the need to perform imaging and spectroscopy in real time. However, I think that the detectors will be a useful supplement to current screening techniques and I am that convinced we will find such systems at airports in the next few years.
Another UK company, TeraView, is exploring an interesting application in the pharmaceutical industry. The company claims that its system can be used to make high-speed measurements of the coating thickness of tablets while the pills are in random motion in a coating pan. For this application, the non-contact, non-destructive nature of the measurement technique is an advantage, plus it gives 3D and chemical information about the contents of tablets and capsules — something not normally possible with conventional monitoring techniques.
“NASA has used THz products developed by US-based firm Picometrix to examine the exterior of the Space Shuttle”
THz technology can also be used to detect defects in materials. For example, in the plastics industry it can be used to find air bubbles in extruded products, check the integrity of welded plastics and to monitor product thickness. NASA has used THz products developed by US-based firm Picometrix to examine the exterior of the Space Shuttle. THz measurements are used to detect flaws and poor adhesion in the sprayed-on foam insulation of the shuttle’s external fuel tank. NASA is also evaluating the use of THz measurements to determine the integrity of the tiles on the shuttle’s heat shield. As a shuttle ages, corrosion can form under the tiles potentially causing them to detach. By examining the layers that attach the tiles to the orbiter, NASA can determine which tiles to replace, and which tiles are still in good working order.
As it is safe to use on humans, THz technology can also, in principle, be used for medical imaging applications and in dentistry. Applications for this technology are endless.
Are there any problems with its use?
Yes, the radiation does have its limitations. Most polar molecules in the gas phase interact with THz radiation, which means that, in principle, it could be used to detect gases. However, the sensitivity of such techniques are not comparable with other, more established, methods of gas detection. The biggest problem for applications of THz radiation is water vapour, which interferes with measurements. This makes it challenging to use THz systems in outdoor settings.
What are the future innovations that are needed to help this industry develop and grow?
The main issues are cost, speed and complexity. THz systems are expensive because the femtosecond lasers used to create the radiation are very costly. The use of low-cost femtosecond fibre lasers, in particular at telecommunication wavelengths, should bring down the overall cost of systems. The use of continuous-wave devices based on electronic or optical generation may also reduce the cost considerably, although these systems cannot be used for all applications.
“In order to be useful in industrial applications, the systems have to become robust, compact and easy to use”
In order to be useful in industrial applications, the systems have to become robust, compact and easy to use. The time it takes to make measurements is another important issue that has to be addressed. In particular, for online process monitoring or security applications, new 2D measuring techniques have to be developed, including fast evaluation algorithms. The ongoing improvement of quantum cascade lasers as THz sources could enable more industrial usage of these systems.
As the technology has many different fields of application, it will not be possible to use a single THz system for all these applications simultaneously. Therefore, all the systems currently under development have their advantages and problems depending on the application being considered. To make most use of single systems, a modular set-up with flexible emitters and detectors is advantageous as it can be easily adjusted to a particular situation without changing the whole THz system. This is one route that many commercial companies and research institutes are following in order to make THz technology useable for different industrial applications.
Prof. Beigang will be making a plenary address on advances in THz technologies at the International Infrared Sensors and Systems Conference 2008 on Tuesday 6 May in Nuremberg, Germany.