Since 1960’s the radio astronomy has been an important application for millimetre waves, but many other applications are imaginable as well. Because of the longer wavelength the millimetre-wave imaging is possible also during a bad weather, but still the wavelength is short enough to produce an accurate image. It is clear that millimetre and terahertz imaging will be more and more interesting in the near future, especially in different safety applications and spectroscopy.
Another interesting field for terahertz waves is biology and medicine. One could think that a natural application of terahertz waves would be telecommunications as larger bandwidths are available at these frequencies. However, it has to be remembered that the propagation properties of these waves do not allow very long transmission distances and also that the current state of the components is not yet suitable for these very demanding telecommunications applications.
APPLICATIONS OF TERAHERTZ TECHNOLOGY
1 Terahertz imaging
Detection and imaging of natural blackbody emission is much more challenging at millimetre wavelengths than in the infra red region because of the much smaller bandwidth available at millimetre wavelengths. Sensitivity is
the major motivation for using active illumination to enhance the contrast of the images. Passive architectures that measure natural radiation can be also used, but the indoor applications, like security screening, requires active illumination of the object to be imaged. Several types of imaging architectures exist. The image is achieved by measuring the power of the millimetre waves received at each pixel in turn. It is possible to build up an image of a scene by scanning a single pixel receiver, or by using an antenna array to receive a large number of
pixels at the same time. Combining these techniques to form an image is also possible.
Security screening for example at the airports was already mentioned, but a similar system using an antenna array as imaging device can be used in airplanes or in ships to make the navigation easier in bad weather conditions like in fog. Detection of land mines is a demanding task world-wide and there is not yet a reliable detection technique to find buried mines. Terahertz imaging has been considered also for this application, but the problem is the poor penetration depth into the ground, especially in wet conditions, so an active system with a significant amount of transmit power would be required. Other problems to overcome are in image recognition.
2 Applications in biology and medicine
There are several research groups around the world who have applied terahertz techniques to disease diagnostics, recognition of protein structural states, monitoring of receptor binding, performing label-free DNS sequencing,
visualising and cataloguing absorption and contrast mechanisms in otherwise uniform tissue and radiation effects on biological samples and biological processes. Two applications that are very promising are measurements of avidin-biotin binding and DNS hybridisation. Since the terahertz signal is both remote and non-destructive, this is a fast and powerful method for determination of the change of state of many biologically important processes. Also a very promising application of terahertz molecular spectroscopy for detecting the presence of unwanted polymorphs in prepared drugs has been developed.
Detection or early characterisation of disease is also one of the hopes for terahertz applications. Wound inspection through dressings or casts is also one promising application for terahertz imagers. Current terahertz technologies in medical and biological field are still in a very early and exploratory stage.
And even if many potential applications can be imagined, one big problem is the difficulty to get funding for these projects of high risk. One big challenge is also to get the medical and engineering communities closer to each other.
3 Spectroscopy applications
Passive millimetre-wave systems have been already developed to remotely map thermal and chemical signatures. This information can be used to investigate potential chemical hazards in the air or for example in the envelopes or even to detect facilities for weapons of mass destruction. Chemical and nuclear facilities may be identified by remotely measuring molecular signatures of airborne molecules emitted from these locations. Most such facilities release a lot of heat as a by-product.
Although thermal information alone is not specific enough to identify a process, it can be an indication to bring other remote verification technologies. Passive millimetre-wave spectroscopy measures rotational emission spectra of molecules. With a heterodyne radiometric technique, millimetre and sub millimetre spectral lines of upper atmospheric molecules have been measured from mountain tops and satellites. Terrestrial measurements are more difficult because the spectral lines are broader due to pressure-broadening phenomenon and the earth surface does not offer uniform background as the sky.
Typically fire detection sensors use optical technology and they do not work well in adverse atmospheric conditions and are incapable of covering sizeable areas. A scanning radiometer using millimetre waves has been developed and it has proven the advantages of remote fire detection even under adverse weather conditions and through smoke, giving fewer false alarms than optical sensors and allowing a larger area of coverage. According to the results a millimetre-wave sensor can also accurately locate a developing fire.
4 Terahertz technology in telecommunications
The interest in short range wireless communications means also a need for higher data rates and large bandwidth with new demanding applications. Standardisation institutions have been working on new schemes for short range wireless communications. New standards like IEEE 802.11a, g+ and HIPERLAN/2 are, however, limited to a data rate of 54 Mbit/s.
This capacity will not be sufficient to cover many future short range applications. One indoor communication scheme is based on impulse radio, generally referred to as Ultra-Wide Bandwidth Radio, and it is expected to achieve data rates up to 500 Mbit/s. It is clear that in near future short range wireless communications systems will operate at frequencies of several tens of GHz. But as the applications become more and more demanding wireless communications systems will have to work at several hundred GHz, thus at terahertz range, to support sufficient bandwidth. If the problem in microwave range is too small bandwidths, the problems above terahertz range, infrared or optical frequencies, are the eye-safety power limits, an inefficient direct detection technique and high ambient light noise.
Thus the terahertz range is the most suitable for future applications and a lot of research is done globally to support these applications. Fundamental hardware to build a commercial wireless terahertz communication system does not exist today. Currently the researchers are far from having room-temperature emitters and receivers in terahertz range that would also be cost effective and small in size. To obtain room-temperature terahertz devices for communication purposes, long term research and many technological breakthroughs are required.