Terahertz (THz) technology has become an innovative and fast growing field of research over the last few years. Its potentials are becoming increasingly obvious in the areas of food control and security technology. Imaging terahertz processes are the main focus of interest in the above areas. 

However, medical technology has proven to be a less suitable field of application, as recent research has shown. But researchers at the Technical University of Braunschweig see a mass market in the area of indoor communication.

Terahertz systems are becoming more and more important in the area of nondestructive test methods. There is a good reason for this. These waves, which can be seen as very high frequency microwaves or very long-wave light, are better than x-rays, for instance, because they can make different contrasts visible and they are also safe.

The technologies on which the various applications are based differ considerably. But the most commonly used are imaging processes and optoelectronic systems. Here, the THz rays are generated by short laser impulses with photoconductive dipole antennas. Cheaper systems with slightly more limited performance can be realized with diode lasers. In this case, the focus is on a fixed frequency rather than a broad spectrum. If higher frequencies or better spatial resolution is required, it is better to use a THz gas laser or a quantum cascade laser.

For imaging processes THz systems use far-infrared rays with frequencies between 0.1 and 10 THz. The ray is focused on the probe with the help of quasi-optical elements. In most cases a THz image is created by scanning the probe in the x and y direction vertically to the beam and evaluating the transmitted or reflected signal for every point. The two-dimensional presentation of all points together then produces the THz image.

In terms of possible applications for this technology, THz cameras based on microwave technology could create new standards in the field of security systems. With this technology it would be possible to detect explosives as well as metal or ceramic weapons beneath clothing and in non-metallic containers, with no harmful side effects for humans. This is because one of the properties of THz waves is that they can penetrate textiles and be efficiently reflected by objects containing water, such as the human body. Several companies are currently working intensively to develop cameras such as this. But government agencies, like the Berlin-based German Aerospace Centre (DLR), are also active in this sector.

Another area of application for THz technology could be the food industry. At present the potentials in the area of nondestructive process and quality control are being researched at the Institute for High Frequency Technology at Technical University Braunschweig among other places. In this connection scientists are also involved in detecting foreign bodies in food. While metal contamination can easily be detected these days with metal detectors, current technology is generally unable to detect non-metallic contamination. For instance, contamination in chocolate can be determined on the basis of a specific refraction index of the material in the THz range.

However, the scientists at Technical University Braunschweig are rather skeptical about the use of terahertz technology in medical diagnostics. In this field a previously underestimated problem arises because the water content that organic samples generally have in this range is too high. This severely limits the penetration depth in the samples.

The scientists in Braunschweig believe that the future potential for THz waves lies in the area of indoor communication. In addition to imaging processes the THz waves also have enormous technical potential for wireless networking of mobile office equipment. However, it will be a few years before this technology can be introduced to this sector of the market.
Scientists at the Institute for High Frequency Technology at Technical University Braunschweig generate terahertz waves with a femtosecond laser
photo: Technical University Braunschweig