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Development of THz spectroscopy and imaging systems

Fig. 1: typical THz lab spectrometer (photo taken in 2002)

A variety of different methods exist to generate and detect THz waves. The most common approach bases on the generation and detection of THz pulses using photoconductive antennas which are gated by ultrafast laser pulses. This technique which is called terahertz (THz) time-domain spectroscopy (TDS) was first demonstrated in 1998. 10 years ago the femtosecond lasers which provide the ultrashort pulses were bulky and expensive. The same was true for the spectrometers (Fig. 1) which could be operated only in research labs.

In the year 2006, the group of Prof. Koch (at this time still at the Technical University of Braunschweig) could demonstrate that low-priced and compact femtosecond fiber lasers can be used to operate THz TDS systems, if photoconductive antennas made of InGaAs are used (see: R. Wilk et al., CLEO 2007, Baltimore (May 2007)).

Fig. 2: portable THz system for plant water status monitoring

During the last years, this approach was further developed in several projects and portable systems have been developed. One example of such a system can be seen in Fig. 2. It is a portable fiber-coupled THz TDS system that can be used to determine the water status of plants. The system still has the size of a refrigerator. Yet, the next generation will fit in a backpack and can be operated by a battery. And of course, fiber-coupled THz spectrometers are meanwhile commercially available

However, also the price of technology is crucial for its acceptance. In the year 2009, the group could demonstrate that one can replace the cost-intense femtosecond laser by a simple laser diode if one accepts a somewhat lower performance. With this approach and a few other inventions which have been made in the group, one can build a low-cost THz system.