Bridging the Terahertz Gap
The terahertz range is the region in the electromagnetic spectrum between radio waves and microwaves on the long-wavelength side and the infrared range on the short wavelength side. The terahertz range is promising for many areas of research such as solid state physics, gas spectroscopy, atmospheric physics, biochemistry and bio-medical research. However, there is still a lack of simple devices to generate terahertz radiation. Therefore researchers at the Forschungszentrum Rossendorf in Dresden, Germany, have recently developed a novel terahertz source and filed a patent for it. Using an intelligent trick, the novel source overcomes weak points of existing sources. The company Gigaoptics GmbH in Konstanz, Germany, will take on the marketing and sales of the terahertz source.
Dresden, 11. April 2005. One Terahertz corresponds to one trillion cycles per second. It is the range of thermal radiation in the frequency range from 300 Gigahertz (GHz) to 30 Terahertz (THz). The use of THz radiation for applications in medical diagnostics and biological analytics is still at the beginning, however the potential seems very promising. THz radiation can replace some X-ray diagnostics, for example caries can be detected with higher contrast as compared to standard X-ray diagnostics. Also skin cancer can be detected in an early phase using terahertz imaging. Furthermore, since clothes are almost fully transparent for THz radiation, terahertz security screening systems for detection of concealed weapons are being developed. At the Forschungszentrum Rossendorf THz radiation is used to study semiconductor materials. Of special interest is the dynamics of electrons in so called semiconductor heterostructures, which are systems of layers of different semiconductor materials. The knowledge of the basic processes which determine the electron dynamics is of key importance for the development of future optoelectronic devices.
The weak point of THz radiation – so far it cannot be generated in a simple and cost-efficient way. Furthermore many approaches for generation do not yield radiation with the intensity that is required for many applications. Hence scientists are talking about a gap, the “terahertz gap” in the electromagnetic spectrum. Worldwide scientists are working on closing the gap, either by extending the operation frequencies of microwave electronic devices towards higher frequencies, developing infrared photonic devices for lower frequencies, or by inventing new device concepts that are unique for the THz frequency range. In any case the goal is to generate intense THz radiation at comparatively low cost. Two main approaches are currently pursued. The first approach is to superimpose two laser beams of slightly different frequencies on an electrically biased semiconductor. In this case, the electrons in the semiconductor emit continuous THz radiation. The second approach is based on the irradiation of semiconductors with short, intense laser pulses. The laser pulses create electrons in the semiconductor, which are accelerated in the electric field between two electrodes on the semiconductor surface. The acceleration of carriers gives rise to the emission of THz radiation.
The invention at the Forschungszentrum Rossendorf is related to the second approach and improves existing solutions significantly. If the electrodes on the semiconductor material (typically gallium arsenide) have a large separation (in the centimetre range), in order to have a large active area for THz generation, then high voltages in the kilovolt range are required. This makes such devices unsuitable for practical applications. If one places the electrodes closely together, the device can be operated at small voltages, but on the other hand the active area - and thereby the THz intensity – are greatly reduced.
The Rossendorf physicists around Thomas Dekorsy (now at the University of Konstanz, Germany) and Stephan Winnerl had a simple, but extremely efficient idea. They fabricated an interdigitated electrode structure on a gallium arsenide wafer. The distance between the electrode “fingers” is 5 micrometer and the total active area is currently 1 cm2. Without a second technological trick, however, no THz radiation is emitted for the follwoing reason: the electric field changes its direction from one gap between two electrodes to the neighboring gap. Hence the electrons in neighboring gaps are accelerated in opposite directions and the emitted radiation is cancelled due to destructive interference. Here the trick comes into play: every second gap between electrodes is masked by a second metallization layer, making it inactive. As a result, the radiation from the active gaps can interfere constructively.
As Stephan Winnerl explains, the THz radiation from the device is coherent – a property that is otherwise found in laser radiation – and covers the frequency range from 0.5 to 3 THz. It is a very sensitive tool to make semiconductors appear in new light. The electronic states in semiconductors can be complex, according to the layer structure of a heterostructure. THz radiation is very well suited to study the dynamics of electrons in such complex states.
Another advantage of the invention at Rossendorf is the scalability of the active area and freedom for the user to adjust the beam diameter in a flexible way. This is an important criterion e.g. for using the THz radiation in imaging systems for biomedical applications. The concept of the THz source has been filed as patent and has recently been published in Applied Physics Letters*. The Konstanz-based company Gigaoptics GmbH will commercialize the terahertz source and will present the devices for the first time at the world’s largest exhibit on optoelectronics in Baltimore, USA, in May.
|The new Rossendorf Terahertz Radiation Source||Principle of the Terahertz Radiation Source|
(*) Article in: Applied Physics Letters 86, 121114 (2005).
|Dr. Stephan Winnerl
Institute of Ion Beam Physics and Materials Research
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