A collaboration between several German research groups has resulted in what is claimed to be the first terahertz imaging spectrometer based on continuous-wave (CW) terahertz radiation. Three of the groups are based in Braunschweig (Institut für Hochfrequenztechnik, at the Technical University of Braunschweig; the Physikalisch-Technische Bundesanstalt; and the Institut für Pathologie at the Städtisches Klinikum) and a fourth group is at the Philipps-Universität in Marburg. The imaging system is designed to be relatively low cost and compact, and is aimed at biological- and medical-imaging applications.
The far-infrared range of terahertz frequencies—which lies between the frequency ranges used by optical spectroscopy and modern microwave telecommunications—has potential for uses that remain relatively unexplored. One of these is imaging; recent work has shown that terahertz imaging yields better resolution than microwave imaging. One problem in developing these systems has been that the spectral range between a few hundred gigahertz and several terahertz is hard to access. Recently, terahertz imaging has been demonstrated using pulsed radiation. The radiation was generated using a biased photoconductive dipole antenna gated by optical femtosecond laser pulses. The gating caused a short-current pulse, leading to the emission of an electromagnetic impulse in the terahertz range.
More recently, attempts have been made to replace the femtosecond gating pulses with two superimposed CW laser beams that are slightly detuned with respect to each other so that CW terahertz waves can be generated. Last year, such a system was developed using a two-color Ti:sapphire laser pumped by an argon-ion laser. Both the femtosecond-laser-based pulsed system and the Ti:sapphire-based CW system are expensive and relatively cumbersome. The newest system developed by the German groups uses cheaper and more robust laser-diode technology.
The laser-diode-based terahertz imaging system uses an electronically tunable external-cavity laser diode as a light source (see Fig. 1). The laser used is a gallium arsenide (GaAs) multiple-quantum-well device operating at 830 nm and producing a maximum output of 35 mW from the tunable cavity. A V-shaped mirror is used as the frequency-selective element. The frequency spacing of the two laser modes can be adjusted by moving the V mirror vertically; the spacing is set to 230 GHz, which is the resonance frequency of the photoconducting dipole antenna used for photomixing.
The two-line laser emission has a total optical power of 29 mW and is focused onto the 5-μm-wide gap of a 50-V dc-biased stripline dipole antenna. The dipole has a length of 400 μm and was deposited on a GaAs substrate grown at low temperature. The epitaxial layer was grown at 300° C and annealed at 600° C for 10 min to achieve carrier lifetimes in the picosecond range, a necessity for photomixing. A standard bolometer is used to detect the terahertz radiation. Four off-axis parabolic mirrors bring the terahertz waves to a focus. The image of the sample is generated by scanning the sample stage through the focus and using a computer-controlled lock-in detector to acquire the transmitted radiation registered by the bolometer, pixel by pixel.
Distinguishing tissue types
A prepared sample of human liver tissue with cancerous areas was used to test the performance of the system (see Fig. 2). The level of detail obtained in this first demonstration of the CW system is not yet as good as that obtained with a pulsed system. The pulsed system used an integration window from 0.2 to 0.5 THz, which was found to give the best contrast; in comparison, the relatively low frequency of the CW system leads to a reduced spatial resolution. In addition, the signal-to-noise ratio was only 75 in this first demonstration of CW terahertz imaging, diminishing the quality of the image.
Martin Koch, head of the Technical University group, believes that the CW terahertz imager has great potential because it is a tunable system that makes possible low-noise transmission images at precisely defined frequencies. This quality is important because in some cases an unambiguous distinction between different types of tissue is only possible from a comparison of two or more terahertz images obtained at different frequencies, says Koch. By tuning the CW frequency to the respective absorption lines, it should be possible to visualize distinct substances, he notes.
After this first demonstration, the researchers want to develop a rugged and compact portable system in which the laser is fiber-coupled to the antenna. They plan to use antennas with higher resonance frequencies. The group also plans to increase the terahertz output power by using a more powerful laser diode, improving the signal-to-noise ratio.