Terahertz Spectroscopy and Imaging
School of Electronic and Electrical EngineeringUniversity of Leeds, Leeds LS2 9JT, U.K.
The terahertz (THz) region (0.3–10 THz) of the electromagnetic spectrum spans the frequency range between the infrared and millimeter/microwave, as illustrated in Fig. 1. Historically this region has not been fully exploited owing to the very limited number of suitable (in particular, coherent) radiation sources and detectors that have been available. In recent years,1 outstanding progress has been made in developing THz components and systems. Highlights include the fabrication of the first THz quantum cascade laser,2 together with the commercialization of THz spectroscopy and imaging systems based on femtosecond-laser technology,3 notably for non-destructive testing in the pharmaceutical industry (for example, investigating polymorphic transformations, and drug distributions in tablets4). Further, three years ago an ultra-broadband THz spectroscopy system was proposed, making use of photoconductive emitters in a reflection geometry. This offered a bandwidth in excess of 20 THz for spectroscopy applications,5 and its performance was demonstrated through measurements of the absorption of polycrystalline adenosine.1 Good agreement was obtained with spectra acquired using the complementary technologies of Fourier-transform infrared, Raman, and neutron inelastic spectroscopy.
Despite these exciting ...