Investigation of hidden structures in art and technology by THz-time-domain imaging

THz radiation passes through a large number of nonconductive materials with little attenuation while conductive materials block electromagnetic waves in that spectral range. This property opens the way for the investigation of hidden structures like circuit paths embedded in three-dimensional structures of, say, ceramic. On the other hand, molecular groups with electrical dipole moment can cause characteristic absorption lines in the THz-spectrum. So a fingerprint-like identification of substances is possible in many cases. This makes it possible to obtain structural and compositionional information simultaneously using tomographic and spectroscopic information provided by THz-Time Domain Imaging. In this paper, the potentials and limits of THz-Time Domain-Imaging are presented with examples from technology and arts. A software package has been developed by the authors to envision the results. Measurements are possible for both time and frequency domains. In both cases, movies that show the electrical field signal of every point of a lateral image scan in transmission versus time or frequency have been calculated. The latter makes it possible to show the distribution of substances across the sample. Single frames are extracted from the movie at frequencies which show absorption minima in the spectrum.