# Terahertz Inspection of Buildings and Architectural Art  
[PERSON]  
1 Faculty Engineering and Health, HAWK University of Applied Sciences and Arts, Von-Ossietzky-Str. 99, 37085 Goettingen, Germany; [EMAIL_ADDRESS] (K.K. ); [EMAIL_ADDRESS] (W.V)  
[PERSON]  
2 Department of Physics and Material Sciences Center, Philipps-Universitat Marburg, Renthof 5, 35032 Marburg, Germany; [EMAIL_ADDRESS] (J.O. ); [EMAIL_ADDRESS] (L.M.S. ); [EMAIL_ADDRESS] (A.J. ); [EMAIL_ADDRESS] (M.K.) [PERSON]  
2 Department of Physics and Material Sciences Center, Philipps-Universitat Marburg, Renthof 5, 35032 Marburg, Germany; [EMAIL_ADDRESS] (J.O. ); [EMAIL_ADDRESS] (L.M.S. ); [EMAIL_ADDRESS] (A.J. ); [EMAIL_ADDRESS] (M.K.) [PERSON]  
2 Department of Physics and Material Sciences Center, Philipps-Universitat Marburg, Renthof 5, 35032 Marburg, Germany; [EMAIL_ADDRESS] (J.O. ); [EMAIL_ADDRESS] (L.M.S. ); [EMAIL_ADDRESS] (A.J. ); [EMAIL_ADDRESS] (M.K.) [PERSON]  
3 Independent Researcher, 6962 Viganello, Switzerland; [EMAIL_ADDRESS]  
[PERSON]  
4 Centro de Investigaciones en Optica A.C., Loma del Bosque 115, Lomas del Campestre, Leon, Guanajuato 37150, Mexico
1. It penetrates a broad selection of materials, including textiles, polymers, ceramics, pigments, wood, etc. 2. It is non-ionizing, therefore it represents no occupational hazard to the users of this technology, and to the chemical, and therefore aesthetic, integrity of artworks. 3. It can produce cross-sectional images of layered structures, such as the ones present in multi-layer paintings. Since about 20 years ago, a hand-full of research groups around the world have explored the potential of terahertz for the inspection of many forms of art, which go from oil-on-canvas paintings to mummies, and even a few examples of architectural art. Furthermore, a few examples of demonstrations of this same technique in the evaluation of the structural integrity of buildings have been presented. In this article we will present an overview of terahertz technology, followed by a detailed literature review of applications of T-Rays in architectural art and structural inspection. Finally, we present some new results of our own, in which we monitored in-situ the evolution of a conservation intervention on some murals from the Konstantinbasilika in Trier, Germany. Overview of Terahertz Technology  
Terahertz time-domain spectroscopy (THz-TDS) was first demonstrated in 1988 by [PERSON] and coworkers [7] who used a colliding pulse mode-mocked dye laser to gate photoconductive antennas on the basis of radiation damaged silicon-on-sapphire (SOS). [PERSON] improved this technique, such that it allows for high-quality spectroscopy in the lower terahertz range [8]. Further improvements became possible with the advent of mode-locked titanium sapphire in the early 1990s and the use of low-temperature grown GaAs (LT-GaAs) as antenna substrate. Figure 1a illustrates the working principle of a typical free-space THz time-domain spectrometer: a Ti:sapphire laser that is pumped by a frequency doubled Nd:YVO\({}_{4}\) laser which produces \(\sim\)100 fs pulses with a repetition rate of around 80 MHz. These laser pulses are used to gate both the generation and detection photoconductive antennas that consist of a semiconductor substrate and a pair of coplanar metal striplines. The laser pulse with a photon energy above the semiconductor's band gap energy is focused in between the two striplines. It optically excites electrons and holes. The striplines of the emitter antenna are biased, such that a constant field existed between them with a field strength of around 5 kV/cm. The optically excited carriers are accelerated in this field, leading to a short current pulse that is the source of the emitted THz pulse. After passing through some optical components, such as silicon hyperspherical lenses, off-axis parabolic mirrors, and the sample, the transmitted THz radiation is focused onto the detector antenna, which also consists of two striplines on a semiconductor material. The detector gated by another 100 fs laser pulse.