| Abstract |
Terahertz radiation lies between infrared and millimeter waves in the electromagnetic spectrum. It spans from 0.1 THz to 30 THz and its photons have an energy amount of a few meV. For a long time, this part of the electromagnetic spectrum was known as the “THz gap” due to the lack of coherent sources and detectors. It was only in the late 1980’s when the first demonstration of coherent emission and detection of broadband THz pulses took place and allowed routinely access to this part of the electromagnetic spectrum. Thus, the unique properties of THz radiation have been revealed, suggesting the development of promising applications.
The main characterisitcs of the THz waves that make them a unique tool for spectroscopic or imaging applications in many scientific fields, from security to applications in medicine and in food industry- are (a) its non-ionizing character due to their low energy (~meV) photons, (b) the fact that many non-metallic and non-polar materials are transparent to THz radiation, due to its long wavelength, (d) are partially reflected from interfaces between non-metallic materials with different refractive indices and (c) the fact that THz radiation is highly absorbed by water.
Due to these properties THz waves can become a very useful and promising tool in fields such as art conservation and civil engineering where a common problem is the presence of moisture in walls. In the case of art conservation the presence of water in depth of some millimeters under a mural painting can be disastrous. THz radiation can provide very useful information: (a) for periodically monitoring the conditions of a fresco in terms of a moisture content map of its surface, (b) for early diagnostics, contributing to the detection of in depth humidity and (c) for giving insight about the absorption dynamics of water (or water-based products) in the areas to be treated during the restoration phase. In the civil engineering field, similar problems arise, where the penetration of water in walls can create cracks/defects and thus decrease the strength of concrete structures. These kind of degradations in buildings can become very dangerous especially in seismogenic regions. Thus, as it has already been mentioned, a tool for early diagnostics of the condition and location of moisture in buildings walls is very useful.
In this thesis, a first attempt to create an experimental routine and the appropriate analysis for the detection and localization of moisture in depth of some millimeters, exploiting THz radiation through a THz-TDS reflection system, is going to be presented. First, the operation of our homemade reflection THz-TDS system and its ability to distinguish different moisture contents for paper samples, with single surface and stratified ones, is demonstrated. Then, a number of different materials are tested in order to choose the most suitable one for our experiments, which we conclude to be the gypsum, a material commonly found in walls. Finally, in the last part of this thesis, the presence and the specific location of moisture is demonstrated both experimentally and theoretically.
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