# Terahertz Inspection of Buildings and Architectural Art
The semiconductor material chosen for this antenna must have a very short carrier life time, such as SOS or LT-GaAs. In contrast to the emitter antenna, the optically excited carriers are not accelerated by an applied external field, but instead by the field of the incident THz pulse. This leads to a weak current which is amplified and monitored. By varying the delay between the gating laser pulse and the incoming THz pulse, one can sample the THz field via a cross correlation process. See [9] for details. In order to accumulate an image of the THz transmission though an extended object, one can, in the simplest case, translate the sample through the focus of the THz beam in a raster pattern [10]. An image is then acquired pixel-by-pixel. Yet, there have been several other schemes demonstrated over the years (see [11] for a review). It is worth mentioning that, although all TDS systems operate conceptually in the manner that we explained here, most of the studies related to cultural heritage with terahertz have been performed with fiber-coupled TDS systems, in those cases, the Ti:sapphire laser is replaced by a rare-earth:fiber laser and most of the laser optics are implemented in optical fibers, instead of free-space. Picometrix were the first demonstrated fibre-coupled THz systems in the year 2000. Such systems allow to position THz antennas more freely. In 2007 [PERSON] and coworkers presented a THz-TDS system based on a cost-effective fibre laser operating at 1.55 \(\upmu\)m [13]. These lasers are based on low-cost telecom components developed for long-term operation. As antenna material, low-temperature grown InGaAs/InAlAs multiple-quantum wells were used. On the basis of this technology, a variety of different companies today offer fibre-coupled THz-TDS systems with a good signal to noise ratio and a reasonable bandwidth [14; 15]. For THz measurements on buildings, a reflection geometry is needed. This technique was introduced in 1997 by [PERSON] and it is called \"THz tomography\" [16]. Both beams, the one coming from the emitter that is focused onto the object under investigation and the one being reflected from the object and propagating to the receiver are focused or collimated, respectively, either with lenses or via off-axis parabolic mirrors, as shown in Figure 1b. At each interface between materials with different refractive index the pulses are reflected. From the pattern of theses \"echoes\", one can reconstruct the inner structure of the object. THz tomography is closely related to optical coherence tomography [17] and it has some similarities to ultrasound imaging, although sound waves are longitudinal pressure waves, while THz are transverse electromagnetic waves. For many years THz tomography was limited to the investigation of flat samples that were moved through the focus of the beam in a raster pattern. In 2017, [PERSON] et al. introduced a fibre-coupled THz  
Figure 1: (**a**) Schematic of a free-space THz time-domain spectrometer in transmission geometry. (**b**) Schematic representation of a reflection geometry of a terahertz system. (**c**) Render of a THz reflection setup mounted on a robotic arm while doing a scan of an arbitrarily shaped object. Panels (**b**,**c**). Adapted with permission from [PERSON] PhD thesis, 2019 [12]. tomography unit attached to a robotic arm [18] as shown in Figure 1c. With this approach, the THz emitter and receiver can be positioned perpendicular and at defined distance to the sample surface. Hence, this system allows for the acquisition of reflection THz tomographic images of samples with an arbitrarily shaped surface.