We present a broad literature review on the uses of terahertz radiation for the inspection architectural art, as well as building structures. As an example of the uses of terahertz in this field, we also include original results on a non-destructive assessment of a conservation intervention on murals of the Konstantinbasilika in Trier, Germany while using terahertz time-of-flight spectroscopy.

In this work we demonstrate that 3D printed Al ₂ O ₃ is a promising material for prototyping and precise fabrication of quasi-optical devices in the terahertz frequency range. The 3D printed Al ₂ O ₃ exhibits a low absorption coefficient (α <; 2cm ^-1 at 1 THz) and a high refractive index (n > 3). The printing resolution in the sub 50 μm range allows for the implementation of structures in the 0.3-3.0 THz range on the subwavelength scale. Furthermore, the printing process enables the realization of crystalline solids, which allows the use of the Al ₂ O ₃ birefringence effect. Here, a Δn ≈ 0.05 was achieved and used for the implementation of λ/2-wave plates working at ~1 THz. The material properties and wave plates were characterized using a terahertz time-domain spectrometer.