“Terahertz pulse imaging of stratified architectural materials for cultural heritage studies” J. Bianca Jackson* a,d , Julien Labaune a , Gerard Mourou a Irl N. Duling b Gillian Walker c , John Bowen c Michel Menu d a Institute de la Lumière Extrême, École Polytechnique, Palaiseau France ; b Picometrix-API, Inc, Ann Arbor MI United States; c School of System Engineering, University of Reading, Reading United Kingdom, Reading United Kingdom; d CNRS-LC2RMF, Palais du Louvre Paris France ILE, ABSTRACT Keywords: terahertz, imaging, spectroscopy, cultural heritage, architecture Introduction In recent years, Terahertz pulse imaging (TPI) has become a novel nondestructive technique for the examination of cultural heritage artifacts. It has the advantage of broadband spectral range, time-of-flight depth resolution, and penetration through optically opaque materials. Fiber-coupled portable systems have also enabled the technique to be taken out of the laboratory and into the field. At many architectural heritage sites, it has been found that decorative renovations for pragmatic, stylistic or religious purposes have resulted in the stratification of generations of architectural artwork. Each layer can provide insight into the history and culture of the people who created it. Thus, by exploiting the “optical” inhomogeneity of these structures, we can analyze terahertz pulse reflections from each interface and reconstruct multi-dimensional images of the sub-surface layers. For various reasons, it is common for these paintings to be obscured by pollution, additional paint, and/or addition layers of paneling or plaster. It is desirable for conservators to reveal these hidden works without risking any valuable surface work or support structure. As a spectral imaging tool, time-domain terahertz pulses have an advantage over other methods because the penetration depth of the terahertz beam is limited primarily by scattering and material absorption. Most materials that are opaque in the UV to near-infrared range of the EM spectrum are semi-transparent to terahertz radiation. Plaster is a notable example of this. While infrared light may penetrate a layer of plaster as thick as 500 m, This work concentrates on the TPI of laboratory models of multilayer mosaics and fresco paintings, specimens extracted from a neolithic excavation site in Çatalhöyük, Turkey, and specimens measured at the medieval Église de Saint Jean Baptiste in Vif, France. Furthermore, terahertz spectroscopic studies were made of the various composite materials— including ochre pigments and lime, gypsum and clay plasters—with the intent to aid future computer simulations of the TPI of similar subjects. Methodology Our time-domain terahertz systems consisted of an ultrafast, optically-excited, photoconductive antenna emitter and receiver pair, oriented in a either a normal or off-axis reflection geometry. The terahertz beam was focused on the first surface plane—for thinly stratified objects—or, on the layer with the next largest interface reflection—for thickly stratified objects. The cross-sectional information was obtained using a scanning optical delay line which enabled the sampling of the terahertz pulse signal in time, and thus the measurement of the time-of-flight of the terahertz pulse in the Z-direction. The structures were then physically scanned en face using a motorized X-Y translation stage.