2023 IMEKO TC-4 International Conference on Metrology for Archaeology and Cultural Heritage Rome, Italy, October 19-21, 2023 Terahertz imaging super-resolution for documental heritage Danae Antunez Vazquez 1 , Laura Pilozzi 2,3 , Eugenio Del Re 1 , Claudio Conti 1,2,3 , Silvia Sotgiu 4 , Federica Delia 5,6 , Mauro Missori 1,2 (1) Department of Physics, University Sapienza, Piazzale Aldo Moro 5, 00185 Rome, Italy (2) Institute for Complex Systems, National Research Council (ISC-CNR), Via dei Taurini 19, 00185 Rome, Italy (3) Research Center Enrico Fermi, Via Panisperna 89a, 00184 Rome, Italy (4) National Central Library of Rome, Viale Castro Pretorio, 105 - 00185 Rome Italy. (5) Recto Verso Conservation Studio, Viale della Venezia Giulia 12b, 00177 Rome, Italy, (6) Academy of Fine Arts of Rome, Via di Ripetta 222, 00186, Rome, Italy. Abstract – THz time-domain spectroscopy (THz-TDS) and pulsed imaging have been demonstrated to be able to provide a non-invasive examination of cultural heritage materials. However, the spatial resolution of THz pulsed imaging is of the order of 1 mm for THz waves, a value that is often not sufficient for the examination of small details on written heritage. This paper will focus on the development of a far-field super-resolution THz imaging system based on a free- standing knife edge and a reflective confocal configuration for the THz beamline. This system has been designed for the recognition of inks, pigments, and dyes used in graphic signs and for the detection of texts buried beneath graphical layers. To optimize the set-up preliminary experiments were realized by imaging the diffraction pattern of a slit, where the freestanding knife set-up showed an improved resolution. After, a reflective set-up was realized and tested on paper samples with graphite patterns. Results demonstrated the super-resolution of THz imaging by showing written features separated by less than the wavelength used. The future direction will be the application of the set-up to real ancient documents for their diagnostics. I. INTRODUCTION Electromagnetic waves are widely used in the nondestructive testing of cultural heritage [1,2]. The application of waves in the terahertz (THz) range, also called the far-infrared range, to diagnostics began about 15 years ago, with applications in spectroscopic imaging and material identification [3]. THz time-domain spectroscopy (THz-TDS) technology, developed in the 1990s [4], uses narrow pulses with pulse widths of the order of 1ps to obtain spectra in the frequency range of 0.2-4 THz [5]. Terahertz imaging has the potential to identify and decipher fragments of ancient writings that are invisible at infrared and visible wavelengths [6]. However, the spatial resolution of THz pulsed im- aging is diffraction-limited to about the wavelength, which is of the order of 1 mm for commonly used THz waves. This work will focus on the development of a far-field super-resolution THz imaging system based on a freestanding knife edge and a reflective confocal configuration for the THz beamline, like those employed in THz-TDS systems [7]. The super-resolution THz imaging system may be applied to the study of the text in ancient documents on paper or parchment substrates, where graphic signs with lateral sizes less than 1 mm are usually found [8]. II. EXPERIMENT PROCEDURE The knife-edge scan is a super-resolution scheme operating with a structured illumination plane at a variable subwavelength distance from the reflecting surface. It achieves super-resolution in its most basic single-wavelength beam-profiling execution, in contrast to advanced super-resolution techniques that generally involve radiation delivery and collection at different wavelengths, deconvolution calculations, fluorescence phenomena, and/or nonlinear interactions with other light fields [9]. In the THz range, knife-edge scans have been implemented to greatly increase the spatial resolution of images of laser-induced broadband source points, and an optically induced virtual knife-edge technique was also demonstrated by structured illumination with a visible laser in the terahertz-emitting (object) plane [10]. 554