Abstract-Terahertz Radiation Transport in Photonic Glasses

Silvia Gentilini,  Mauro Missori,   Neda Ghofraniha,   Claudio Conti,

https://onlinelibrary.wiley.com/doi/10.1002/andp.202000005

Multiple scattering of electromagnetic (EM) waves arises in disordered media with a refractive index varying on the scale of the wavelength. The diffusion approximation is a powerful tool to treat multiple scattering as a photon random walk, neglecting resonant phenomena. However, as the light intensity varies on a scale much smaller than the transport mean free path, resonances may occur in media formed by finite‐size scatterers and break the diffusion approximation. The energy and phase velocity are very useful tools to reveal the onset of the resonant transport regime. In this paper the study of the propagation of terahertz (THz) waves through 3D random media by employing terahertz time‐domain spectroscopy (THz‐TDS) is addressed. Specifically, measurements of the electric field transmitted by samples of different thicknesses made of 1 mm diameter silica spheres dispersed in a paraffin matrix at different filling fractions are reported. This investigation has provided an accurate measurement of the EM field phase and, hence, information on the radiation propagation velocity that has enabled the first observation of a photonic glass at the THz range.

Abstract-Confocal Imaging at 0.3 THz With Depth Resolution of a Painted Wood Artwork for the Identification of Buried Thin Metal Foils

Chiara Ciano, Mariano Flammini, Valeria Giliberti, Paolo Calvani, Eugenio DelRe,  Fabio Talarico, Mauro Torre,  Mauro Missori, Michele Ortolani

https://ieeexplore.ieee.org/document/8362985/

A compact confocal terahertz microscope working at 0.30 THz based on all-solid-state components is used to locate buried thin metal foils in a painted wood artwork. Metal foils are used for decoration, and their precise localization under the pictorial layer is relevant information for conservation scientists and restorers, which can neither be obtained by X-ray radiography nor by spectroscopic imaging in the infrared, as we directly show here. The confocal microscopy principle based on the spatial pinhole concept is here implemented by positioning the first focus of an ellipsoidal reflector at the phase center of horn antennas coupled to Schottky diode detector and emitter mounted in rectangular waveguide blocks, together with an optical beamsplitter. The second focus of the reflector is mechanically scanned inside the sample in three dimensions. The predictions of diffraction theory for a confocal microscope at an imaging wavelength of 1.00 mm with numerical aperture of 0.53 are verified experimentally (1.2 and 2.8 mm for the lateral and the axial resolution, respectively). These values of resolution allow a precise determination of the position of buried metal foils in an ancient piece of art hence making restoration interventions possible.

Abstract-Cancellation of Fabry-Perot interference effects in terahertz time-domain spectroscopy of optically thin samples

Renato Fastampa, Laura Pilozzi, and Mauro Missori

https://journals.aps.org/pra/abstract/10.1103/PhysRevA.95.063831

Terahertz time-domain spectroscopy is increasingly used in many fields of research. For strongly absorbing materials with refraction index close to 1, optical parameters at terahertz frequencies are most conveniently quantified using transmission measurements through thin samples. Unfortunately, extracting optical parameters from raw data implies the use and/or development of complicated numerical data processing procedures. In this work we present an efficient computational procedure for extracting the optical parameters in very thin samples $(\lesssim 100\mu \text{m})$ from transmission terahertz time-domain spectroscopy. In our procedure, we are able to successfully remove from raw data the Fabry-Perot interference effects, which are commonly recognized to be the leading cause of inaccuracy in the extracted parameters, introducing fictitious oscillations in their frequency dependence. The procedure is based on the Davidenko method to identify the roots of complex functions used to numerically solve the implicit equation obtained by equating the experimental and theoretical transfer functions. The advantage of the method is the possibility of obtaining the roots using the numerical solution of a system of real differential equations using standard mathematical packages. In addition, we show that complete removal of the Fabry-Perot oscillations is achieved by including in the computational procedure, besides the sample thickness, the instrumental error on the starting instant of the terahertz signal sampling. This error could be common to many terahertz time-domain systems, especially those using optical fibers. This correction is necessary in general to preserve the terahertz spectroscopic features in the extracted optical parameters for strongly absorbing materials with refraction index close to 1, such as water, biological matter, and several organic materials.

Abstract-Stabilization of the Tensile Strength of Aged Cellulose Paper by Cholinium-Amino acid Ionic Liquid Treatment

Eleonora Scarpellini, Michele Ortolani, Alessandro Nucara, Leonetta Baldassarre, Mauro Missori, Renato Fastampa, and Ruggero Caminiti

J. Phys. Chem. C, Just Accepted Manuscript

DOI: 10.1021/acs.jpcc.6b06845

Publication Date (Web): October 6, 2016

Copyright © 2016 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/acs.jpcc.6b06845

In this study, a chemical stabilization method, that preserves the tensile strength of artificially aged paper pre-treated with an aqueous solution of ionic liquid, is presented. Pure cotton cellulose papers samples were soaked with cholinium glycinate ionic-liquid solution either before or after the artificial aging process, which was based on thermal degradation in dry air. The tensile strength of artificially aged paper was measured by using the double-folding technique. The role of ionic liquids was investigated by mid-infrared and terahertz time-domain absorption spectroscopy. It was found that the tensile strength of pre-treated samples is higher than that of other aged samples. A model for changes in the cellulose structure caused by oxidation and by the binding of ionic liquid molecules is proposed, basing on the analysis of the mid-infrared absorption bands in the carbonyl/carboxyl region at 1590-1750 cm−1. Terahertz spectroscopy data indicate that the ionic liquid molecules penetrate in the larger size porosity, acting as binders among cellulose fibers to maintain the tensile strength of paper.