Abstract—We report on the absorption properties of hybrid sol-gel materials based on the 3-glycidoxypropyltrymethoxysilane (GPTMS) precursor in the spectral region 0.5–7 THz. THz transmission properties of such materials are quite good and, even if not comparable with the most transparent polymers, still their chemico-physical, structural, and electro-optical properties make them attractive alternative systems for applications in the THz field. An attempt to explain the relationship between compositional and structural characteristics of the investigated materials with their transmission behaviour is presented. I. INTRODUCTION n the last years the THz region has attracted growing interest, both for the theoretical relevance (in molecular spectroscopy, for instance) and for the several practical applications in high-speed communication and security imaging. However, suitable techniques to control and generate THz waves are still lagging behind, pushing the research to develop THz technologies for electronics and photonics. In particular, the realization of guided systems for THz transmission is of fundamental importance to solve problems of efficient radiation manipulation, and integration on a single chip device of sources and detectors, in the so called “THz gap” (0.3-10 THz). Many materials have already been studied in this spectral region. In this work, an investigation on hybrid sol-gel systems has been carried out in order to explore their potentialities as transparent media in the THz region. Hybrid organic inorganic sol-gel materials present the outstanding advantage of joining together the properties of glassy and polymer systems, giving the possibility to widely tailor material electro-optical and structural features to experimental requirements. We report on the transmission properties and absorption coefficients of hybrid organic-inorganic sol-gel materials prepared starting from an organically modified alkoxide containing an epoxy ring, the 3- glycidoxypropyltrimethoxysilane (GPTMS) precursor. An attempt to explain the relationship between compositional and structural characteristics of the investigated materials with their transmission behaviour has been given. The lowest absorption coefficients of such hybrid sol-gel materials in the THz region are found when unpolymerized epoxy rings are present, and in the case of systems that are poor of un-reacted hydroxyl groups (-OH) interacting through hydrogen bonds. II. EXPERIMENTAL GPTMS was hydrolyzed both in acidic and basic conditions. Acidic conditions were realized by adding HCl (1N) in molar ratio GPTMS:H 2 O:HCl = 1:3:0.3, while basic conditions was obtained by adding NaOH (1N) in molar ratio GPTMS:H 2 O:NaOH = 1:3:0.003. Both solutions were left to react for 1h under reflux at 80 °C. Sols were filtered with a 0.2 μm Millipore filter. Epoxy polymerization of GPTMS-based hybrid systems was achieved by using a Lewis acid catalyst, 1% weight on total solution. Samples were prepared by casting the solution into Teflon molds, and applying a thermal treatment at 70 °C for 24h. Flat slabs of typical thickness ranging from hundreds μm to a few mm are obtained. Infrared absorption spectra in the range 4000-400 cm -1 were recorded by Fourier Transform Infrared spectroscopy (FTIR) (Perkin-Elmer 2000) on similar films deposited by spin coating on silicon wafers (100), and thermally treated at 70 °C for 24h, with a resolution of 4 cm -1 . Ultraviolet and visible absorption spectra the range 200-2000 nm have been recorded with a UV/VIS/NIR spectrophotometer (Jasco V-570) on sample slabs, with a resolution of 1 nm. THz transmission measurements have been performed with a Bruker IFS66v Fourier Transform Interferometer at the SISSI infrared beamline of the ELETTRA storage-ring, by using both synchrotron and conventional (Globar) sources. A He-cooled bolometer detector with a filter in the range 0-250 cm- 1 was used, together with a 50 μm mylar beamsplitter. Measurements have been taken at a 4 cm -1 spectral resolution. III. RESULTS Four kinds of sol-gel systems have been examined in the present study, that were synthesized starting from a GPTMS precursor: GPTMS in acidic synthesis (GH), GPTMS in basic synthesis (GB), and the respective polymerized systems (polymerized GH and polymerized GB). Epoxy polymerization of GPTMS-based hybrid systems consists in the ring opening of the cyclic ether group that, through a cationic polymerization mechanism, leads to the formation of polyethylene oxide (PEO) oligomeric chains in the hybrid network. FT-IR spectra of the investigated systems are reported in Figure 1. The decrease of the epoxy C-H stretching peaks at 3060 and 2995 cm -1 , and of the asymmetric ring stretching peak at 915 cm -1 indicates the ring opening reaction. P. Prosposito a , L. Brigo b , G. Della Giustina b , A. Perucchi c , S. Lupi d , and G. Brusatin b a Dipartimento di Fisica, Università di Roma Tor Vergata, INSTM and MINAS-Lab, Via della Ricerca Scientifica 1, 00133 Rome, Italy b Dipartimento di Ingegneria Meccanica – Settore Materiali, Università degli Studi di Padova, Via Marzolo 9, 35131 Padova, Italy c Sincrotrone Trieste S.C.p.A., Area Science Park, I-34012 Basovizza, Trieste, Italy d CNR-IOM Laboratorio TASC and Dipartimento di Fisica, Università di Roma La Sapienza, Piazzale Aldo Moro 2, I-00185 Rome, Italy Transmittance of hybrid sol-gel materials in the THz region I 978-1-4244-6656-6/10/$26.00 ©2010 IEEE