Novel blue sensitive polymeric materials for optical data storage R. Castagna 1* , L. Criante 1 , F. Vita 1 , D. E. Lucchetta 1 , S. Frohmann 2 , T. Feid 2 , S. Orlic 2 , F. Simoni 1 1 Dipartimento di Fisica e Ingegneria dei Materiali e del Territorio and CNISM – MATEC Lab, Università Politecnica delle Marche, Via Brecce Bianche, 60131 Ancona – Italy 2 Institute of Optics, Technical University Berlin, Berlin, Germany ABSTRACT We report the properties of a new polymer-guest photosensitive mixture based on two monomers with different properties as basic components. The material has been characterized by recording holographic gratings using a low power blue laser (405 nm) and a spectro-photometric technique to get the optical properties of the grating during and after recording. The detected grating shows a very high sensitivity (of the order of 10 3 cm/J) and a surprising “anti- shrinkage” phenomenon (red-shift of the Bragg reflection grating wavelength). Keywords: optical storage, shrinkage, blue sensitive materials 1. INTRODUCTION Holography is considered the elective technique for the next generation of high-density optical data storage applications [1-7]. For this purpose, the most studied materials are photo-polymers which have many interesting optical and mechanical characteristics [5-25]. However, the requirements for holographic data storage (HDS) are very severe and many problems are connected with the use of polymers in this field. In fact, the European Union COST Action P8 2006 [1] and the Information Storage Industry Consortium [2] set the following constraints, to be satisfied in the blue-violet range (λ = 405 nm), for the ideal HDS material: a) high sensitivity (> 10 3 cm/J); b) high spatial resolution (7000 lines/mm); c) low shrinkage (< 1%); d) high transparency before and after polymerization (over 80%); e) time and temperature stability. Actually, the perfect material that satisfies all the above described characteristics still does not exist. For instance, holographic polymeric mixtures (HPM) based on a free radical photo-polymerization approach (e.g. the ones containing acrylate monomers) can reach high sensitivity values, but have problems connected with polymeric shrinkage. On the contrary, epoxide-based mixtures undergoing cationic polymerization typically present lower shrinkage but also lower sensitivity values. This is due to the chemical mechanism of cationic ring opening polymerization (CROP) reactions in which the intermolecular distance lost during the polymerization process (~3 Å) is compensated by the epoxy ring opening. In this work, we present new composite photo-polymeric materials, in which free radical and cationic polymerization reactions act at the same time. We called this approach Polymer-Guest Polymer-Host (PGPH) in contrast to other similar approaches in which the phase separation is realized by not polymerizable compounds [22-24]. We demonstrated this new formulation can be conveniently used in high-density HDS by recording and characterizing high- resolution reflection gratings. 2. MATERIALS AND METHODS Our photosensitive mixture contains the epoxy-aromatic resin tri-phenyl-o-methane-tri-glycidil-ether (TPMTGE, from Aldrich), known to be a low molecular weight glass-forming liquid, in combination with the poly- acrylate monomer di-pentaerythritol-hydroxy-penta/hexa-acrylate (DPHP/HA, from Aldrich), in a 25:65 weight ratio. The mixture was sensitized to λ = 405 nm by the use of a free radical photo-initiator, 2,4,6-tri-methyl-benzoyl- phosphine-oxide (Irgacure 819, from CIBA Chemical Specialty) and a cationic photo-initiator, iodonium-(4- methylphenyl)-[4-(2-methyl-propyl)-phenyl]-hexa-fluoro-phosphate (Irgacure 250, from CIBA Chemical Specialty). The former, present in the solution with a 1% weight concentration, has its maximum absorbance at 360 nm and is able to Organic 3D Photonics Materials and Devices II, edited by Susanna Orlic, Proc. of SPIE Vol. 7053, 70530J, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.797992 Proc. of SPIE Vol. 7053 70530J-1 2008 SPIE Digital Library -- Subscriber Archive Copy