High fidelity holographic recording with cyclic allylic sulfide monomer

A new photopolymer system for holographic recording is presented. It exploits the free-radical ring opening polymerization of a cyclic allylic sulfide monomer, 7-methylene-1,5-dithiacyclooctan-3-yl acetate (MDTOA). Such a system allows for the transferring of the sinusoidal holographic pattern with great fidelity, independently on the rigidity of the host matrix. For this purpose, experiments were carried out in a cellulose acetate butyrate (CAB-531-1, Eastman) matrix, using Irgacure 784® as photoinitiating system. The films show excellent optical properties in terms of uniformity and transparency (complete bleaching). The measured diffraction efficiency curves indicate that the written pattern is nearly ideal and it well matches with the theoretical curves computed according to the Kogelnik model. The stability during time of the gratings is monitored and a decrease of efficiency is evident. This means that the formed polymer chains tend to countermigrate smoothing the refractive index modulation. The formation of a crosslinked network is therefore mandatory to obtain stable gratings.