Modelling photopolymer behavior as optical recording medium

Abstract

Since photopolymers were first used as a holographic recording medium in the 1970s, one of the most important goals to be achieved was a quantitative understanding of their behaviour. In general, photopolymers are composed of one or more monomers, a binder and a dye that absorbs light in a certain region of the spectrum. The absorbed photons initiate polymerisation in the bright regions creating a concentration gradient compensated by the diffusion of chemical molecules described by Fick’s law. The first models to simulate the etching process in this type of materials were proposed in the early 1990s; they were very simple models where the authors assumed a harmonic in the monomer concentration, a constant rate of polymerisation and diffusion without direct influence of the refractive index of the monomer and the polymer. From this point on, improvements started to be made in the so-called “diffusion models”. One of the most important breakthroughs, proposed by John Sheridan’s research group in 2000, is the non-local behaviour of the photopolymer due to the finite size of the polymer chains. This phenomenon affects the limit of holographic recording resolution. Our research group at the University of Alicante (Spain) had the honour to collaborate with John Sheridan’s group developing new advances in the modelling of photopolymers as an optical recording medium, such as the threedimensional expansion of the models, and the explanation of surface variations or models to explain the recording of diffractive optical elements in this type of materials. In this papSer, we review this collaboration to improve diffusion models applied to photopolymers.