The term photopolymer refers to a system of inorganic molecules that polymerize during light excitation and exhibit changes in the refractive index. Characteristics of photopolymers are the outstanding light sensitivity a high dynamic range as well as long time stability. Photopolymers are dry, grainless, self-developing storage media unlike the well proven silver halides. Due to the low production costs, they are an ideal material for write-once, read-many (WORM) media for the consumer market. The high light sensitivity offers refractive index changes even with exposures in the nanosecond time domain. The polymer samples were developed in cooperation with the "Center for Industrial and Engineering Optics" at the "Dublin Institute of Technology".

In general a photopolymer is comprised of the following ingredients:

1. A dye for the photoinitialization process. The absorption is adapted for a small bandwidth of the pump laser line.
2. An electron donor, that produces free radicals after undergoing a reaction with dye molecules.
3. A monomer, that produces poylmer chains after reacting with free radicals.
4. A binder to embed all the components in a semifluid matrix.

The photopolymerization process starts upon absortpion photons from the dye and moves to an excited state. The dye molecules react with the electron donor and produces free radicals. In general a radical is not reactive enough to start the polymerization process on its own. The main process starts when a free radical reacts with a monomer to produce the first part of a polymer chain. Theof polymer chain growths due to successive connection of monomers to the reactive chain.

Inhomogeneous polymerized material varies its index of refraction due to density changes, segregation of chemical compounds and molar refractive index changes. Molar refractive index changes through reconfiguration of bonding states are less common in most photopolymers and are usually neglected. Changes in density are produced by reduced distance between molecules after being linked to the main polymer chain. In case of a sinusoidal light exposure, monomers diffuse from maximum to minimum intensity. The diffusion is due to a gradient in monomer concentration and the density changes in the material.