This chapter presents a brief overview of the sequence of phenomena that are involved in efficient photorefractive response in polymer composites and glasses patterned by the creation of space charge fields. It particularly concerns those materials with low glass transition temperatures, at or near to ambient. The sequence of processes occurring in a photorefractive material can be summarized as:

• Light is absorbed and an ion-carrier pair is created. • The ion is relatively immobile but the carrier can drift away under

the influence of a field, toward a darker region. • Mobile carriers are attracted to recombine with ions and the carrier

population reaches a steady level quickly. • Ions are left behind in the brighter regions while excess carriers

collect in darker regions. • An electric field builds up between bright and dark regions, which

acts to oppose further accumulation. • This electric field modifies the refractive index, leading to diffraction.