Photorefractive Polymers and Their Applications

Abstract

Photorefractive polymers exhibit large refractive index changes when exposed to low power laser beams. When the optical excitation consists of two interfering coherent beams, the periodic light distribution produces a periodic refractive index modulation. The resulting index change produces a hologram in the volume of the polymer film. The hologram can be reconstructed by diffracting a third laser beam on the periodic index modulation. In contrast to many physical processes that can be used to generate a refractive index change, the photorefractive effect is fully reversible, meaning that the recorded holograms can be erased with a spatially uniform light beam. This reversibility makes photorefractive polymers suitable for real-time holographic applications. The mechanism that leads to the formation of a photorefractive index modulation involves the formation of an internal electric field through the absorption of light, the generation of carriers, their transport and trapping over macroscopic distances. The resulting electric field produces a refractive index change through orientational or non-linear optical effects. Due to the transport process, the index modulation amplitude is phase shifted with respect to the periodic light distribution produced by the interfering optical beams that generate the hologram. This phase shift enables the coherent energy transfer between two beams propagating in a thick photorefractive material. This property, referred to as two-beam coupling, is used to build optical amplifiers. Hence, photorefractive materials are also playing a role in imaging applications. Discovered and studied for several decades mainly in inorganic crystals and semiconductors, the photorefractive effect has not yet found wide spread commercial applications.