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. This can be attributed to the difficulties associated with the growth of crystals, and to the high cost of optical and optomechanical components necessary for the development of complete optical systems. With the emergence of novel low cost plastic optical components that can be mass produced by techniques such as injection molding, the cost and the weight of optical components is decreasing rapidly. This trend together with the advances made in fabricating integrated laser sources at lower cost provide a great momentum to the development of new optical processing technologies. As real-time optical recording and processing media, photorefractive polymers are expected to play a major role in these technologies. The optical, physical, and chemical properties of photorefractive polymers are outlined and discussed. Current material classes and their respective merits and future challenges are presented together with examples of applications.