A Low Glass Transition Temperature Photorefractive Polymer System

Organic materials are an important branch of optoelectronic materials. Because of their various technological applications, organic materials have been extensively studied. The crucial aim of the research in this field is to develop new light sensitive materials which have properties such as high efficiency, high speed, high sensitivity, low cost and long working life. Non-linear optical materials are of particular relevance in this field, because they can manipulate and transform several features such as frequency, phase or direction of propagation of an optical signal under proper conditions.As a very important kind of non-linear optical materials, photorefractive materials are among the most promising materials used in optoelectronic and photonic devices. Photorefractive materials are photoconductors in which a space-charge field can be created by the space redistribution of charges photogenerated by a non-uniform illumination. Then the refractive index can be modulated if the material exhibits non-linear optical properties and/or spontaneous (or field-induced) birefringence. There are many applications based on photorefractive effect, such as data storage, holography and image processing.Photorefractive materials can be divided into two groups:inorganic crystals and organic materials. Organic materials are superior to inorganic crystals, since they have many advantages, such as low cost, short time to produce and easy manipulation. Among organic materials, polymers are better.This research project was focused on the photorefractive characterization of a low Tg polymer. We chose PVK which has a Tg of 200℃as the substrate. Plasticizer ECZ was doped into the polymer to lower its Tg. The new Tg was chracterized by DCS and it was found to be 20℃, which means that the Tg of the system was successfully decreased. Then some inorganic nano-crystal Gd2O3 was added into the polymer to enhance the photorefractive performance with the help of orientation enhancement effect.The polymer system has good photoconducting properties, and its photorefractive properties were characterized by Multi-Wave Mixing technique and grating moving technique. The direction of the energy exchange between the two beams was inverted by inverting the electric field polarity. The grating build-up time for the laser wavelength of 632.8nm was more than 1000 seconds, and the grating build-up time was successfully decreased to 8 seconds by using laser wavelength of 533nm. Finally, via transient ellipsometry, the orientational enhancement effect was characterized to be the only mechanism to modulate the refractive index of the polymer system.