Improving Sensitivity of Photorefractive Polymer Composites for Holographic Display Applications

Abstract

This work presents recent progress in the area of organic photorefractive polymer composites. These materials have been previously shown to be a suitable medium for dynamic holographic displays, with multiple colors and single frame writing times on the order of seconds. However, these materials still require large electric fields and high intensity lasers to function effectively. Recent advancements in improving these areas are discussed, including a review of the history and state-of-the-art in photorefractive polymer composites.The addition of electron traps via low loading of the electron-transporting molecule Alq3 is shown to dramatically improve the diffraction efficiency and reduce the required field. The grating formation also proceeds faster by more than one order of magnitude, leading to an increase in sensitivity by a factor of 3. The dynamics of these materials also show evidence of competing gratings indicative of bipolar charge transport and trapping.The addition of an amorphous polycarbonate (APC) buffer layer is reported to have a similar effect on the steady-state diffraction efficiency, and the further doping with a fullerene derivative (PCBM) allows a 3x increase in the efficiency in the reflection geometry, which is normally poor due to the small grating spacing. These composites reveal the fundamental limits of the reflection geometry, based on the physics of high frequency gratings. A reversal in the direction and increase in the magnitude of the two-beam coupling energy transfer is also observed.The use of interdigitated coplanar electrodes, instead of the standard uniform electrodes in a parallel-plate geometry, is shown to result in large diffraction efficiency with symmetric writing beams due to the increased projection field. The efficiency is similar to that achieved in the standard samples with large slant angles and much better than those geometries typically used in applications, with the benefit that the writing beams do not have to be slanted with respect to the sample normal. Different electrode widths are examined and the trade-offs discussed. This device makes beam injection simpler and allows one to bring the benefits of highly slanted geometries, common to small area setups, to the large-area applications.