Characterization of blue sensitive holographic polymer dispersed liquid crystal for microholographic data storage

We present the results of an extended investigation performed on holographic reflection gratings recorded in blue sensitive polymer dispersed liquid crystals in view of a possible application in optical data-storage. The analysis has been performed by means of real-time spectroscopy during the polymerization process. The analysis of transmission spectra during the recording process showed a fast photopolymerization, resulting in the rapid formation of a reflection peak. Gratings characterized by high spatial frequency (> 7000 lines/mm), high diffraction efficiency (up to 50%) and large refractive index modulation (Delta n similar to 0.008) have been obtained. Unfortunately, the monomer conversion into a polymer network is generally characterized by a strong packing, with a consequent reduction in volume (polymerization shrinkage). This phenomenon induces a significant blue-shift of the reflected wavelength. With the aim of better understanding and possibly reducing the shrinkage, a new high-sensitive experimental technique has been developed to measure shrinkage effects in multi-component polymeric mixtures.