The photorefractive properties and the phase stability of polymer composites are dependent on the detail of the alkyl chain substituent attached to the electro-optic dye within the composite. Photorefractive composites based on poly (N-vinylcarbazole) (PVK), sensitized with trinitrofluorenone (TNF) and mixed with a concentration of 47.5 wt. % of electro-optic dye have been tested for photorefractive performance. Two alternative azo dyes of identical molecular weight have been used to produce alternative composites; both dyes were modified to suppress spatial isomerism and incorporated an eight carbon alkyl chain at the electropositive end of the chromophore: either a straight octyl chain or a branched ethylhexyl chain was substituted. The reorientational enhancement of photorefractive performance is similar in the composites resulting from these dyes. The dye with a straight octyl chain led to a composite with improved holographic performance. The dye with a branched ethylhexyl chain led to a composite exhibiting lower diffraction efficiency, but with superior phase stability. A tentative explanation is offered for these differences based on the shape of the alkyl substituent and its effect on a trapping mechanism involving the dye molecules and the sensitisor anions in PVK:TNF-based photorefractive composites.
Polymer photorefractive devices are formed by sandwiching an electro-optic dye/photoconducting polymer composite between two indium tin oxide coated glass substrates Holographic information may be written in the material using two coherent optical beams. Device diffraction efficiencies of 60 %, holographic growth times of a few hundred milliseconds and two beam coupling gain coefficient of greater than 200 cm-1 have been reported[1]. High gain and low manufacturing costs of polymer devices are clear advantages over photorefractive crystals, although typical holographic grating risetimes of a few hundred milliseconds are restrictive.
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