Large dynamic ranges in photorefractive NLO polymers and NLO-polymer-dispersed liquid crystals using a bifunctional chromophore as a charge transporter

We report on the synthesis and characterization of two photorefractive side -chain polymethacrylates, functionalized with amino-donor-containing chromo phores. Carbazole units were neither copolymerized nor added to the polymer s as separate components, and charge transport was provided by the chromoph ores. The polymer glass transition temperature was lowered to room temperat ure with appropriate plasticizers. After the addition of a sensitizer, (2,4 ,7-trinitro-9-fluorenylidene)malononitrile (TNFM) or C-60, a charge-transfe r (CT) complex is formed between the chromophore and sensitizer, and photoc onductivity can he observed upon excitation of the CT complex at 680 and 78 0 nm. The excellent photorefractive properties at, both wavelengths were pr obed using the techniques of four-wave mixing and two-beam coupling. We sho w that the grating buildup speed is dominated by the charge generation. Bec ause of the absence of carbazole units, the chromophore number densities in the samples are very high (up to 15.3 x 10(20) cm(-3)), leading to refract ive index modulation amplitudes exceeding 5 x 10(-3) at only 50 V/mum and 7 80 nm, which is among the best results obtained so far in polymeric photore fractive media. Meanwhile, the fully functionalized polymers offer the majo r advantage of a very good phase stability. As a proof of principle, we hav e also prepared polymer-dispersed liquid crystals (PDLC) using the bifuncti onal NLO polymers as the polymer matrix. PDLCs were prepared by a thermally induced phase separation after addition of 20 wt % of a liquid-crystal mol ecule that shows a nematic phase at room temperature. In the PDLC, a refrac tive index modulation amplitude of 1.8 x 10(-2) was observed at 780 nm and 50 V/mum. To the best of our knowledge! this exceeds the refractive index m odulation amplitudes observed so far in fully functionalized polymers at 78 0 nm.