Real-time holographic grating by means of photoresponsive polymer liquid crystals with a flexible siloxane spacer in the side chain

The formation of real-time holographic gratings was investigated in an azob enzene polymer liquid crystal (PLC) possessing a siloxane spacer in the sid e chain. Irradiation of thin films of the PLCs created a phase grating whic h is ascribed to periodic induction of the nematic-to-isotropic (N-I) phase transition due to photochemical trans-cis isomerization of the azobenzene moiety (photochemical phase transition). It was found that the N-I phase tr ansition temperature of the PLC with the siloxane spacer was lower than tha t of the PLC without the siloxane spacer owing to flexibility of the siloxa ne units. This property resulted in a decrease in the irradiation temperatu re, leading to effective induction of the photochemical N-I phase transitio n by suppression of thermal cis-trans back-isomerization of the azobenzene moiety. Thus formation of the grating in the PLC having the siloxane spacer could be achieved by low intensity of the interference light compared to t hat in conventional PLCs including only alkyl spacers. Moreover, a grating with narrower fringe spacing could be created by depression of propagation of perturbation due to trans-cis photoisomerization of the azobenzene moiet y in the LC phase.