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

Abstract

The formation of real-time holographic gratings was investigated in an azobenzene polymer liquid crystal (PLC) possessing a siloxane spacer in the side chain. Irradiation of thin films of the PLCs created a phase grating which 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 transition temperature of the PLC with the siloxane spacer was lower than that of the PLC without the siloxane spacer owing to flexibility of the siloxane units. This property resulted in a decrease in the irradiation temperature, leading to effective induction of the photochemical N–I phase transition 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 that 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 moiety in the LC phase.