Kinetics of Light-Induced Patterning in Liquid Crystal Elastomers (**joint talk with I-CAMP)
Seminar Room 1, Newton Institute
Liquid crystal elastomers doped with molecules that change conformation on absorption of a photon show very large changes photoelastic response. This can be investigated by recording a holographic grating in the material with the use of two crossed UV laser beams. Angular dependence of the diffraction efficiency in the vicinity of the Bragg peak can be analyzed using a numerical model that takes into account the propagation of writing beams and rate equations for the local concentration of the absorbing trans conformer, and spatial distribution of the trans and cis conformers can be obtained. Stress dependence on the irradiation time can be analyzed in a similar way. Strongly nonlinear relationship between the concentration of the cis isomers of the azomesogens and the refractive index modification of the material, which is characteristic for the phase transition region, results in non-monotonous time dependence of the diffraction efficiency of a probe beam. From this effect the sensitivity of the nematic transition temperature on the molar fraction of the cis isomers is determined. The relation between the cis isomer molar fraction and nematic order also provides a possibility for recording hidden holograms, which can be made visible by cooling the sample from the paranematic to the nematic phase.