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Polarization dependence of photo-mechanical behavior of monodomain liquid crystal polymeric materials

Polarization dependence of opto-mechanical behavior of monodomain photochromic glassy liquid crystal (LC) polymers under polarized ultraviolet light (PUV) is studied. Trans-cis photo-isomerization is generally known to be most intense at 'parallel illumination' (polarization parallel to LC director), as light-medium interactions are active when polarization aligns with trainsition dipole moment. We show that at parallel illumination though cis isomers are converted from trans the most near surface, they can be the least below certain light propagation depth. Membrane force, an average effect of trans-cis conversion over propagation depths, shows a monotonic polarization dependence, i.e. maximum at parallel illumination, which agrees well with experiment [1]. However, under strong illumination, cis fraction/photo-contraction distribution through depths shows deep penetration, switching over the polarization dependence in photo-moment, which is related to photo-contraction gradient ---- photo-moment can be maximum at 'perpendicular illumination' (polarization perpendicular to director) under strong light. We give both intuitive explanation and analytical demonstration in thin strip limit for the switchover.