Effect of a surface polarized layer on the optical properties of a nematic cell

The optical properties of a cell of nematic liquid crystal presenting a tilted surface polarization submitted to anelectric field are theoretically investigated. The origin of the surface polarization is assumed to be related to theinteraction of the nematic molecules with the substrate and to the polar order due to the cell filling procedure.The analysis shows that the coupling of the surface polarization with the external field has different effects onthe two surfaces limiting the sample. On one of them, the coupling gives rise, in an unbounded sample, to acontinuous variation of the easy angle, whereas on the other one it is responsible for a first-order like transitionof the easy axis orientation. For a sample of finite thickness our analysis shows that in the presence of a couplingbetween the two surfaces, operated by the elastic properties of the liquid crystal, the abrupt variation of the easyaxis on one of the surfaces induces also on the other surface a discontinuity on the nematic surface orientation.The optical transmission of the cell versus the amplitude of the applied voltage, for an impinging monochromaticlinearly polarized light, presents a discontinuity for the critical voltage at which the first-order transition of thesurface tilt angles appears.