Orientational Photorefractive Effect in Undoped and CdSe Nanorods-Doped Nematic Liquid Crystal-Bulk and Interface Contributions

In the work, we revisited the orientational photorefractive effect generated in nematic liquid crystal (NLC) in a degenerate four-wave mixing interaction configuration. A detailed calculation of the bulk laser-induced space-charge fields and analytical expressions for the reorientation angles and the required dc threshold fields are obtained and compared with experimental measurements. While the bulk theory could describe the general features of the self-diffraction and threshold fields, there are significant deviations from experimental observation, particularly in an observed lowering of the threshold voltage by the optical intensity. This effect is due to an optically induced surface-charge modulation. A theoretical analysis including both the bulk and the surface-charge-modulation effects allows us to obtain better agreement with experimental observations of the threshold voltages on the optical intensity and sample thicknesses, and to estimate the relative contributions from the bulk and surface effects. Recently observed enhanced photorefractive effects of CdSe nanorods-doped NLCs are also reported to corroborate these analyses and to provide guidelines to optimize the photorefractive effect for dynamic holographic application.