Dynamic Holographic Storage Based On Orientational Photorefractive Effects In C₆₀-Doped Nematic Liquid Crystals

Nematic liquid crystals (NLCs) have attracted great attention due to their special optical properties that have been studied for decades. In recet years, nonlinear optical responses of nematic liquid crystals and their applications have created another revolution in scientific research worldwide. This is due in part to the fact that the low cost, easy fabrication, high sensitivity and large orientational nonlinearity. This nonlinear response of nematic liquid crystals is dramatically enhanced by adding certain impurities or dopants. It has been recognized that the optical properties may also be easily manipulated electronically and controlled using light. In this thesis, we investigated the properties of coupling, amplification and diffraction of the thin grating produced in NLCs theoretically and experimentally, and researched into the nonlinear optical phenomena in NLCs under the application of a dc field.Based on the conventional photorefractive effect that the charge distribution within the sample. We propose a surface-mediated photorefractive effect that the modulated surface-charge distributions near the interface. A possible physical mechanism involving the surface-mediated effect is assumed, and gives a charge injection model to illustrate the dynamics of ions on surface under the application of a dc voltage. The resulting electric field exhibits two modulated components, one is parallel to the grating vector (Ï€/ 2 phase shifted) and the other is orthogonal to the cell's surfaces (in phase), undergoing exponential decay.Photorefractive diffraction gratings were studied in cells of homeotropicall-y aligned 4-cyano-4 -pentyl-biphenyl (5CB) liquid crystals doped with buckminsterfullerene (C₆₀) based on two beam coupling (TBC) experiment. These Raman-Nath gratings were induced by the interference modulation of two coherent optical beams in conjunction with an applied dc field across the sample thickness. The various aspects of two-beam coupling phenomena including dynamic self-diffraction, time response, beam amplification properties, and dc voltage dependence of thin grating are investigated. In the 20μm thick NLC sample (grating spacing wasΛ≈24μm), the orientational PR grating presented a first-order diffraction as high as 40%, and the distribution of the diffractive intensities was extremely asymmetric. These diffraction characteristics are strikingly different fromthose for the thin grating with a symmetric, sinusoidal profile. Thus, we believe that the high first-order diffraction obtained from the thin grating in NLCs is probably induced by the not rigorously sinusoidal profile of the space-charge field, leading to the asymmetric distribution of the diffractive intensities. The asymmetric profile of the space-charge field most probably related to the surface-charge layer between the NLC film and the electrode.A new method to demonstrate the dynamic holographic image storage effect and the diffraction enhancement under the influence of an externally applied dc voltage is presented in read out process. We observed clear holographic images due to the transient diffraction efficiency enhancement. The hidden gratings are due to a light-induced surface-charge modulation, and amplified by the Carr-Helfrich effect in the presence of electric field produced by dc voltage. A qualitative analysis of the phenomenon, according to which the effective bulk voltage is modulated due to light-induced surface modification is proposed. The simulation results are in good agreement with the experimental results.In addition, in thin C₆₀-doped homeotropic alignment NLC film, the self-phase modulations (SPM) were investigated. Two different type of concentric diffraction rings were observed in the far field. Based on the Kirchhoff-Fraunhofer's diffraction integral, the numerical simulation results show that the diffraction rings were determined by the two parameters. Furthermore, an anisotropic diffraction patterns were observed in the far field. The phenomena originated in the radial direction dependence of an additional directional phase shift induced by Gaussian beam and diffraction patterns are strongly dependent on the polarization of the incident beam. A theoretical model is proposed to clarify the mechanism of the anisotropic diffraction pattern formation by using the Kirchhoff diffraction integral. The simulation results showed that our theoretically analyzed was consistent with the experimental results.