Optical Nonlinearity And Holographic Storage In Azo-Dye-Doped Nematic Liquid Crystal Films

Nematic liquid crystals have large optical nonlinearity due to anisotropic refractive index. Optical properties can be greatly improved by doping azo dyes into liquid crystals. In recent years, nonlinear optical processes in dye-doped nematic liquid crystals have received considerable attention with their low cost, ease of preparation and modification, and low external electric field, high optical nonlinearity and so on. In this thesis, nonlinear optical properties of two typical azo-dye (DR1 and MR) doped liquid crystals, especially holographic characteristic, are investigated with no external electric field and magnetic field.A theoretical model for holographic storage and diffraction is developed in azo-dye doped liquid crystal film. The formation of the grating in this liquid crystal film is based on the trans-cis isomerization of azo dye molecules and the reorientation of liquid crystal molecules, and the refractive index modulation of the grating is achieved due to refractive index anisotropy of liquid crystal molecules. In these films, the dynamic changes in anisotropic absorption and anisotropic refractive index can lead the remarkable changes of optical intensity and phase, therefore, the intensity grating and polarization grating can be formed. The dielectric tensors of the intensity grating and the polarization grating are obtained by using the dielectric tensor based on anisotropic absorption and a self-consistent steady-state Maxwell's equation. The diffraction efficiency expressions of the intensity grating and the polarization grating are achieved by substituting the dielectric tensors of the intensity grating and the polarization grating, respectively, into the self-consistent steady-state Maxwell's equation and using the slowly varying amplitude approximation and their boundary conditions. These two expressions show the dependence of the diffraction efficiency on the material parameters, such as dye concentration, sample thickness, and recording parameters, such as recording intensity, recording light polarization, recording angle and so on.In DR1-doped liquid crystal films and MR-doped liquid crystal films, absorption and two-wave mixing are studied, and then the optimum material parameters and recording parameters of holographic recording are obtained. Under the optimum experimental conditions, the maximum diffraction efficiencies are 11.8% and 18.2% in DR1-doped and MR-doped liquid crystal films, respectively.The formation of the grating in azo-dye doped liquid crystal films depends not only on the recording intensity but also on the polarization direction of the recording light. Due to the anisotropy in the absorption of dye molecules, the absorption of the film to the exciting light is strongest and the diffraction efficiency should be the maximum with the polarization direction of the light parallel to the director vector of the azo-dye doped liquid crystal film. In reverse, the absorption of the film is weakest and the diffraction efficiency should be the minimum with the polarization direction perpendicular to the director vector of the film. In addition, the absorption of the film and the diffraction efficiency of holographic grating increase with the increase of the dye concentration. The experimental results in DR1-doped liquid crystal film are in good agreement with the theoretical simulation results. However, the experimental results in MR-doped liquid crystal film show that the diffraction efficiency of the grating reaches the maximum with the minimum anisotropic absorption but not the maximum anisotropic absorption like DR1-doped liquid crystal film. The compare analysis of different polarized transmission light intensities as a function of the polarization of the incident light (488nm) between these two kinds of films shows that there is rotation effect in MR-doped liquid crystal film. Then, it is demonstrated that the mechanisms of the grating formation in both DR1-doped and MR-doped liquid crystal films are same in nature, and the theory in this thesis suits these two kinds of azo-dye doped liquid crystal films.Holographic gratings in DR1-doped and MR-doped liquid crystal films are investigated, and hologram recording and reconstruction are achieved in these two kinds of films. In DR1-doped liquid crystal films, only transient grating and transient hologram can be formed, and the formation time and erasure time are in the order of millisecond, which can be applied in holographic display and other holographic optical devices with fast response. In MR-doped liquid crystal films, permanent grating and permanent hologram can be recorded, and image edge enhancement can appear with a long enough recording time, which can be applied in holographic storage, image processing and so on. Multiple grating storage is obtained in MR-doped liquid crystal film, and 13 gratings are stored at a location by angular multiplexing and peristrophic multiplexing. Then, multiple hologram recording and reconstruction are achieved in MR-doped liquid crystal film, and two holograms are stored at a single location by angular multiplexing and by peristrophic multiplexing, respectively. When spatial multiplexing is applied along with angular multiplexing or peristrophic multiplexing, more than 50 holographic pages can be stored in MR-doped liquid crystal film with the area of 5 cm2. Theoretical analysis shows that the number of holograms at a single location can be improved to 24, i.e., the storage density of 0.12 Gbits/cm2. High-order reconstructed images are also investigated.In the thesis, the theoretical and experimental results have deep significance for applications of DR1-doped and MR-doped liquid crystals in holographic image display, dynamic holography, optical image processing, high-density optical information storage and other optical data processing.