We study the dynamical behavior of the holographic grating formed in polymer-dispersed liquid crystal ( PDLC ) films. The grating is written by two coherent Ar+ laser ( l=514.5nm ) , while probed by a He-Ne laser ( l=632.8nm ).

The laser-induced holographic gratings in the azo-dye-doped liquid crystals have been investigated by changing the temperature, and the concentration of the sample, the intensity of the two writing beams, and the polarization of probe beams. The cw Nd:YAG laser has been employed as the writing beams and the cw He-Ne laser has been employed as a real-time probe beam to detect the first order diffraction singals. The gratings are the results of photo-isomerization of azo dye and the structure alignment of liquid crystals. The model has been established to explain the first order diffraction signals, the mechanism of gratings formation and the temperature dependence.

The study of the first-order diffraction efficiency and the mechanism of formation have been investigated on dye-doped liquid crystals (DDLC) and liquid crystals with azo-dye-doped polymer film. The thesis mainly contains three experimental parts by changing the temperature of sample and the polarizations of writing and probing beams. The first part includes the study of temporal profiles of diffraction efficiency for transient gratings and their temperature and polarization dependence in azo-dye-doped liquid crystals. The dynamics of molecular reorientation of transient gratings can be understood by analyzing the build-up time of the peak efficiency and the relaxation decay of the first-order diffraction. The study of the polarization and temperature dependence allows us to understand the underlying mechanism of laser-induced transient gratings. The second part is concentrated in the diffusion process of photoexcited dye in a planar liquid crystal host. The experiment result reveals that the diffusion coefficient is larger for the molecular director along the grating vector than the perpendicular case and the diffusion will be faster as temperature increases. The third part is focused on the mechanism of formation and the temperature dependence of holographic grating for the liquid crystals with azo-dye-doped polymer film. The temporal profile of the first-order diffraction intensity shows a dip at the temperatures of nematic phase. The dip of the first-order diffraction intensity is temperature dependent and can be explained to be the light scattering due to the photothermal effect. The transient behavior in the dip of transmitted probe beam is also temperature dependent. The surface modulation has been measured by using atomic force microscope (AFM). The depth of surface relief grating of liquid crystals with azo-dye-doped polymer film is deeper than that of azo-dye-doped polymer film and the first-order diffraction efficiency is also larger for the liquid crystals with polymer film.

In this study, a high power Q-switch pulse laser has been used as the writing beams. The laser-induced holographic gratings in the DR1-doped liquid crystal samples and the DR1-PMMA polymer thin films were investigated by changing the temperature of samples and the angles of two writing beams. The He-Ne cw laser has been used as a real-time probe beam to detect the first order diffraction signals. Without external field, the gratings are the results of concentrations and diffusions of azo dye isomers. The diffusion model has been utilized to analyze the first order diffraction signals in order to understand the mechanics of gratings and the effect of temperature and angle.

The laser-induced holographic gratings in the Azo dye-doped PMMA thin films with liquid crystal were investigated by changing the temperature of the sample, the angle of two writing beams and the rubbing. The high power Q-switch pulse laser has been used as the writing beams and the He-Ne cw laser has been used as a real-time probe beam to detect the first order diffraction singals. The grating are the results of photo-isomerization of azo dye and diffusions of liquid crystal. The model has been established to analyze the first order diffraction signals in order to understand the mechanism of grating and the effect of temperature and angle.

The aim of this project was to develop a passive optical compensating arrangement that would allow the formation and continued stability of interference patterns over a long timescale and also to investigate optical wave scattering in thick gratings at extreme angles of scattering. A novel passive arrangement based on a Sagnac interferometer is described that produces interference patterns more stable than those produced by a conventional arrangement. An analysis of the arrangement is presented that shows it to be an order of magnitude more stable than an equivalent conventional approach. The excellent fringe stability allowed holographic gratings with small periods (~ 0.5 μm) to be written in photorefractive lithium niobate with low intensity writing fields (~mW/cm2) produced by a He:Ne laser, despite long grating fabrication times (~ 1000 s). This was possible because the optical arrangement compensated for phase shifts introduced by translational and rotational mirror motion caused by environmental perturbations. It was shown that the rapid introduction of a phase shift in one of the writing fields can change the direction of energy flow in the two-wave mixing process. It was found that the improvement in stability of the modified Sagnac arrangement over a conventional interferometer decreased when the crossing angle was increased and that the point about which the mirrors are rotated greatly affects the stability of the arrangement. For a crossing angle of 12 degrees, the modified Sagnac arrangement is more than twice as stable when the mirrors are rotated about their midpoints, rather than their endpoints. Investigations into scattering in the extremely asymmetrical scattering (EAS) geometry were undertaken by scattering light from a 532nm Nd:YAG laser off gratings written in photorefractive barium titanate and lithium niobate. Despite the difficulties posed by background noise, there was very good agreement between the observed scattered field and that predicted by a previously established theoretical model. Thus, this work represents the first experimental observation of EAS in the optical part of the spectrum.