Study On Two-Wave-Coupling Characteristics In Nematic Liquid Crystal Doped With Silver Nanoparticles

The remarkable nonlinear optical properties of nematic liquid crystals (NLCs) have attracted much attentions and made them become a kind of important nonlinear optical materials. The nonlinearity can be enhanced greatly by doping photo-excitable dopants. In this thesis, we investigated the effect of dopants of the silver nanoparticles and dye-molecules coated silver nanoparticles on the photorefractive nonlinear properties in the NLCs samples theoretically and experimentally, such as the diffraction properties of the thin gratings recorded in NLCs and the beam coupling, beam amplification in two beam coupling experiments.We calculated the effective dielectric permittivity tensor for a suspension of small metallic nanoparticles immersed into NLCs. The results indicated that the dielectric permittivity anisotropy is enhanced. For a photorefractive (PR) LCs, the orientational PR effect is generally ascribed to a modulation of photo-generated charge density as a consequence of the Carr-Helfrich effect in the bulk, which reorients the LCs molecular director through the space-charge field, in turn, leads to a modulation of the index of refraction and the holographic grating formed. And the magnitude of the space-charge field will be increased due to the enhancement of dielectric permittivity anisotropy in this nanoparticles doped NLCs. Meanwhile, we calculated the enhancement of local field inside the sample due to the excitation of surface plasmon polaritons of silver nanoparticles, and the enhancement of local field will contribute to the generation of space-charge field and photorefractive nonlinear effect in this nanoparticles doped NLCs.We prepared dye-molecules coated silver nanoparticles doped and silver nanoparticles doped planar alignment liquid crystal cells. The orientational PR effect was observed in the sample using two-beam coupling experiment which the writing beam was in 532nm and the probe was in 632.8nm. The dependence of diffraction efficiency on the applied voltage,incident pump power, incident power ratio and grating constant were studied. Experimental results indicate that the optical nonlinearity of NLCs can be enhanced significantly by doping silver nanoparticles or dye-molecules coated silver nanoparticles. The maximum first-order diffraction efficiency obtained in the dye-molecules coated silver nanoparticles-doped NLCs cell reaches to 15.1% with 3.4V and obtained in silver nanoparticles-doped NLCs cell reaches to 16.24% under an external voltage of 4.1V.2D-array transverse pattern was observed in the two beam coupling experimental for dye-molecules coated silver nanoparticles doped NLCs due to the transverse instability. And PR two-beam coupling experiment was also performed in the samples, gain coefficient as high as 1664 cm-1 was obtained at a low dc voltage of 3.4 V for Ag-doped NLCs. But low gain coefficient, 34cm-1, was observed for dye-molecules coated silver nanoparticles doped NLCs. These results are of benefit to the applications of PR NLCs in holography and the study of electro-optical characteristics in NLCs.