線狀液晶之非線性光學--簡併四波混合及光學相位共軛

Abstract

摘要 本論文廣泛地探討了在垂直排列線狀液晶薄膜中，前向波及反向波作用下之簡併四波 混合特性。對於繞射效率最佳化因子，包括外加電場、液晶材料參數、實驗幾何條件 等，以及光場和外加電場對相位共軛光品質的影響作了詳細的分析研究。 液晶之三階非線性效應源自於光場誘導的分子轉向，而此分子轉向將引起非線性的折 射係數變化。在理論推導當中，我們同時考慮了液晶中所有可能之分子形變（包括斜 展、扭轉、彎曲形式），得到四波混合之一般解及特殊解。數值計算及小分子轉角近 似解皆顯示，扭轉形變對最佳偏壓有決定性之影響，而此最佳電壓值同時亦決定於斜 展及彎曲形變對分子轉向的差異作用。如果光誘相位光柵之最大振幅超過一定值，則 可以在兩個不同電壓下得到相同之繞射效率峰值。最大相位振輻為光場強度、光柵週 期、扭轉彈性係數等之單調變化函變，但隨樣品厚度先增而後減。 在實驗中，我們使用了連續波氬離子電射為光源，激發一種室溫下為液晶態之材料MB BA，而以一千赫芝的似靜電場作為外加偏壓。所有測量分析皆以自製之自動化系統執 行。由結果顯示，實驗測量與理論分析預測十分吻合。我們同時也以空間濾波法測量 了相位共軛光之反射率及其傳真度。實驗結果顯示，傳真度可由外加電場來改善但隨 光場增大而劣化。我們以分子擾動引起的雜散光及液晶中相伴發生的非線性自相位調 制作用，對上述現象做了合理的解釋。 /////// Properties of degenerate four7 wave mixing process, including both the forward7 and backward7 wave interaction geometries, in homeotropically aligned nematic liquid7 crystal films are extensively studied in theis dissertation. The optimization of electric bias, material and geometric parameters on the diffraction efficiency as well as the influence of optical and electric fields on the quality of optical phase conjugation are analyzed in a detailed manner. The third7 order nonlinearity of liquid crystals originates from the laser7 induced molecular reorientation which can lead to a nonlinear refractive7 index change. A theoretical model is derived to describe the periodically distorted structure of molecular reorientation, the formation of a phase grating and its associated diffraction. Both the basic and analytical solutions are obtained by taking into account all possible elastic deformations （i.e., splay, twist and bend types）in nematic liquid crystals at the same time. It is pointed out that the determination of optimal biasing field is crucially influenced by the twistdeformation in the reorientational grating and is also subjected to the competing influences of splay and of bend elastic torqures on the molecular reorientation, while the diffraction efficiency is concerned. If the maximum amplitude of phase grating is large enough, double diffraction peaks of the same efficiency can be obtained at two distinct voltages. It is shown both by numerical calculations and analytic predictions that the optimal amplitude of the induced phase grating is a monotonically increasing function of the grating period, the optical intensity, and the ratio of the bend elastic constant to that of twist. However, the dependence of the phase amplitude on the sample thickness is quite unique. There always exists an optimal thickness for the peak phase amplitude. The numerical calculations are carefully checked and are shown to be valid as compared with the applicability limit of Raman7 Nath theory. In the experiments, a CW argon7 ion laser was used to pump a room7 temperature nematic substance, N7 （47 methoxybenzylidene）7 47 butylaniline （MBBA）. A the measurement was automatically carried out by utilizing a home7 made computerized system. The experimental results are good agreement with the numerical predictions. The efficiency and fidelity of optical phase conjugation in nematics are also investigated by using a spatial7 filtering technique. It is shown in the experimental results that the fidelity is improved by an applied electric field, but is degraded with the increment of optical intensity. The background scattering noise, due to the thermal fluctuation of NLC directors, and the accompanying nonlinear self7 phase modulation effect are supposed to have the most important influences on the beam quality and this explanation agrees well the observed phenomena.