Studies of Biphotonic Effect in Azo-Dye-Doped Liquid Crystal Films and Their Applications

• Main Authors: Ko-Ting Cheng, 鄭恪亭
• Other Authors: Andy Ying-Guey Fuh
• Format: Others
• Language: en_US
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/40392188422198730852

博士 === 國立成功大學 === 物理學系碩博士班 === 95 === Azo derivatives, such as azobenzene, azo dyes ..., etc. have been studied intensively in recent decades. Many researchers use such materials to develop the photo-alignment techniques in liquid crystal devices because of their particular anisotropic optical properties. Among these, the applications of azo dye adsorption in a dye-doped liquid crystal (DDLC) film induced using a single pump beam, the so-called one-photonic effect, have attracted substantial attention over the past years. Many optical devices, including displays, holographic grating, optical lens, optical filter, polarizer and so forth, have been developed in this system. The mainly mechanism in this photo-alignment of liquid crystals is the light-induced dye adsorption, which conclusively results from the positive/negative torque effect, photoisomerization effect, adsorption/desorption. In this thesis, we study the biphotonic effect, which is caused by pumping two photons of different wavelengths (one red and one green light) in a DDLC film. The aims of the thesis work are to study the related interesting physics, such as electrical, thermal, optical and chemical effects occurred in the system. Briefly, the thesis mainly consists of two topics; one is the applications of photoalignment in LCs, and the other is the fabrications of controllable holographic polarization/intensity gratings using biphotonic technique based on azo dye-doped liquid crystal films are the in this thesis. In the first part, we propose a biphotonic laser-induced ripple structures (BLIRSs), analyzed using scanning electron microscopy and atomic force microscopy to alignment LCs. A BLIRS is formed by competing red and green light interfering fields, generated by the interference of incident red and green light with the corresponding surface scattering lights. We also investigate that the ratio of the intensity of red light to that of green light, the wavelength and polarization of the lights and the ambient temperature markedly affect the formation of the BLIRS. Moreover, a biphotonic erasure method to erase partially the photoalignment layer on an indium-tin-oxide (ITO)-coated glass slide resulted from the adsorbed azo dyes doped in a liquid crystal film. The experimental results also indicate that the intensity ratio and relative polarization should affect the efficiency of biphotonic erasure technique. The second part is the study of the formation and the dynamic of biphotonic gratings (BGs) in DDLC films. We successfully generate a biphotonic polarization grating, which is electrically switchable and thermally erasable. Additionally, the dynamics of biphotonic intensity holographic gratings (BIHGs) are studied. The results clearly indicate that the grating consists of two contributions; one is the bulk-reorientation effect, and the other is the surface-adsorption effect.