The Opto-thermal Effect on the Encapsulated Cholesteric Liquid Crystal and Its Application to the Light Intensity Modulator

• Main Authors: Kin-Min Yang, 楊楷民
• Other Authors: Hui-Chi Lin
• Format: Others
• Language: zh-TW
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/rateqv

碩士 === 國立虎尾科技大學 === 光電與材料科技研究所 === 101 === In this thesis, the transmittance of the micro-encapsulated cholesteric liquid crystal was changed by the pump beam. The opto-thermal effect, induced by the PEDOT absorbing layer, results in the variation of the transmittance on the cholesteric liquid crystal thin films. We discussed the possibility of utilizing this material for the application of flexible electronic papers by evaluating the thickness of cholesteric liquid crystal layer and the PEDOT layer, respectively. The results showed that the patterning effect is better for the sample of 12 μm thick cholesteric liquid crystal layer than that of 25 μm at the same excitation conditions and the same thickness of the PEDOT layer. And the patterning effect is better for the sample of 7.5 μm thick PEDOT than that of 5.5 μm at the same excitation conditions and the same thickness of the cholesteric liquid crystal layer. We also successfully achieved a gray-scale pattern on cholesteric liquid crystal using shutter to control the light excitation time. To explore the reason why the cholesteric liquid crystal appeared a transmittance variation when pumped, two experiments were checked: (1) Under the optical field within the pump intensity range of this experiment, the transmittance of cholesteric liquid crystal were irrelevant to the angles between the pump and probe polarizations. (2) As for the opto-thermal effects, a simple heat-conducting model was proposed to fit with the experimental results. The theoretical and experimental results agree well with this model. The curves of the transmittance of cholesteric liquid crystal evolving with time showed that the higher the pump intensity, the larger the slope is in the transient state. Besides higher the pump intensity, the larger transmittance of the cholesteric liquid crystal has in the steady state. When the pump intensity is higher than 0.4 W/cm2, the recovery of the transmittance of exhibits a delay after the pump is turned off. The curve of the steady state transmittance versus the pump intensity can be divied into three regions: the slowly varying region, working region, the saturated region. When the samples were pumped with the intensities within the working area, the probe intensity varied linearly with the pump intensity. The transmittance of cholesteric liquid crystal can be modulated by the pump intensity. This demonstrates the possibility of applying this technique on the light modulators. The results showed that the probe beam exhibits almost no distortion at the rising part and the falling part of triangular pump intensity profile when thickness of PEDOT layer is 2.5 μm.