聚合物分散型液晶之光學相位研究

Abstract

偏振無關之液晶相位調製器有許多光學應用，目前偏振無關之液晶相位調製器主要分為：1)雙層型2)剩餘型3)雙層+剩餘混合型，及4)電場引發之克爾效應型。本實驗室在2015年發表的文章中提出另一種混合型偏振無關之液晶相位調製器：奈米型聚合物分散型液晶(domain~250nm)的光學相位來源為液晶分子轉動與電場引發之克爾效應，但是由先前的實驗結果，克爾效應造成的相位遠小於液晶分子轉動之偏振無關相位。本論文研究動機為：在奈米型聚合物分散型液晶中，克爾效應之相位是否永遠小於液晶分子轉動之相位?還是兩者相位有可能達到相近的大小?藉由研究此問題，我希望可以更加了解奈米型聚合物分散型液晶中，液晶在外加電場下的轉動與光學相位的關係。 本文提出在尺度夠小(~200nm)的奈米型聚合物分散型液晶中具有一種混合型的偏振無關相位，此混合型的偏振無關相位包含兩種相位：1)外加電場引發的克爾效應造成之相位與2)外加電場下，液晶分子在高分子包圍的液晶球中轉動所造成之相位。外加小電場下，入射光會先感受到克爾效應造成之相位；當電場逐漸增加，克爾效應的相位會消失，取而代之的是由液晶分子轉動所造成之相位。根據實驗結果，液晶球尺寸在200nm左右時，克爾效應之相位約為0.1π(徑度)，但轉動造成之相位可從0.24π(徑度)~ 0.42π(徑度)，此相位與奈米型聚合物分散型液晶之相分離程度有關。當尺度超過~350nm的奈米型聚合物分散型液晶中，不存在克爾效應造成之相位，完全都是由轉動造成之相位，相位可從0.22π(徑度)~ 0.63π(徑度)。 由此研究我們可得知在奈米型聚合物分散型液晶中，克爾效應之相位與液晶分子轉動之相位取決於液晶球之大小，此兩種相位在某些條件下是可能達到數量級接近的。並且我們得知克爾效應之相位發生在臨界電壓(Threshold voltage)之前，接著外加大電壓時，液晶開始轉動使得克爾相位消失。

There are many applications using polarization-independent LC phase modulators. Many researches proposed polarization-independent LC phase modulators, such as 1) double-layered type, 2) residual phase type, 3) combination of double-layered and residual phase type and 4) Kerr effect type. In 2015, we proposed a new type of polarization-independent LC phase modulator in nano-PDLC(droplet size~250 nm) which indicates the optical phases come from LC orientation and Kerr effect-induced birefringence. According to previous results, optical phases coming from Kerr effect is much smaller than the one coming from LC orientation. Hence, in this paper, we would like to investigate that in nano-PDLC, is it possible to make optical phases coming from Kerr effect be the same order compared with the optical phases come from LC orientation? In this paper, we proposed that the polarization-independent optical phases in nano-PDLC can be divided into two categories. 1) For droplet size about 200 nm, the optical phases come from LC orientation and Kerr effect. Under small voltages, Kerr phase dominates. With increasing voltages, optical phases come from LC orientation instead of Kerr effect. According to experimental results, Kerr phase is around 0.1π(rad) and orientational phase ranges from 0.24π(rad)~ 0.42π(rad). 2) For droplet size larger than 350nm, the optical phases come from LC orientation, without any Kerr phase. Through this research, we concluded that in nano-PDLC, Kerr phase depends on the droplet size. Under certain conditions, Kerr phase and orientational phase can compete. We also concluded that Kerr phase occurs before threshold voltage. After threshold voltage, orientational phase increases and Kerr phase gradually disappears.