Liquid crystal thin films obtained by spincoating and photopolymerization are widely used nowadays for geometric phase optical components. The liquid crystal-air interface during photopolymerization plays a crucial role as it should not disturb the photoalignment induced by the other interface. When photopolymerizing the liquid crystal layer in vacuum, processing parameters need to be optimized carefully to avoid alignment issues at the air interface. In this work we demonstrate that domains are formed due to a tilt angle which is induced at the air interface under certain conditions. A measurement method is developed to quantify the tilt angle at the interface. The method consists of optical analysis of the domains using crossed-polarizer transmission microscopy with an oblique sample holder. By careful analysis of the obtained tilt angles for different parameters, it is assumed that a non-negligible homeotropic anchoring strength at the air interface is responsible for the domain formation. The results are explained with a theoretical model that assumes a finite anchoring strength at both the photoaligned interface and the air interface.

中文翻译：

---

垂直排列强度在聚合液晶层空气界面的作用

通过旋涂和光聚合获得的液晶薄膜目前广泛用于几何相位光学元件。光聚合过程中的液晶-空气界面起着至关重要的作用，因为它不应干扰由另一个界面引起的光取向。在真空中对液晶层进行光聚合时，需要仔细优化工艺参数，以避免空气界面出现对准问题。在这项工作中，我们证明了域的形成是由于在某些条件下在空气界面处引起的倾斜角。开发了一种测量方法来量化界面处的倾斜角。该方法包括使用带有倾斜样品架的交叉偏振器透射显微镜对域进行光学分析。通过对不同参数获得的倾斜角的仔细分析，假设在空气界面处不可忽略的垂直锚固强度是域形成的原因。结果用一个理论模型来解释，该模型假设在光对准界面和空气界面处的锚定强度都是有限的。