Augmented and virtual reality displays require the use of thin liquid crystal cells, with thickness of the order of 1-2 µm. Here, we demonstrate a method, based on cross-polarised intensity measurements coupled to a Frank-Oseen model of the liquid crystal alignment, to characterise such thin cells and the interactions between the liquid crystal and the cell substrates. We first use wedge shaped cells to calibrate the measurement process and show that the cross-polarised intensity data can be used to reliably characterise cells with a non-uniform thickness profile. Previously, we have shown that we can characterise optically thin cells (cells with a phase lag of less than 2π). In this report we apply the method to characterise the optical properties of cells with a non-uniform thickness profile and geometrically thin cells. We show that reducing the thickness of the cell increases the pretilt.
We present a versatile characterisation of liquid crystal devices, including those integrated with organic photovoltaics. Photovoltaic thin film serves as an alignment layer and also generates an electric field under illumination that reorients liquid crystals for self-activated or autonomous operation. Apart polar alignment angle, anchoring energy, thickness uniformity, the photovoltaic properties, such as the photovoltage generated and photoconductivity, are captured and the map of the spatial changes of the parameters can be created. The method is applicable to other liquid crystal systems, such as doped liquid crystals and optically thin cells, with phase lag as small as π.
The overall performance of liquid crystal devices is determined by a large number of interlinked features. We demonstrate that an easy-to-implement methodology and optical technique can provide a comprehensive characterisation and mapping of liquid crystal systems, capturing their static as well as dynamic properties. It has also been successfully applied to thin liquid crystal cells, planar and twisted cells. The technique is not only a powerful tool for optimising the choice of materials for each specific application, but also offers a great insight in the polarisation dynamics of light propagating in the anisotropic and multi-layer liquid crystal systems.
Current liquid crystal technologies often rely on the use of optically thin cells and new liquid crystals. The characterisation of their parameters, such as elastic constants, including twist elastic constant and pretilt, key to control the liquid crystal response, poses several challenges. We present an optical method that successfully characterises such liquid crystal devices, is relatively simple yet a powerful probe of their static and dynamical properties. The method is demonstrated for the cells with the total phase lag smaller than 2pi and for experimental liquid crystals, where optical and dielectric properties are only partially known and for estimating K2.
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