Rutkowska Katarzyna A, Kozanecka-Szmigiel Anna
Faculty of Physics, Warsaw University of Technology, 00-662 Warszawa, Poland.
Sensors (Basel). 2020 Nov 27;20(23):6789. doi: 10.3390/s20236789.
Tunable diffraction gratings and phase filters are important functional devices in optical communication and sensing systems. Polarization gratings, in particular, capable of redirecting an incident light beam completely into the first diffraction orders may be successfully fabricated in liquid crystalline cells assembled from substrates coated with uniform transparent electrodes and orienting layers that force a specific molecular distribution. In this work, the diffraction properties of liquid crystal (LC) cells characterized by a continually rotating director pattern at the cell substrates and in the bulk, are studied theoretically by solving a relevant set of the Euler-Lagrange equations. The electric tunability of the gratings is analyzed by estimating the changes in liquid crystalline molecular distribution and thus in effective birefringence, as a function of external voltage. To the best of our knowledge, such detailed numerical calculations have not been presented so far for liquid crystal polarization gratings showing a director pattern. Our theoretical predictions may be easily achieved in experimental conditions when exploiting, for example, photo-orienting material, to induce a permanent LC alignment with high spatial resolution. The proposed design may be for example, used as a tunable passband filter with adjustable bandwidths, thus allowing for potential applications in optical spectroscopy, optical communication networks, remote sensing and beyond.
可调谐衍射光栅和相位滤波器是光通信和传感系统中的重要功能器件。特别是偏振光栅，能够将入射光束完全重定向到一级衍射级，它可以在由涂覆有均匀透明电极和取向层的基板组装而成的液晶盒中成功制造，这些取向层迫使分子具有特定的分布。在这项工作中，通过求解一组相关的欧拉 - 拉格朗日方程，从理论上研究了液晶（LC）盒在盒基板处和体内具有连续旋转指向矢图案的衍射特性。通过估计液晶分子分布以及有效双折射随外部电压的变化，分析了光栅的电可调性。据我们所知，对于具有指向矢图案的液晶偏振光栅，目前尚未有如此详细的数值计算。当利用例如光取向材料以高空间分辨率诱导液晶永久取向时，我们的理论预测在实验条件下很容易实现。例如，所提出的设计可以用作具有可调带宽的可调谐带通滤波器，从而在光谱学、光通信网络、遥感等领域具有潜在应用。
