基于琼斯矩阵的液晶偏振光栅扭曲角及厚度的测量方法

Method for measuring the twist angle and thickness of liquid crystal polarization grating based on Jones matrix

液晶偏振光栅是一种基于几何相位原理的新型光栅器件,可以通过调控液晶光轴的空间分布来实现对入射光相位、偏振态等的调制。传统的液晶偏振光栅在正入射及波段范围较小时可以达到很高的衍射效率,但其在宽波段范围内难以保持很高的衍射效率,大角度入射时衍射效率下降明显。多层扭曲结构的提出使这些问题可以得到解决,但引入了新的问题,如何精准控制扭曲角及厚度的大小。基于此问题,本文提出了一种新型的基于琼斯矩阵的液晶偏振光栅的扭曲角及厚度的测量方法,使用差值拟合的方法消除了由于整个波段折射率变化引起的误差,可以准确得到液晶片的扭曲角和厚度。分析了扭曲液晶片和扭曲液晶偏振光栅的异同,基于扭曲液晶片的测量结果可以得到相同条件下制备的液晶偏振光栅的扭曲角及厚度。实验结果表明对厚度的测量误差小于2%,对扭曲角的测量误差小于±0.5°。该方法能够快速和方便地实现对扭曲角及厚度的精准测量,有很高的测量准确性及稳定性,并分析了手性剂的浓度和扭曲角之间的关系,对扭曲液晶偏振光栅的制备方法提供了理论依据。

Liquid crystal polarization grating is a new type of grating device based on the principle of geometric phase.It can modulate the phase and polarization state of incident light by adjusting the spatial distribution of the optical axis of liquid crystal.Traditional liquid crystal polarization gratings can achieve high diffraction efficiency at normal incidence and a small spectral bandwidth,but it is difficult to maintain a high diffraction efficiency with broad spectral bandwidth,and the diffraction efficiency decreases significantly when incident at a large angle.The proposed multi-layer twisted structure allows these problems to be solved,but introduces a new problem,how to accurately control the twist angle and thickness.Based on this problem,this paper proposes a new method for measuring the twist angle and thickness of a liquid crystal polarization grating based on Jones matrix.The difference fitting method eliminates the error caused by the refractive index change of the entire waveband,and can accurately obtain the twist angle and thickness of the liquid crystal cell.The similarities and differences between the twisted liquid crystal film and the twisted liquid crystal polarization grating are analyzed.Based on the measurement results of the twisted liquid crystal film,the twist angle and thickness of the liquid crystal polarization grating prepared under the same conditions can be obtained.The experimental results show that the thickness measurement error is less than 2%,and the twist angle measurement error is less than±0.5°.This method can quickly and conveniently realize the accurate measurement of twist angle and thickness,with high measurement accuracy and stability.The relationship between the concentration of the chiral agent and the twist angle is analyzed,which provides a theoretical basis for the preparation method of multi-twist liquid crystal polarization grating.