A Planar Fresnel Lens in Reflection Type Based on Azo-Dye-Doped Cholesteric Liquid Crystals Fabricated by Photo-Alignment.

Bing-Yau Huang,Chie-Tong Kuo,Jia-De Lin,Tsung-Hsien Lin,Tzu-Yeh Chen,Ting-Hui Chen

doi:10.3390/polym12122972

Bing-Yau Huang, Chie-Tong Kuo + Show 4 more

Open Access

https://doi.org/10.3390/polym12122972

Copy DOI

Abstract

This paper presents a focusing efficiency and focal length tunable planar Fresnel lens in reflection type based on azo-dye-doped cholesterol liquid crystal film. The Fresnel-like pattern of a pumping beam can be formed by a Sagnac interferometer. When the azo-dye molecules are irradiated by the pumping beam, the photoalignment effect will be induced in the bright (odd) zones due to the trans–cis photoisomerization of azo-dye molecules. Thus, the structures of cholesteric liquid crystals in the odd zones will reorient from the imperfectly planar textures to the perfectly planar textures. The different structures of cholesteric liquid crystals in two adjacent zones will give rise to phase difference for the reflected light and thus function as a Fresnel lens. The focusing efficiency of the proposed Fresnel lens can be controlled by the applied voltages and affected by the polarization state of incident light. Moreover, various focal lengths of the Fresnel lens can be achieved by rewriting a different center radius of the Fresnel-like pattern.

Highlights

Cholesteric liquid crystals (CLCs) are widely used in many optical components, such as gratings [1,2], displays [3,4], lasers [5], and photonic crystals [6,7], because CLCs have special optical properties and can be controlled by electric field
The above issue can be overcome by using LCs or CLCs as the optical material, which can be controlled by applying an external voltage
The electrically controlled LC Fresnel lens is operated by the external electric field modulated refractive index or structure of LCs in two adjacent zones of the Fresnel pattern and the focusing efficiency can be arbitrarily adjusted, which can be produced by using several methods, such as photomasking [15]

Summary

Introduction

Cholesteric liquid crystals (CLCs) are widely used in many optical components, such as gratings [1,2], displays [3,4], lasers [5], and photonic crystals [6,7], because CLCs have special optical properties and can be controlled by electric field. The transreflective liquid crystal (LC) lens can be produced by using CLCs as optical material in an LC cell [8]. The focusing efficiency and focal length of the Fresnel lens fabricated by these ways are fixed and lack tunability. The above issue can be overcome by using LCs or CLCs as the optical material, which can be controlled by applying an external voltage. The electrically controlled LC Fresnel lens is operated by the external electric field modulated refractive index or structure of LCs in two adjacent zones of the Fresnel pattern and the focusing efficiency can be arbitrarily adjusted, which can be produced by using several methods, such as photomasking [15]

Methods

Results

Conclusion