Electrically Tunable Fresnel Lens in Twisted-Nematic Liquid Crystals Fabricated by a Sagnac Interferometer.

Bing-Yau Huang,Tsung-Hsien Lin,Chie-Tong Kuo,Tian-Yi Jhuang

doi:10.3390/polym11091448

Bing-Yau Huang, Tsung-Hsien Lin + Show 2 more

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https://doi.org/10.3390/polym11091448

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Abstract

This paper presents an electrically tunable Fresnel lens in a twisted nematic liquid crystal cell fabricated by using a Sagnac interferometer. When the Fresnel-patterned green beam, formed by the Sagnac interferometer, is irradiated on the azo-dye doped liquid crystal mixture, the azo-dye molecules undergo trans–cis photoisomerization and then generate the photo-alignment effect in the bright (odd) zones. The director of the liquid crystal molecules in the odd zones reorients the direction perpendicular to the polarization direction of the linearly polarized green beam. The various structures of liquid crystals in the odd and even zones will result in a phase difference and thus, a Fresnel lens can be generated. The experimental results show that the proposed Fresnel lens has a high diffraction efficiency of 31.5% under an applied alternating-currents (AC) voltage. The focal length of the Fresnel lens can also be tuned by thermally erasing the photo-alignment effect of the azo dyes and rewriting by a different Fresnel-like pattern.

Highlights

Fresnel lenses are of enormous interest due to their wide range of applications [1,2,3,4,5,6,7,8,9,10]
Fresnel lenses are fabricated by thin-film deposition and electron-beam lithography [11,12], which leads to some disadvantages, such as the diffraction efficiency being fixed and not being adjustable
The mixture of the azo dye-doped LCs (ADDLCs) is injected into the empty cell, which is produced with two planar indium-tin oxide (ITO) glass plates separated by plastic spacers of 12 μm

Summary

Introduction

Fresnel lenses are of enormous interest due to their wide range of applications [1,2,3,4,5,6,7,8,9,10]. Fresnel lenses are fabricated by thin-film deposition and electron-beam lithography [11,12], which leads to some disadvantages, such as the diffraction (focusing) efficiency being fixed and not being adjustable. Such issues can be overcome by using liquid crystals (LCs), which can be controlled by an electrical field. Many methods for fabricating LC Fresnel lens have been proposed, such as using photomasking [13] and patterned alignment of LCs [14]. The common disadvantage of the above-mentioned LC Fresnel lenses is the fixed focal length

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