Chiral nematic liquid crystal microlenses

Piotr Popov,Elizabeth K Mann,Lawrence W Honaker,Antal Jákli,Mona Mirheydari

doi:10.1038/s41598-017-01595-6

Piotr Popov, Elizabeth K Mann + Show 3 more

Open Access

https://doi.org/10.1038/s41598-017-01595-6

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Abstract

Nematic liquid crystals (NLCs) of achiral molecules and racemic mixtures of chiral ones form flat films and show uniform textures between circular polarizers when suspended in sub-millimeter size grids and immersed in water. On addition of chiral dopants to the liquid crystal, the films exhibit optical textures with concentric ring patterns and radial variation of the birefringence color. Both are related to a biconvex shape of the chiral liquid crystal film; the rings are due to interference. The curvature radii of the biconvex lens array are in the range of a few millimeters. This curvature leads to a radial variation of the optical axis along the plane of the film. Such a Pancharatnam-type phase lens dominates the imaging and explains the measured focal length of about one millimeter. To our knowledge, these are the first spontaneously formed Pancharatnam devices. The unwinding of the helical structure at the grid walls drives the lens shape. The relation between the lens curvature and material properties such as helical pitch, the twist elastic constant, and the interfacial tensions, is derived. This simple, novel method for spontaneously forming microlens arrays can also be used for various sensors.

Highlights

Nematic liquid crystals (NLCs) of achiral molecules and racemic mixtures of chiral ones form flat films and show uniform textures between circular polarizers when suspended in sub-millimeter size grids and immersed in water
Chiral nematic Liquid crystals (LC) (N*) materials were obtained by adding chiral dopants CD1 or CD2 to the nematic LC (NLC) material 4-cyano-4′-pentylbiphenyl (5CB) obtained from Sigma-Aldrich and used without further purification
When the LC in N* phase is held in air, which provides homeotropic anchoring, a so-called “fingerprint texture”[25] forms [see Fig. 2 (b)]

Summary

Introduction

Nematic liquid crystals (NLCs) of achiral molecules and racemic mixtures of chiral ones form flat films and show uniform textures between circular polarizers when suspended in sub-millimeter size grids and immersed in water. This curvature leads to a radial variation of the optical axis along the plane of the film Such a Pancharatnam-type phase lens dominates the imaging and explains the measured focal length of about one millimeter. The relation between the lens curvature and material properties such as helical pitch, the twist elastic constant, and the interfacial tensions, is derived This simple, novel method for spontaneously forming microlens arrays can be used for various sensors. The unique optical properties of Pancharatnam devices have been utilized in making high efficiency compact optical lenses by providing an appropriate profile across an aperture[21, 22] In such lenses the optical axis is in the plane of the film with an azimuthal angle β that spatially varies along the radial direction. We characterize the geometry and imaging properties of such lenses, and show that the lens shape is induced by molecular chirality and is driven by the liquid crystal-grid interaction

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