Spontaneous light-induced Turing patterns in a dye-doped twisted nematic layer

Ignacio Andrade-Silva,Marcel G Clerc,Stefania Residori,Gregorio González-Cortés,Mario Wilson,Umberto Bortolozzo

doi:10.1038/s41598-018-31206-x

Abstract

Optical pattern formation is usually due either to the combination of diffraction and nonlinearity in a Kerr medium or to the temporal modulation of light in a photosensitive chemical reaction. Here, we show a different mechanism by which light spontaneously induces stripe domains between nematic states in a twisted nematic liquid crystal layer doped with azo-dyes. Thanks to the photoisomerization process of the dopants, light in the absorption band of the dopants creates spontaneous patterns without the need of temporal modulation, diffraction, Kerr or other optical nonlinearity, but based on the different scales for dopant transport processes and nematic order parameter, which identifies a genuine Turing mechanism for this instability. Theoretically, the emergence of the stripe patterns is described on the basis of a model for the dopant concentration coupled with the nematic order parameter.

Highlights

Non-equilibrium processes often lead to the formation of spatial periodic structures developed from a homogeneous state through the spontaneous breaking of symmetries[1,2,3,4]
We consider a twisted nematic liquid crystal cell, namely, the liquid crystal molecules have mutually orthogonal planar anchoring onto the two glass substrates that constitute the confining walls of the cell
We are able to describe the emergence of the stripe patterns on the basis of a model for the concentration of azo-dye dopants in the excited state coupled with the order parameter of the twisted nematic layer

Summary

Introduction

Non-equilibrium processes often lead to the formation of spatial periodic structures developed from a homogeneous state through the spontaneous breaking of symmetries[1,2,3,4]. We show that stripe domain patterns between different nematic states (i.e. molecules that are locally alternated between regions of higher and lower orientational order) can spontaneously arise in a dye-doped twisted nematic liquid crystal layer when illuminated under appropriate conditions In this case, the pattern formation www.nature.com/scientificreports/. We are able to describe the emergence of the stripe patterns on the basis of a model for the concentration of azo-dye dopants in the excited state (cis-state) coupled with the order parameter of the twisted nematic layer This model allows us to identify the mechanism of pattern emergence, which is due to the different scales for transport processes of dopants and order parameter, i.e. it corresponds to a Turing instability. The Turing-Swift-Hohenberg equation is a paradigmatic model for pattern formation in several contexts, such as hydrodynamics, chemistry, plant ecology, nonlinear optics, and elastic materials[4]

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