Non-linear optical measurement of the twist elastic constant in thermotropic and DNA lyotropic chiral nematics

Liana Lucchetti,Tommaso P Fraccia,Fabrizio Ciciulla,Tommaso Bellini

doi:10.1038/s41598-017-05136-z

Liana Lucchetti, Tommaso P Fraccia + Show 2 more

Open Access

https://doi.org/10.1038/s41598-017-05136-z

Copy DOI

Abstract

Throughout the whole history of liquid crystals science, the balancing of intrinsic elasticity with coupling to external forces has been the key strategy for most application and investigation. While the coupling of the optical field to the nematic director is at the base of a wealth of thoroughly described optical effects, a significant variety of geometries and materials have not been considered yet. Here we show that by adopting a simple cell geometry and measuring the optically induced birefringence, we can readily extract the twist elastic coefficient K22 of thermotropic and lyotropic chiral nematics (N*). The value of K22 we obtain for chiral doped 5CB thermotropic N* well matches those reported in the literature. With this same strategy, we could determine for the first time K22 of the N* phase of concentrated aqueous solutions of DNA oligomers, bypassing the limitations that so far prevented measuring the elastic constants of this class of liquid crystalline materials. The present study also enlightens the significant nonlinear optical response of DNA liquid crystals.

Highlights

Liquid crystals (LC) are a state of matter whose structured anisotropic and yet fluid nature has enabled an enormous variety of applications, most of which relying on the easy coupling of their symmetry axes to external fields
We report here experiments performed on N* cells in planar geometry, where the chiral axis of cholesterics develops along z, the direction normal to the cell surfaces
The typical cell response to the pump optical field can be observed in Fig. 2b, where both the transmitted probe and the pump signals are shown as a function of time for the choice of polarizers and analyzer orientations described above, in the case of DNA samples

Summary

Introduction

Liquid crystals (LC) are a state of matter whose structured anisotropic and yet fluid nature has enabled an enormous variety of applications, most of which relying on the easy coupling of their symmetry axes to external fields. Measuring elasticity in DNA LC has turned out to be cumbersome because of a combination of difficulties: DNA LC are intrinsically chiral, making it difficult to use typical light scattering based analysis; no strategy has emerged yet to control the surface alignment of DNA LC phases, ruling out the possibility of using Freedericksz transition methods; coupling with electric fields is weak and disturbed by the large density of intrinsic counterions present in the solution. All these limitations are here overcome by using an optical field and by exploiting a geometry in which surface alignment is not crucial

Methods

Results

Discussion

Conclusion