Optical trapping of director structures and defects in liquid crystals using laser tweezers

Ivan I Smalyukh,Paras N Prasad,Andrey N Kuzmin,Daniel S Kaputa,Aliaksandr V Kachynski

doi:10.1364/oe.15.004359

Ivan I Smalyukh, Paras N Prasad + Show 3 more

Open Access

https://doi.org/10.1364/oe.15.004359

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Abstract

We demonstrate optical manipulation of structures and defects in liquid crystals (LCs). The effective refractive index depends on the LC molecular orientations and the laser beam's polarization. We use the orientation-mediated refractive index contrast for the laser trapping in LCs with a homogeneous composition, but with spatially-varying patterns of molecular orientations. Tightly-focused polarized beams allow for optical trapping of disclinations and their clusters, dislocations and oily streaks, cholesteric fingers and focal conic domains, etc. We calculate the optical gradient forces for typical structures and explain the trapping properties at low laser powers. We also show that when a high-power beam causes local molecular realignment, the laser trapping properties change for two reasons: (1) the refractive index pattern and optical gradient forces are modified; (2) additional elastic structural forces arise to minimize the elastic free energy.

Highlights

Optical trapping with tightly focused laser beams has been broadly applied to objects as diverse as biological molecules, colloidal particles, and living cells [1,2,3]
Molecular orientations in liquid crystals (LCs) form a wealth of complex three-dimensional (3-D) patterns of the director N (r ), that depend on the treatment of confining surfaces, presence of inclusions and chiral agents, flow and temperature changes, etc
We have demonstrated optical trapping of director structures in liquid crystals

Summary

Introduction

Optical trapping with tightly focused laser beams has been broadly applied to objects as diverse as biological molecules, colloidal particles, and living cells [1,2,3]. The manipulation of multiple objects has been demonstrated by the use of holographic tweezers and time-shared trapping [2,3,4,5]. The resulting optical gradient forces pul the high-index objects towards the focus with the highest intensity, and a stable trapping is achieved when they overcome the scattering and gravity forces. These objects (such as particles or biological cells) have a material composition and, refractive indices different from that of the surrounding medium, as needed for noncontact manipulation [3]

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Discussion

Conclusion