Light-controlled movement of isotropic droplets in smectic films

Abstract

This paper reports on optical trapping of micrometre-sized isotropic inclusions in free-standing smectic A* films. Droplet manipulation and trapping potential in such a two-dimensional anisotropic system show that optical trapping has two distinct regimes with unique separation dependence, governed by long-range and short-range trapping forces and enhanced diffusivity at the free surfaces. Molecular ordering in the surface layers of isotropic inclusions, at the liquid crystal–air interface, in addition leads to a new field of light-controlled particle dynamics. For low laser powers, translational motion of a droplet along the laser polarisation is observed. Above the threshold laser power, the transfer of optical angular momentum to the inclusion via linearly polarised light leads to circular-like motion. As the optical torque for a given intensity is counterbalanced by the elastic torque of the smectic film, this motion results in finite angle steps.

Keywords:

• free-standing smectic films
• isotropic droplets
• optical tweezers

Acknowledgements

This work was supported by Alexander von Humboldt foundation. The author acknowledges useful discussions with Dr M. Ravnik and Professor C. Bechinger.