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