Optical angular manipulation of liquid crystal droplets in laser tweezers

The high sensitivity of liquid crystals to external fields, especially electromagnetic fields, confer to them fascinating properties. In the case of light fields, their large optical nonlinearities over a broad spectrum have great application potential for all-optical devices. The linear optical properties of liquid crystals, such as their high refractive index, birefringence and transparency, are also of great practical interest in optofluidics, which combines the use of optical tools in microfluidic environments. A representative example is the laser micromanipulation of liquid crystalline systems using optical tweezing techniques. Liquid crystal droplets represent a class of systems that can be easily prepared and manipulated by light, with or without a nonlinear light-matter coupling. Here we review different aspects of quasi-statics and dynamical optical angular manipulation of liquid crystal droplets trapped in laser tweezers. In particular, we discuss to the influence of the phase (nematic, cholesteric or smectic), the bulk ordering symmetry, the droplet size, the polarization state and power of the trapping light, together with the prominent role of light-matter angular momentum exchanges and optical orientational nonlinearities