Twisting and tweezing liquid crystals with lasers

Exciting new directions for liquid crystals (LCs) are emerging on the length scale of the wavelength of light. Two complementary micron-sized systems are formed by LC droplets and by dispersions of colloidal particles in LCs. The dimensions of each of these systems are ideal for laser tweezer manipulation, allowing a new range of photon-addressed LC systems to be envisaged. Trapping and moving micron-sized particles in LCs is a beautiful approach that can build novel colloidal photonic materials. However, it is also a unique way of studying fundamental LC properties, particularly anisotropic viscosity coefficients in the low Ericksen regime, which can be accessed by laser trapping. Rather few nematic materials have been studied using laser traps; we describe two different approaches to deduce the viscosity coefficients of nematic mixtures. Micron-sized LC droplets are emerging as intriguing photonic systems in their own right. Angular momentum can be transferred from laser traps to droplets, with specific polarization properties and droplet geometries resulting in a variety of novel photon-driven effects. Fast optical switches, rotating at speeds >1kHz, can be produced from nematic droplets in circularly polarized beams. Both droplet geometry and beam polarization influence the droplet rotation, allowing control of the phenomenon. Surprisingly, a chiral nematic droplet can sometimes undergo continuous rotation in a linearly polarized trap, a phenomenon caused by optically-induced changes in chirality. We describe this remarkable effect which demonstrates how the control of chirality through polarization can result in an optically driven transducer.