Optical trapping and manipulation exploiting liquid crystalline systems

Abstract

This thesis and all the research contained within, pretends to develop new ideas and concepts on liquid crystals (LC) and optical trapping and manipulation. The combination between optical tweezers and LC systems promises unique and exciting results. The content on the thesis is presented for those with some experience in the elds of liquid crystal and optical manipulation, and for those who are interested in begin to learn about these matters, proposing an overview of much existing work and a correlation between di erent science branches like soft matter, photonics and optical control. Two main research lines has been developed involving liquid crystalline systems and polarized optical tweezers. In the rst part, nematic LC droplets in water have been adopted to study the mechanical properties of light elds with a polarization gradient, i.e. optical tweez- ers based on polarization holographic techniques with non conventional trapping in an extended interferometric optical trap. For this purpose, LC emulsions in wa- ter were prepared, obtaining droplets with radial or bipolar director con guration, which result in optically isotropic or anisotropic particles. Exploiting the vecto- rial nature of the light and its interaction with LC droplets, an unconventional opto-hydrodynamical control and trapping has been demonstrated. The planned experiments shown that a hydrodynamic force, known as Magnus force, never con- sidered in optical micromanipulation experiments, can play an important role in the optical micromanipulation and should be considered whenever particles are forced to spin and dragged in a uid. In the second part, the study was mainly focused into developing an innovative and versatile soft matter object, namely chiral-solid microspheres. They were created by combining very simple self-assembling and photoinduced processes of the soft mat- ter, i.e. photopolymerizing cholesteric LC droplets in water emulsion. The ability to control the internal helical geometry using chemical agents in the precursor LC emulsion enables to obtain solid microspheres with radial, conical, or cylindrical con gurations of the helical structures that exhibit unique optical properties. Their exclusive capabilities were demonstrated by optical manipulation experiments in- volving optical tweezers. A unique and dichotomous behavior has been revealed by polarized circularly polarized tweezers: an attractive or repulsive optical force is ex- erted by varying the light polarization. Moreover, the application of the chiral-solid microspheres as optical microresonators for creating microlasers was also demon- strated. The high performance as well as the novel and exclusive properties make these chiral microparticles good candidates for developing new concepts in colloidal materials science, microphotonics, microlasers, optical trapping and manipulation, micro- and opto uidics and microsensors.