Self-organized chiral microspheres; optical control and applications

Multifunctional colloidal micro and nano-particles with controlled architectures have very promising properties for applications in bio and nanotechnologies. Various distinctive optical features of new micrometric solid chiral particles, created via very simple self-assembling processes of soft matter, have been demonstrated. The microspheres show high performances and strongly innovative photonic capabilities in optical manipulation and as microlasers, suggesting new strategies in colloidal materials science, microphotonics devices, micro and optofluidics, microsensors, etc. The key for the creation of these solid microspheres with exotic chiral structures is based on the combination of the following phenomena: self-organization of helicoidal structures in liquid crystal (LC) monomers; confinement of LC microdroplets in water emulsion; physicochemical control of the molecular orientation of the LC at the water interface, and light induced polymerization process. The internal configuration can be controlled in the precursor LC droplet taking advantage of the large flexibility of the mesophase and its sensitivity to external agents. The investigations demonstrate that the supramolecular helical structures are fully preserved in the polymerized microspheres. Depending on the LC anchoring conditions at the interface, the internal configuration of these microparticles can be sorted on the basis of their symmetry: radial, conical or equatorial. These microspheres represent a new concept of chiral colloidal particles, as demonstrated in optical manipulation experiments and as microlaser application [1,2]. For the optical manipulation the microspheres behave as an optically isotropic or anisotropic dielectric particles with variable reflection and transmission properties. In fact, due to the selective reflection band connected to the helical structure, they may be attracted (as isotropic dielectric particles) or repelled (as metallic particles) by the optical trap, depending on the particular circular polarization state. On the other hand, optically anisotropic particles show an exotic dynamics. Depending on the light handedness, they behave like Janus spherical particles with dissimilar optical properties, meaning that the surface of the dielectric particles is partly transparent and partly reflecting. Concerning the laser application, emission from dye doped chiral microsphere demonstrates an innovative solid-state photonic microdevice. These solid chiral microparticles can play either a passive and/or an active role in processes involving lab-on-a-chip technologies. Interesting potential applications are foreseen in micro and optofluidics, microphotonics and materials science References [1] G. Cipparrone, A. Mazzulla, A. Pane, R. J. Hernandez and R. Bartolino, Advanced Materials 23, 5704 (2011). [2] R.J. Hernández, A. Mazzulla, A. Pane, K. Volke-Sepulveda and G. Cipparrone, Lab on a Chip 13, 459 (2013)