Chiral microparticles in circular polarized light traps

Control of self-assembled chiral microparticles (MP) has been demonstrated by means of polarized optical trapping. These objects show strongly innovative photonic capabilities in optical manipulation suggesting new approaches for microphotonic devices, optofluidics, microsensors. The MP formation is mostly due to the combination of the following mechanisms: the helicoidal self-organization of the liquid crystal (LC) monomers, the physicochemical control of the LC molecules orientation at the interface, and the light induced polymerization process. The internal MP configuration can be controlled in the precursor LC microdroplet taking advantage of the large flexibility of the mesophase and by its sensitivity to external agents. Investigations demonstrate that the supramolecular helical structures already present in the droplets are fully preserved in the polymerized MP. The anchoring conditions at the interface impose the internal molecular configuration allowing the MP to be sorted on the basis of their symmetry: radial, conical and cylindrical. These microspheres represent a new concept of chiral colloidal particles, as demonstrated in optical trapping experiments and in microlaser application [1,2,3]. In optical manipulation investigations the MP behave as optically isotropic or anisotropic dielectric elements. In fact, due to their selective reflection band, connected to the specific chiral arrangement, they might be attracted to (like a dielectric particles) or repelled from (like a conductive particles) the trapping light spot, depending on the circular polarization handedness. In addition, optically anisotropic MP show a more complex dynamics acting as Janus particles that possess dissimilar optical properties, i.e. their surface have both transparent and reflecting regions depending on the particular polarization state. A model based on the transversal and scattering forces generated by the trap gives a qualitative explanation of the observations. Laser emission from dye doped MP has been obtained, demonstrating that these chiral objects also work as innovative solid-state photonic microdevices. Solid chiral microparticles 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, Chiral Self-assembled Solid Microspheres: a Novel Multifunctional Microphotonic Device, Advanced Materials 23, 5773 (2011). [2] R.J. Hernández, A. Mazzulla, A. Pane, K. Volke-Sepulveda and G. Cipparrone, Attractive-repulsive dynamics on light-responsive chiral microparticles induced by polarized tweezers, Lab on a Chip 13, 459 (2013). [3] A. Mazzulla, G. Cipparrone, R.J. Hernández, A. Pane, R. Bartolino, Self-Organized Chiral Microspheres, Mol. Cryst. Liq. Cryst. 576, 15 (2013).