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Tightly focused light creates optical trapping sites. Periodic spatial arrangement of optical trapping cites forms optical lattice, which is capable of more complex particle manipulation, such as sorting and controlled transportation. Although extensive research of optical lattice has been carried out at micron scale based on far-field optics, e.g. holographic or interference techniques, the nanoscale counterpart has never been realized experimentally mainly due to the diffraction limit. Plasmonic nanostructures can localize and enhance light at nanoscale and offers unique opportunity to construct optical lattice with much higher density of trapping sites beyond the limit of far-field optics. In this work, we have for the first time reported an optical lattice created by well-designed periodic plasmonic nanostructures, as shown in the figure above. We demonstrate transport type of motion of nanospheres as well as the stacking of nanoparticels around the strongly localized potential of a primary lattice unit.
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Optical manipulation of very small particles has long been challenging due to reduced gradient force. Rotation of particles by light is even more difficult since that requires the particle to be absorbing or to exhibit large polarizability and optical anisotropy. Recently surface plasmon-enhanced optical near field has been used to effectively trap small particles. However, rotation and spinning of isotropic dielectric and transparent particles by optical near field remain challenging. In this work, we report the first experimental demonstration of selective trapping or rotation of isotropic dielectric micro-particles using one single plasmonic device, a plasmonic Archimedes spiral. Depending on the handedness of the input circularly polarized light, we show selective generation of a near field focusing hot spot for particle trapping or a plasmonic near-field vortex for particle rotation. Such functionality is of great interest and may find applications in various fields, such as protein folding analysis and local mixing in microfluidic channels. We anticipate various applications in the areas of optical manipulation, colloidal science, lap on a chip, and even atomic physics.
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