Reversible Photoinduced Formation and Manipulation of a Two-Dimensional Closely Packed Assembly of Polystyrene Nanospheres on a Metallic Nanostructure

Nanostructure-enhanced optical trapping of polymer beads was investigated by means of fluorescence microspectroscopy. It was found that trapping behavior was quite sensitive to the particle size as well as excitation light intensity. We present a 2D closely packed assembly of polystyrene nanospheres on a gold nanostructure that is triggered by gap-mode localized surface plasmon (LSP) excitation. We discuss the trapping mechanism from the viewpoints of not only the radiation force but also of the thermal force (thermophoresis and thermal convection) induced by near-infrared laser irradiation. Thermophoresis worked as a repulsive force whose direction was opposed to that of the radiation force. On the other hand, thermal convection acted in favor of trapping: It supplied nanospheres toward the LSP excitation area. By suppressing the repulsive force, the assembled trapped nanospheres took the form of hexagonal shapes on a gold nanostructure. By optimizing irradiation parameters, we achieved 2D manipulation of nanospheres on a substrate. Our method has advantages over the conventional optical tweezers technique because of its weak light intensity, and could be a promising method of creating and manipulating a 2D colloidal crystal on a plasmonic substrate.