Optical Trapping, Assembly, and Surface Fixation of Nanoparticles in Liquid

Abstract

The intense electromagnetic field produced by a focused laser beam attracts microparticles, dispersed in liquid, into the focal spot. This is known as optical trapping and used to manipulate microparticles dispersed in liquid. If the size of the particles is much smaller than that of the focal spot, i.e. the order is of wavelength, a number of particles are trapped in the focal spot. This means that the focal spot of the laser beam acts as a microcage for nanoparticles and the confined nanoparticles form a microassembly. Nanoparticles are attractive materials for their characteristic chemical, physical, and optical properties. We have studied “optical assembly” of nanoparticles and found the following unique properties. (1) By a single particle counting measurement, it was revealed that the assembling process of polystyrene nanoparticles is dependent on laser power and concentration. (2) It was demonstrated that the assembly structure and optical properties of gold-nanoparticles can be controlled by optical force.