Light propagation in optical trapping assembling of colloidal particles at an interface

We conduct the optical trapping and assembling of polystyrene particles at the glass/solution interface by utilizing tightly focused 1064 nm laser of high power. Previously we reported that this leads to form the assembly sticking out horns consisting of single row of aligned particles through light propagation. Here, we demonstrate the laser power dependence of this phenomenon. With increasing the laser power, the particles are started to distribute around the focal spot and form the assembly larger than focal spot. The shape of the assembly becomes ellipse-like and the color at the central part of the assembly in transmission images is changed. This indicates that the assembly structure is changed, and trapping laser is started to propagate through the adjoining particles leading to horn formation. Strong laser power is necessary to elongate the horns and to align them straightly. We expect that this study will offer a novel experimental approach for assembling and crystallization of nanoparticles and molecules exclusively by optical trapping.