Microbead dynamics in optical trap assisted nanopatterning

Optical near-field techniques allow one to overcome diffraction by positioning an optical element in close proximity to the surface of interest. In optical trap assisted nanopatterning, this optical element is a microbead optically trapped above the substrate in a liquid environment. Using high-speed microscopy, we show that under certain conditions, the laser pulse creates a gas bubble under the bead and that this bubble displaces the bead before disappearing. The bead then returns to its original position under the action of the scattering force of the optical trap. We measure the bead vertical trajectory and extract its terminal velocity in order to calculate the magnitude of the trapping force exerted on the bead. This work opens the way to a better understanding of the bead-surface interactions under laser irradiation and, therefore, contributes to the development of near-field techniques.