Dynamics of microparticle launching and loading into optical traps

Levitated optomechanics is opening new opportunities for quantum measurement in mesoscopic systems and for understanding the transition between quantum and classical mechanics. But loading microparticles into optical traps for measurements in vacuum remains an obstacle to the development of the field, in part because the process of launching particles so that they can be efficiently trapped remains poorly understood. We measure the dynamics of particle motion from vibrational excitation and launch from a surface to trapping in air. We demonstrate the direction of launch from a resonantly vibrating glass coverslip to be highly anisotropic and the measured escape velocity of tens to hundreds of millimeters per second to be insensitive to the amplitude of vibrational excitation. For a single particle launched repeatedly, escape velocities less than 50 mm per second are trapped with the highest probability. Trajectories from the launch point to the trap are found to follow a roughly two-step process in air: rapid deceleration due to drag, followed by drift into the trapping region that is accurately modeled by the Stokes-Einstein relation. These results illuminate the launch process in air, supporting the development of controlled launching of microparticles to enable application in a vacuum. This also enables selecting particles for trapping to optimize characteristics such as shape or Mie spectra, as well as repeated measurement of single particles to ensure reproducibility.