Optically Induced Forces Imposed in an Optical Funnel on a Stream of Particles in Air or Vacuum

Airborne nanoparticles may be manipulated with light via $p{{0}{0ex}}h{{0}{0ex}}o{{0}{0ex}}t{{0}{0ex}}o{{0}{0ex}}p{{0}{0ex}}h{{0}{0ex}}o{{0}{0ex}}r{{0}{0ex}}e{{0}{0ex}}s{{0}{0ex}}i{{0}{0ex}}s$, a thermal force due to uneven illumination of a particle. The authors use a funnel-shaped hollow-core laser beam to trap graphite particles at different heights, depending on their masses. This allows for calibration, and prediction of the trajectories of objects in the optical funnel. Such precise, touch-free positioning could be used to place a biomolecule in the focus of an x-ray free electron laser, for the recording of its time-varying structure in a ``molecular movie''.