A group of physicists has built what may be the world's gentlest cannon, a device that pushes tiny particles along using laser light at about a hundredth the walking speed of the average human. The researchers described their experimental setup in a paper published in the September 10 issue of Physical Review Letters.

The device harnesses thermal forces that arise when a suspended particle is heated by light absorption on only one side, causing the surrounding air molecules to bounce off the particle with greater velocity on the hot side than on the cold side. To produce macroscopic motions from those forces, Vladlen Shvedov of the Australian National University (ANU) in Canberra and the Taurida National University in Ukraine and his colleagues used a so-called optical vortex pipeline, essentially a beam of laser light with a dark core, forming a sort of cylindrical pipe of light.

Optically-induced thermal forces push a particle—in this case micron-scale projectiles, either carbon particles or hollow glass beads—from the walls of the cylinder, trapping the particle in the dark core while gradually nudging the tiny projectile down the pipeline toward a target surface. The study's authors used the technique to transport particles up to 1.5 meters, but it took some particles nearly three minutes to cover the distance.

The researchers demonstrated their technology by spelling out the initials ANU on a target (see first video below). In the demonstration, glass beads of 60 to 100 microns were pushed a distance of 50 centimeters with placement accuracies of 10 microns.


In the second video, the group used a glass target, which reflects part of the cylindrical laser beam, producing a repulsive thermal force near the glass surface. Just before the target, where the two thermal forces counterbalance, the particle comes to a stop, hovering in midair above the glass. The particle can then be moved around over the surface by redirecting the cylindrical laser beam with a movable mirror.


Videos courtesy of The American Physical Society/Vladlen G. Shvedov, Andrei V. Rode, Yana V. Izdebskaya, Anton S. Desyatnikov, Wieslaw Krolikowski, and Yuri S. Kivshar</