The beam moved spherical glass shells one fifth of a millimeter in diameter across a distance of up to 20 centimeters (7.87 in), which is around 100 times further than previous experiments at this scale, using only a single hollow laser beam that's bright around the edges and dark in its center.
The ANU researchers previously developed a similar device that moves very small particles over long distances using an optical vortex that created something called photophoretic force, which pushes the particle into a dark hollow in the center of the beam as the momentum of the photons drives it forward.
This new technique builds on the previous study, with energy from the laser heating the surrounding air particles to create hotspots on one side of the glass shell's surface. The heat drives air particles away, causing the tiny shell to recoil – thus propelling it in the opposite direction. And by altering the polarization of the laser beam on the fly, switching for instance from axial (shaped like a star) to azimuthal (like a ring), the researchers can cause the shell to change direction or stop.