A new laser tractor beam has the ability to attract and repel millimeter-scale objects over longer distances than previously possible. “Demonstration of a large-scale laser beam like this is a kind of holy grail for laser physicists,” said Dr. Wieslaw Krolikowski, a professor in Australian National University’s Research School of Physics and Engineering. He and fellow ANU researchers built the tractor beam using a hollow laser beam capable of moving gold-coated glass particles one-fifth of a millimeter in diameter over a distance of up to 20 cm. This is about 100 times farther than previous, similar experiments, the researchers said. Dr. Vladlen Shvedov, left, and Dr. Cyril Hnatovsky adjust the hollow laser beam in their lab. Courtesy of Stuart Hay/ANU. In the study, the new tractor beam displayed reliance on the energy of the laser as it heated the particles, as well as the surrounding air. This is different from conventional tractor beam techniques, which use photon momentum to impart motion. As energy from the laser hit a particle and traveled across its surface, the energy was absorbed, creating hotspots on the surface. Air particles that collided with those hotspots heated up and shot away from the surface, causing the particle to recoil in the opposite direction. The team manipulated the particle by moving the hotspot, achieved by controlling the laser beam’s polarization. “We have devised a technique that can create unusual states of polarization in the doughnut-shaped laser beam, such as star-shaped (axial) or ring-polarized (azimuthal),” said research fellow Dr. Cyril Hnatovsky. “We can move smoothly from one polarization to another and thereby stop the particle or reverse its direction at will.” The research was published in Nature Photonics (doi: 10.1038/nphoton.2014.242). For more information, visit www.anu.edu.au.