A doughnut-shaped beam of green light can pull glass beads for tens of centimeters, the longest distance ever reported for a tractor beam of light.
Regardless of what science fiction would suggest, it’s hard to get an object to move in the opposite direction as the light illuminating it. Light scattering off an object provides a push. To counter that effect and generate a pull, Australian National University physicist Wieslaw Krolikowski and his team used air temperature and pressure. They illuminated 50-micrometer-wide gold-coated glass beads with a hollow green laser beam and then adjusted the light’s polarization, or orientation of the light’s vibration. When the researchers directed the polarization toward the center of the beam, the targeted bead moved forward, away from the light source. But when they made the polarization tangential to the beam, the light heated the back of the bead, which also lowered the pressure of the surrounding air. Moving from high to low pressure, the bead nudged backward.
In a study published October 19 in Nature Photonics, Krolikowski’s team reports pushing, pulling and trapping the beads by varying the polarization. Krolikowski says he envisions using tractor beams to precisely move biological samples into position for study by powerful microscopes.