For decades, scientists have harnessed the force exerted by light to move tiny objects, such as cells and viruses. Now, physicists have taken such manipulation literally a thousand times further than before, devising a "light pipe" that can transport a tiny particle a meter and set it on a surface with an accuracy of 20 micrometers—less than the width of a human hair. The technique could have applications that include simulating the physics of interstellar space and sniffing out airborne traces of contraband.
"My whole group was blown away by this," says Miles Padgett, an experimental physicist at the University of Glasgow in the United Kingdom. "We've been talking about what we might do with this clever idea."
Pushing and pulling on things with light has become a standard laboratory technique. For example, so-called optical tweezers can move and stretch DNA, cells, viruses, and other microscopic objects. In a typical experiment, one end of a biomolecule—say, a protein or a stretch of DNA—in a liquid binds to a glass plate whereas the other binds to a small glass or plastic bead. Researchers then pull on the bead with a tightly focused beam of laser light. The electric field in the light shifts charges within the bead to polarize it. The bead can then minimize its energy by moving to where the light is brightest, creating a tug called the "gradient force." But optical tweezers can move an object only about a millimeter, as they involve shining a laser through a microscope.
To move stuff farther, Vladlen Shvedov, Andrei Rode, and colleagues at Australian National University in Canberra used a different bit of physics called the photophoretic force. It arises when light warms a particle in air or another gas on one side. The warm side of the particle heats the air near it, causing the air molecules to speed up. When those zippier molecules bump into the particle, they hit it harder than the slower molecules striking the particle's cool side. So the particle gets shoved out of the light with a force that can be far stronger than the gradient force.
To put this force to work, the researchers formed a tube of laser light called a vortex beam. They then fed into the tube either bits of carbon or hollow glass spheres coated with carbon, with sizes ranging from 20 to 120 micrometers and masses less than 50 nanograms. When a particle would wander away from the dark hole in the middle of the beam, the light would warm one side of it and push it back toward the center. And because the light in the beam was flowing in one direction, it would gently push the particle down the pipe, as the team reports in a paper in press at Physical Review Letters.
The technique has some limitations. The particles must be lightweight and must conduct heat poorly so that they can stay warm on one side and cool on the other. Still, it should be possible to transport biomolecules and even cells over large distances without touching or contaminating them, Rode says. The experimenters transported particles the full length of their optical table, 1.5 meters. "I can't see anything that would stop us from delivering [a particle] over 10 meters," Rode says.
"It is very cool," says Kishan Dholakia, an experimenter at the University of St. Andrews in the United Kingdom. Dholakia says he suspects the 1.5-watt laser beam would overheat cells, but he sees other possibilities. Photophoretic forces push around interstellar dust, so the technique might be useful for laboratory-based astrophysics research, he says. Padgett envisions taking the rig out of the lab, perhaps to sniff out contraband by snaring molecules out of the air: "Maybe we should have one of these above the baggage carousel at the airport to inspect suspect suitcases."