Particles hitch a ride inside a beam of light

Since its introduction in the 1970s, optical trapping has become a mainstay in science research. It has been used to tug at strands of DNA, to levitate aerosols, and to cool atoms to microkelvin temperatures. It has not, however, been very effective at manipulating light-absorbing particles, particularly in air or other gases. That’s because conventional traps rely on radiation pressure, the momentum imparted by refracting light, to pull a particle into an optical beam. But photophoretic forces, which result from the imbalanced momentum exchange between an anisotropically heated particle and its surrounding gas, tend to push an absorbing particle away from intense light. Andrei Rode and colleagues at the Australian National University in Canberra have now developed a technique that exploits photophoretic forces to confine and move light-absorbing particles in air. And unlike conventional techniques that manipulate particles over just millimeter distances, theirs could conceivably send particles gliding across an entire room. The key was to use a Laguerre–Gaussian beam, in which a donut-like ring of light surrounds a dark core. A particle that finds its way into the core, such as the 50-µm carbon-coated glass sphere shown here, gets trapped in the transverse (x–y) plane and propelled in the direction of light propagation (z) by photophoretic pushes. Two beams aimed from opposite directions can hold a particle in place, a potentially useful technique to isolate particles such as carbon soot—a suspected global warming contributor—for sensitive measurements. (V. G. Shvedov et al., Phys. Rev. Lett. 105, 118103, 2010 .)—Ashley G. Smart