An optical tractor beam sorts microscopic particles

Light typically pushes particles forward thanks to radiation pressure. But it can also pull them backward, even in a beam of uniform intensity. The trick, according to theory, is to use a non-diffracting beam whose light scatters off the particles mostly in the forward direction. By conservation of momentum, the particles recoil backward. However, even slight changes in the particles' size, surface roughness, or refractive index can reverse the scattering direction. Exploiting that sensitivity, researchers led by Pavel Zemánek from the Institute of Scientific Instruments of the Academy of Sciences of the Czech Republic have now designed a simple scheme to separate subtly different types of particles—no moving parts required. Key to the achievement was realizing that two plane waves, steeply angled toward each other, could generate enough forward scattering to pull microscopic particles—tractor-beam style. They also realized that the waves' polarization, with electric fields pointed parallel or perpendicular to the forward direction, could be used as a knob to reverse the particles' motion on demand; even differences in scattering force on the scale of piconewtons between orientations can switch the direction. As sketched in the figure, the researchers reflected a Gaussian laser beam from the mirrored bottom of an aqueous cell containing two sizes of polymer spheres. Interference of the incident and reflected waves formed fringes parallel to the bottom. The spheres were sorted within two seconds of the light's polarization being switched to the perpendicular orientation—one size pushed and the other pulled along the fringes. (O. Brzobohatý et al., Nat. Photonics 7, 123, 2013.)—R. Mark Wilson