A single gold nanoparticle can translate light polarization into direction of propagation within ultrathin glass fiber. This “chiral waveguide coupler” can direct up to 94 percent of incident light in the desired direction, and so could function as an optical switch, according to researchers at the Vienna University of Technology. In free space, the rotational axis of circularly polarized light points in the same direction as the light wave propagates. Light hits a gold particle, which then emits light into a glass fiber in only one direction. Courtesy of the Vienna University of Technology. But when circularly polarized light is confined within a nanophotonic waveguide, its rotational plane pivots 90°, an effect called spin-orbit coupling. “Then, the direction of propagation is perpendicular to the spin, just like a bicycle, moving into a direction which is perpendicular to the axes of the wheels,” said professor Dr. Arno Rauschenbeutel. Rauschenbeutel's team irradiated the gold nanoparticle with a laser in such a way to make it emit circularly polarized light. As a result of spin-orbit coupling, the light propagates down the fiber in just one direction; specifically, left-hand circularly polarized light goes left, while right-hand circularly polarized light goes right. The research was published in Science (doi: 10.1126/science.1257671). For more information, visit www.tuwien.ac.at.