Propeller or wheel? In circularly polarized light, the vector which represents the electric field of the light wave (blue arrows in above figure) rotates helically in the direction of propagation. Such an electromagnetic wave has longitudinal angular momentum. If two circularly polarised waves rotating in opposite directions meet at a focal point, light with purely transverse angular momentum is generated. Its electric field vector rotates about an axis perpendicular to the direction of propagation like a bicycle spoke.
Light in general can exert incredible forces. According to the rules of quantum mechanics, light is an electromagnetic wave, as well as a stream of photons. Since it has momentum, a transparent particle through which a light beam falls experiences a recoil when the photons leave it. Although the force which a photon exerts in this process is almost infinitesimal, the effect of innumerable light particles in intense and tightly focused laser beams adds up in such a way that objects up to a few micrometres can be held in an optical trap or moved in a specific way. Biologists, for example, use this effect in optical tweezers to fix cells and rotate them at the focus of a microscope. To this effect, scientists working with Gerd Leuchs, Director at the Max Planck Institute for the Science of Light, are now creating new possibilities for them.
The team has created a photonic wheel, i.e. light with purely transverse angular momentum: the electric field of the electromagnetic wave rotates about an axis whose orientation is perpendicular to the direction of motion, just like the axis of a wheel. Until now, physicists have mainly been familiar with light with longitudinal angular momentum where the electric field rotates like a propeller around an axis aligned along the direction of motion. “The possibility that light can have purely transverse angular momentum when averaged over the complete cross-section of the beam had not been realised before,” says Peter Banzer, who made a significant contribution to the discovery.
This is because, as the Erlangen-based physicists have now shown both theoretically and practically, it is indeed possible to generate light with purely transverse angular momentum - and what’s more it is surprisingly easy to do so. “Once it’s down on paper, it looks easy,” says Gerd Leuchs. But somebody has to come up with the idea in the first place. The researchers are now developing this idea using circularly polarised light. A wave of circularly polarised light turns like a screw around the direction of beam propagation and has propeller-like longitudinal angular momentum. Light with circular polarisation can be generated with the aid of a birefringent crystal, for example.