Focus: Smoke Rings in Light

A newly discovered optical vortex forms a ring around many intense laser pulses but was never noticed before. In a typical optical vortex, light waves twist around a dark line, or hole, through the center of a light beam. Researchers have now uncovered an entirely different type of optical vortex that forms a ring, or toroid, around a self-focusing light pulse. Light waves curl around the vortex, like the air currents around a smoke ring. These so-called spatiotemporal optical vortices (STOVs) are a new optical phenomenon that may be useful in areas such as microscopy or the optical transmission of information. The rotation around an optical vortex does not concern the direction of the oscillating electric field (polarization), but rather its phase [1]. For example, the maximum of the field might initially be located just above the central axis of a light beam, and then a short time later it’s to the right of the axis, then below. This rotating phase gives the light an orbital angular momentum that can cause a charged particle within the beam to rotate.