New method of creating twisted light may allow fibers to carry more information

September 24, 2013

These images show the spiral structure of the coherent light emitted from a beam of electrons accelerated to nearly the speed of light and arranged into a helix by a simple laser. Left: An intensity map. Note the dark center where the light cancels itself out, resulting in a cross section resembling a doughnut. Right: A phase structure map, clearly showing the helix described by the light. (Credit: Hemsing, et al.)

Scientists at SLAC have found a new method to create coherent beams of twisted light — light that spirals around a central axis as it travels.

The method has the potential to generate twisted light in shorter pulses, higher intensities, and a much wider range of wavelengths (including X-rays) than is currently possible.

First described two decades ago, twisted light is attracting attention from researchers in fields as diverse as telecommunications, quantum computing, condensed matter research and astronomy because of one unique property:

Researchers have demonstrated that it can transmit more information through fiber optic cables than the current industry standard.

Creating twisted light with an electron beam

Until now, researchers created twisted light by shooting laser beams through masks or holographic gratings. But a team of accelerator physicists from SLAC and UCLA has shown they can create it with a beam of electrons, in much the same way SLAC’s Linac Coherent Light Source (LCLS) X-ray laser uses electrons to generate pulses of X-ray laser light.

Illustration of the experiment (not to scale). The unmodulated relativistic electron beam interacts with a linearly polarized laser in a helical undulator, which gives the electrons an energy kick that depends on their position in the focused laser beam. The e-beam then traverses a longitudinally dispersive chicane that allows the electrons with higher energy to catch up to those with lower energy (momentum compaction). The result is a ‘helically microbunched’ beam that then radiates light with
OAM at the fundamental frequency in the planar undulator. (Credit: Erik Hemsing et al./Nature Physics)

There are several advantages to generating corkscrew light beams this way, said SLAC postdoctoral researcher Erik Hemsing, who is lead author of a paper in the September issue of Nature Physics. Free-electron lasers can generate light in a vast range of wavelengths and in extremely short, bright pulses,  opening up the possibility of generating orbital angular momentum (OAM) light at the X-ray wavelengths of, for example, LCLS.

In the case of corkscrewing light, researchers send two pulses — one containing electrons, the other laser light — through an undulator simultaneously.

The combination of laser pulse and undulator imprints an energy pattern on the electrons. As they pass through another grouping of magnets called a chicane, the electrons reposition themselves like race cars on a curve, and hit the next straightaway arranged in the shape of a corkscrew. This “helically microbunched” arrangement of electrons then enters a second undulator that causes them to wiggle and emit spiraling light.

Intense beams of spiraling X-ray light could also open the door to new condensed matter research, said Hemsing.