Angular Acceleration Observed in Helical Light Beams

The orbital angular momentum of laser light traveling along a helical path through space can accelerate and decelerate as it spins into the distance.

This is the first time that angular acceleration has been observed in light, according to a team of researchers from Africa and Germany. The phenomenon could lead to new applications in structured light fields, they said.

By combining two structured light fields in the form of Bessel beams (a), the resulting beam (b) spins as it travels. The rate of spin can be controlled. Images courtesy of Andrew Forbes/Wits School of Physics.

Orbital angular momentum is altered by twisting a light beam's wavefront into a helical shape. Usually this twist is smooth, like a spiral staircase with regular steps.

"Our novelty was to realize that by twisting the helicity of these beams in a nonlinear fashion, the result would be a propagation-dependent angular velocity," said professor Andrew Forbes of the University of the Witwatersrand, who led the project.

This acceleration can be controlled with a single parameter that is readily tuned with a digital hologram written to a standard LCD screen, the researchers said.

In fact, the light's orbital angular momentum speeds up and slows down as it travels, periodically switching from one mode to the other. Following its helical path through space, the helix appears to wind up very tightly as it accelerates and winds down very loosely as it decelerates.

Angular momentum is nonetheless conserved through an energy exchange mechanism across the field, the researchers wrote in Physical Review A (doi: 10.1103/PhysRevA.91.043821).

The team expects this phenomenon to be of interest as a tool to study some fundamental physical processes with light, as well as a tool to optically drive microfluidic flow.

For more information, visit www.wits.ac.za.