An international team of researchers have discovered a new property of light called "self-torque".
First, let's take a step back. Over the last few decades, physicists have discovered that it is possible to twist the wavefront of a light beam, giving it angular momentum. This looks a bit like a spiral staircase, with the beam wrapping around an empty middle. When such a beam is targeted at something, you see a bright donut. These beams are said to have orbital angular momentum (OAM), a property not dependent on polarization (which instead is about the geometry of the electromagnetic oscillations.)
As reported in the latest issue of the journal Science, researchers have discovered that it's possible for an OAM light beam to exhibit "self-torque". As the beam moves forward, its twist goes from wider to narrower, a bit like a screw. If such a beam is projected onto a flat surface, it would appear in the shape of a croissant or crescent.
“Remarkably, in addition to many well-known properties such as intensity, wavelength, and polarization, light can be twisted and thus it can possess angular momentum,” co-author Carlos Hernandez-garcia, a senior researcher at the University of Salamanca, said in a video released with the paper.
The discovery happened when the team was studying light beams produced in certain high-energy optical processes. These are often used to generate extreme ultraviolet light or powerful X-rays in order to study tiny and fast molecular processes. The team discovered that self-torque light beams are a natural product of these processes.
“I remember when the Salamanca team first told us about the self-torque of light in this experiment, we really scratched our heads a lot about this,” said study author Kevin Dorney, a postdoctoral researcher associate at Kapteyn-Murnane Group, JILA. “Orbital angular momentum is not such a trivial subject to begin with, so when you add a time dependence into the profile of these beams, things get very interesting.”
“With self-torque, we take two visible donut beams, separate them by just a little bit in time and then make the harmonics with that. When we do that, the beam that is emitted actually looks like a croissant. And this croissant contains over an octave of orbital angular momentum values along the light pulse. And that’s the unique property of the self-torque of light," Dorney added.
The team are now interested in studying how the beams behave and if there is any application to such a discovery.
“This is the first time that anyone has predicted or even observed this new property of light, so the immediate applications are not obvious,” co-lead author Laura Rego, of the University of Salamanca, said in the video. “From the fundamental-science point of view, this new structural property adds a new degree of freedom to study the dynamics of the light-matter interaction.”
One day, it might even be possible for researchers to develop a way to modulate this self-torque like we can modulate frequencies of other sorts.