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.
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