Researchers in Spain and U.S. have announced they’ve discovered a new property of light — “self-torque.” Their experiment fired two lasers, slightly out of sync, at a cloud of argon gas resulting in a corkscrew beam with a gradually changing twist. They say this had never been predicted before.
Pulses with a twist and torque
Structured light beams can serve as vortex beams carrying optical angular momentum and have been used to enhance optical communications and imaging. Rego et al. generated dynamic vortex pulses by interfering two incident time-delayed vortex beams with different orbital angular momenta through the process of high harmonic generation. A controlled time delay between the pulses allowed the high harmonic extreme-ultraviolet vortex beam to exhibit a time-dependent angular momentum, called self-torque. Such dynamic vortex pulses could potentially be used to manipulate nanostructures and atoms on ultrafast time scales.
Science, this issue p. eaaw9486
Light beams carry both energy and momentum, which can exert a small but detectable pressure on objects they illuminate. In 1992, it was realized that light can also possess orbital angular momentum (OAM) when the spatial shape of the beam of light rotates (or twists) around its own axis. Although not visible to the naked eye, the presence of OAM can be revealed when the light beam interacts with matter. OAM beams are enabling new applications in optical communications, microscopy, quantum optics, and microparticle manipulation. To date, however, all OAM beams—also known as vortex beams—have been static; that is, the OAM does not vary in time. Here we introduce and experimentally validate a new property of light beams, manifested as a time-varying OAM along the light pulse; we term this property the self-torque of light.
Although self-torque is found in diverse physical systems (e.g., electrodynamics and general relativity), to date it was not realized that light could possess such a property, where no external forces are involved. Self-torque is an inherent property of light, distinguished from the mechanical torque exerted on matter by static-OAM beams. Extreme-ultraviolet (EUV) self-torqued beams naturally arise when the extreme nonlinear process of high harmonic generation (HHG) is driven by two ultrafast laser pulses with different OAM and time delayed with respect to each other. HHG imprints a time-varying OAM along the EUV pulses, where all subsequent OAM components are physically present. In the future, this new class of dynamic-OAM beams could be used for manipulating the fastest magnetic, topological, molecular, and quantum excitations at the nanoscale.
Self-torqued beams are naturally produced by HHG, a process in which an ultrafast laser pulse is coherently upconverted into the EUV and x-ray regions of the spectrum. By driving the HHG process with two time-delayed, infrared vortex pulses possessing different OAM, and , the generated high harmonics emerge as EUV beams with a self-torque, , that depends on the properties of the driving fields—that is, their OAM content and their relative time delay (td)—and on the harmonic order (q). Notably, the self-torque of light also manifests as a frequency chirp along their azimuthal coordinate, which enables its experimental characterization. This ultrafast, continuous, temporal OAM variation that spans from to is much smaller than the driving laser pulse duration and changes on femtosecond (10−15 s) and even subfemtosecond time scales for high values of self-torque. The presence of self-torque in the experimentally generated EUV beams is confirmed by measuring their azimuthal frequency chirp, which is controlled by adjusting the time delay between the driving pulses. In addition, if driven by few-cycle pulses, the large amount of frequency chirp results in a supercontinuum EUV spectrum.
We have theoretically predicted and experimentally generated light beams with a new property that we call the self-torque of light, where the OAM content varies extremely rapidly in time, along the pulse itself. This inherent property of light opens additional routes for creating structured light beams. In addition, because the OAM value is changing on femtosecond time scales, at wavelengths much shorter than those of visible light, self-torqued HHG beams can be extraordinary tools for laser-matter manipulation on attosecond time and nanometer spatial scales.
Light fields carrying orbital angular momentum (OAM) provide powerful capabilities for applications in optical communications, microscopy, quantum optics, and microparticle manipulation. We introduce a property of light beams, manifested as a temporal OAM variation along a pulse: the self-torque of light. Although self-torque is found in diverse physical systems (i.e., electrodynamics and general relativity), it was not realized that light could possess such a property. We demonstrate that extreme-ultraviolet self-torqued beams arise in high-harmonic generation driven by time-delayed pulses with different OAM. We monitor the self-torque of extreme-ultraviolet beams through their azimuthal frequency chirp. This class of dynamic-OAM beams provides the ability for controlling magnetic, topological, and quantum excitations and for manipulating molecules and nanostructures on their natural time and length scales.
As for what this implies; nanoscale manipulation can be used in both physics and engineering. Quantum physics research requires the manipulation of absolutely tiny structures (nanostructures), this kind of technology could allow us to build new kinds of atom traps and spin-state systems. Basically, it lets us build better experiments. On the engineering side, it could be used for making new types of nano-electro-mechanical-systems (NEMS), photonic integrated circuits, and it might even allow us to finally build a practical spintronic system.
I know a lot of that may sound like confusing, but that’s just because everything in my field has fancy names. It all boils down to making new and exciting experiments!
‘Corkscrew’ light could turbocharge the Internet
Different-shaped beams could increase fibre-optic capacity, easing Internet congestion.
Twisty beams of light could boost the traffic-carrying capacity of the Internet, effectively adding new levels to the information superhighway, suggests research published today in Science1.
Internet traffic is growing exponentially and researchers have sought ways to squeeze ever more information into the fibre-optic cables that carry it. One successful method used over the last 20 years essentially added more traffic lanes, using different colours, or wavelengths, for different signals2. But to compensate for the added lanes, each one had to be made narrower. So, just as in a real highway, the spacing could get only so tight before the streams of data began to jumble together.
In the last few years, different groups of researchers have tried to encode information in the shape of light beams to ease congestion, using a property of light called orbital angular momentum. Currently, a straight beam of light is used to transmit Internet signals, but certain filters can twist it so that it corkscrews around with varying degrees of curliness as it travels.
Now, a team of researchers from Boston University in Massachusetts and the University of Southern California in Los Angeles has found a way to keep the different light beam shapes separated for a record 1.1 kilometres.
The researchers designed and built a 1.1-kilometre-long glass cable, the cross section of which had a varying index of refraction — a measure that describes how fast light can travel in a particular medium. They then sent both twisty and straight beams of light down the cable.
The team found that the light output matched the input — light beams of each shape were not getting muddled together. The varying index of refraction apparently affected each light shape uniquely, so that different shapes moved at different speeds down the cable. “That meant that I could keep them separated,” says Siddharth Ramachandran, an electrical engineer and leader of the Boston University team.
The work published today used clockwise and anticlockwise versions of twisted light with a specific curliness, but Ramachandran says that the team has since done other research that suggests that about ten different beam shapes can be used to convey information.
That is exciting because each shape could potentially act as an entirely new level of traffic on the information superhighway. On each level, streams of data could be further divided into narrow lanes of colour, maximizing flow. “We showed a new degree of freedom in which we could transmit information,” says Ramachandran.
Translating the work from the lab to the real world will take time, however, in part because current Internet cables carry only straight beams of light. A more immediate goal, says Ramachandran, might be to install cables that are capable of carrying twisty light on the short distances between servers on giant ‘server farms’, used by large Web companies such as Facebook.
Miles Padgett, an optical physicist at the University of Glasgow, UK, is impressed with the work and is optimistic about its potential. “One day, more bandwidth will mean we can all Skype at the same time,” he says.