An international team of physicists has theoretically predicted and experimentally generated light beams with a new property that they call the self-torque of light. The discovery, reported in the journal Science, could help us understand physics in binary black holes as well as how to control dynamics in molecules and nanostructures.
Two time-delayed infrared vortex laser pulses (upper-left) impinge on a gas target to generate coherent, EUV light with a time-dependent orbital angular momentum (right): the self-torque of light. Image credit: Kevin Dorney, JILA.
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.
In order to realize an entirely new property of light, manifested as a time-varying OAM along the light pulse, JILA physicist Kevin Dorney, University of Salamanca’s Dr. Laura Rego and their colleagues exploited the quantum physics inherent to the high harmonic generation (HHG) process.
“To create that high harmonic generation with light, an intense, femtosecond laser pulse is upshifted to high frequencies of the driving laser by essentially creating a nanoscale radiating antenna from an atom that is in the process of being ionized,” they explained.
“When properly phase-matched, bright, coherent laser-like beams can be generated that span from the extreme ultraviolet (EUV) to the soft X-ray regions of the electromagnetic spectrum.”
“The HHG process is coherent, meaning it must conserve both energy and momentum during the upconversion process, which is the secret to creating self-torqued EUV beams.”
This breakthrough showed that EUV light produced via HHG can indeed possess a time-dependent OAM.
And in doing so they discovered self-torque of light, which is like a wrench speeding up as it’s tightening a bolt. However, this wrench is composed of photons and twists around faster than an electron orbits a hydrogen nucleus.
“Normally, when HHG is driven with a ‘doughnut’ laser beam, the conservation rules dictate that each harmonic has an OAM equal to the harmonic number times the OAM of the driving laser,” Dorney said.
“However, if we drive the HHG process with a time-delayed pair of laser pulses possessing different OAM, then things get really interesting.”
“The time-delayed pulse pair results in the emission of EUV light with a time-varying OAM, which progresses smoothly through an octave of OAM states.”
“As the OAM content dynamically varies along the EUV pulse, the light beam continues to accelerate and thus possesses a self-torque.”
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Laura Rego et al. 2019. Generation of extreme-ultraviolet beams with time-varying orbital angular momentum. Science 364 (6447): eaaw9486; doi: 10.1126/science.aaw9486
