When a high-intensity laser interacts with plasma, the charged particles typically oscillate back and forth like waves on the ocean. But what if the laser itself could twist like a whirlpool? Researchers have now demonstrated a rotating, spring-shaped laser pulse, opening up new possibilities for fusion energy, particle acceleration, astrophysics and beyond.
In new research published in Nature Photonics, scientists from Lawrence Livermore National Laboratory (LLNL) and the University of California, Irvine demonstrated the first high-intensity "light spring" laser.
Unlike conventional laser beams, a light spring rotates around its central axis at a controllable rate. If shone onto a wall, the beam pattern would trace out circles over time.
"For a long time, people have used high-power and high-intensity laser pulses to drive plasmas," said LLNL scientist and lead author Andrew Longman. "But those lasers are almost always in their simplest configuration. It's basically a round spot - nothing especially interesting. It's like hitting the plasma with a hammer. We're interested in laser pulses that are structured in both space and time because they let us do unusual things, like stir the plasma."
The beam's rotation can be tuned over a wide range of speeds - including apparent speeds that exceed the speed of light without violating relativity. This capability could enable researchers to drive new types of plasma waves that have never before been explored experimentally.
The breakthrough depended on some of the most precise freeform optics ever produced by LLNL's National Ignition Facility and Photon Science directorate. Researchers first split a broadband laser into two beams using specialized beamsplitters: one containing shorter ("blue") wavelengths and the other longer ("red") wavelengths. Each beam then reflects from a custom, nanostructured mirror.
To the naked eye, those mirrors appear perfectly flat. In reality, each contains an extremely subtle spiral pattern etched into its surface. If the 6-inch mirrors were scaled up to a mile across, the spiral step would be only about half an inch high.
"For some of these freeform optics, across the entire optic, the difference between the design and the finished part was only about five nanometers," said author Tayyab Suratwala, the program director for Optics and Materials Science and Technology at LLNL. "We're talking down to almost the atomic scale."
That result required critical expertise in grinding and polishing large optics with unprecedented precision.
After reflecting from these mirrors, a second beamsplitter recombines the two beams, aligning them precisely in space and time. The result is a new type of laser pulse that resembles a twisting strand of DNA.
The laser was assembled and tested at UC Irvine by a team including former LLNL intern Danny Attiyah and professor Franklin Dollar.
"In Ghostbusters, they have the proton packs that fire one beam down the middle and another that spirals around it," Dollar said. "At high intensities, we only knew how to make the middle beam. Now we can make the spiraling beam too."
Simulations suggest that light springs can efficiently drive helical plasma waves that behave like tiny electromagnetic coils. Using these high-precision optics, a tabletop-scale laser system could potentially generate magnetic fields exceeding 100 Tesla - comparable to some of the strongest magnetic fields ever produced in a laboratory.
"The nice thing was that it was all done on a laser that can just literally fit on your tabletop," said Longman. "You don't need to have these large-scale multi-million-dollar lasers in order to do original work."
Magnetic fields this strong could provide a new platform for studying how matter behaves under extreme conditions. Researchers could use them to investigate plasma processes relevant to astrophysical environments and explore how intense magnetic fields alter the behavior of atoms and the light they emit.
One potential application for light springs is plasma-based particle acceleration. In conventional plasma accelerators, electrons can eventually outrun the laser pulse that is accelerating them, limiting how much energy they gain. Simulations suggest that the helical plasma waves driven by light springs could overcome this limitation. Because the wave rotates as it propagates, electrons may remain in the accelerating region for longer distances, allowing them to reach higher energies.
"We can really crank up the fields for how fast we accelerate particles," said Dollar. "We can get similar energies to what you might get out of a giant accelerator facility in something like a centimeter."
The scientists continue to test the laser and explore its wide range of applications. They are also working on a design that could merge all the optics into a single component - an achievement that would make light springs far easier to implement and scale up.
Suratwala and his team of optics experts are ready to help.
"If we make the capability, then people will take advantage of it," he said. "Our group does a lot of this, where we develop a technology and then see it get used. That's incredibly fulfilling. That's the driver that makes us want to do this kind of work."
This work was supported in part by LLNL's Laboratory Directed Research and Development program.
