Physicists Show That a Quantum Particle Made of Light and Matter Can Be Dragged by a Current of Electrons
A pair of studies in Nature show that a quasiparticle, known as a plasmon polariton, can be pulled with and against a flow of electrons, a finding that could lead to more efficient ways of manipulating light at the nanoscale.
Light was thought to move at a fixed rate until 1851, when a French physicist—the first to accurately clock the speed of light—showed it could also be slowed or accelerated simply by shining a light beam with or against the flow of moving water. Decades later, Einstein seized on Hippolyte Fizeau’s landmark water-pipe experiments in developing his theory of relativity.
Now, new research in Nature shows that a quasiparticle made of waves of photons and electrons—a plasmon polariton—has a similar ability to change speeds when immersed in an electrical current flowing through a sheet of graphene. But there’s a hitch: the polaritons appear to more easily shift gears in one direction—to a slightly slower speed—when traveling against the flow of electrons.
The finding is a big deal for plasmonics, a field with a rock-star name dedicated to finding new and efficient ways of controlling light down at the nearly invisible scale of individual atoms—for optical computing, nanolasers, and other applications, including imprinting patterns into semiconductors. Polaritons have two perks. Their relatively slow speed compared to photons makes them a good proxy for manipulating light. Polariton waves are also minuscule; dozens can squeeze into the wavelength of one photon.
Dmitri Basov, a physics professor at Columbia, has devoted most of his lab to studying their behavior. “Polaritons possess the best virtues of electrons and photons,” he said. “They’re compact but still quantum, which means they can be manipulated on ultra-fast time scales.”
In the recent Nature study, Basov and his colleagues recreated Fizeau’s experiments on a speck of graphene made up of a single layer of carbon atoms. Hooking up the graphene to a battery, they created an electrical current reminiscent of Fizeau’s water streaming through a pipe. But instead of shining light on the moving water and measuring its speed in both directions, as Fizeau did, they generated an electromagnetic wave with a compressed wavelength—a polariton—by focusing infrared light on a gold nub in the graphene. The activated stream of polaritons look like light but are physically more compact due to their short wavelengths.
The researchers clocked the polaritons’ speed in both directions. When they traveled with the flow of the electrical current, they maintained their original speed. But when launched against the current, they slowed by a few percentage points.
An Unexpected Result
“We were surprised when we saw it,” said study co-author Denis Bandurin, a physics researcher at MIT. “First, the device was still alive, despite the heavy current we passed through it—it hadn’t blown up. Then we noticed the one-way effect, which was different from Fizeau’s original experiments.”
The researchers repeated the experiments over and over, led by the study’s first-author, Yinan Dong, a Columbia graduate student. Finally, it dawned on them. “Graphene is a material that turns electrons into relativistic particles,” Dong said. “We needed to account for their spectrum.”
A group at Berkeley Lab found a similar result, published in the same issue of Nature. Beyond reproducing the Fizeau effect in graphene, both studies have practical applications. Most natural systems are symmetric, but here, researchers found an intriguing exception. Basov said he hopes to slow down, and ultimately, cut off the flow of polaritons in one direction. It’s not an easy task, but it could hold big rewards.
“Engineering a system with a one-way flow of light is very difficult to achieve,” said Milan Delor, a physical chemist working on light-matter interactions at Columbia who was not involved in the research. “As soon as you can control the speed and direction of polaritons, you can transmit information in nanoscale circuits on ultrafast timescales. It’s one of the ingredients currently missing in photon-based circuits.”
Optical isolators are currently used to limit the bounce-back of light in everything from lasers to the fiber optic cables in broadband. But they’re bulky and incompatible with modern nanocircuits, making polaritons, with their potential to be shut off in one direction, so appealing.
Plasmonics researchers are also excited about the detailed images to come out of the experiments. They show that polaritons can serve as nanoscale probes, they said, triggering and recording electron-electron interactions in a system. This information provides clues about how graphene and other quantum materials will behave in the real world.
“The images are effectively a read-out of the material properties of graphene,” Delor said.
'The Enablers of Nanoptics'
“I like to call polaritons the enablers of nanoptics,” says James Schuck, a mechanical engineer and plasmonics researcher at Columbia Engineering who was not involved in the work. “They’re useful for probing all sorts of materials at the nanoscale.”
Most of the experiments were done during the pandemic; the researchers wore masks and gloves and disinfected the lab after each visit. “There was no slow-down for quantum physics,” says Basov, with a laugh, evoking Fizeau.
The French physicist’s name was later inscribed on the Eiffel Tower; not for the effect that bears his name, but for precisely measuring the speed of light. Fizeau’s work was popularized in a lecture series at Columbia in 1906, as Basov likes to remind students. Fizeau was also an early photographic experimenter. Some of his ghostly daguerreotype views of the rooftops of Paris are held by The Metropolitan Museum of Art, not far from the Columbia campus.