The Longstanding Puzzle: Why Can’t We See the Optical Hall Effect?

For over a century, scientists have known that electric currents bend in a magnetic field—a phenomenon known as the Hall effect. In magnetic materials like iron, this effect is strong and well understood. But in ordinary, non-magnetic metals like copper or gold, the effect is much weaker.

In theory, a related phenomenon—the optical Hall effect—should help scientists visualize how electrons behave when light and magnetic fields interact. But at visible wavelengths, this effect has remained far too subtle to detect. The scientific world has known it was there, but lacked the tools to measure it.

“It was like trying to hear a whisper in a noisy room for decades,” said Prof. Amir Capua. “Everyone knew the whisper was there, but we didn’t have a microphone sensitive enough to hear it.”

Cracking the Code: A Closer Look at the Invisible

Led by Ph.D. candidate Nadav Am Shalom and Prof. Amir Capua from the Institute of Electrical Engineering and Applied Physics at Hebrew University, in collaboration with Prof. Binghai Yan from the Weizmann Institute of Science, Pennsylvania State University, and Prof. Igor Rozhansky from the University of Manchester, the study focuses on a tricky challenge in physics: how to detect tiny magnetic effects in materials that aren’t magnetic.

“You might think of metals like copper and gold as magnetically ‘quiet’—they don’t stick to your fridge like iron does,” explained Prof. Capua. “But in reality, under the right conditions, they do respond to magnetic fields—just in extremely subtle ways.”

The challenge has always been how to detect these tiny effects—especially using light in the visible spectrum where laser sources are readily available. Until now, the signal was simply too faint to observe.

Turning Up the Volume on Magnetic Whispers

To solve this, the researchers upgraded a method called the magneto-optical Kerr effect (MOKE), which uses a laser to measure how magnetism alters light’s reflection. Think of it like using a high-powered flashlight to catch the faintest glint off a surface in the dark.

By combining a 440-nanometer blue laser with large-amplitude modulation of the external magnetic field, they dramatically boosted the technique’s sensitivity. The result: they were able to pick up magnetic “echoes” in non-magnetic metals like copper, gold, aluminum, tantalum, and platinum—a feat previously considered near-impossible.

Why It Matters: When Noise Becomes a Signal

The Hall effect is a pivotal tool in the semiconductor industry and in studying materials at the atomic scale: it helps scientists figure out how many electrons are in a metal. But traditionally, measuring the Hall effect means physically attaching tiny wires to the device, a process that is time-consuming and tricky, especially when dealing with nanometer-sized components. The new approach, however, is much simpler: it merely requires to shine a laser on the electrical device, no wires needed.

Digging deeper, the team found that what appeared to be random “noise” in their signal wasn’t random at all. Instead, it followed a clear pattern tied to a quantum property called spin-orbit coupling, which links how electrons move to how they spin—a key behavior in modern physics.

This connection also affects how magnetic energy dissipates in materials. These insights have direct implications for the design of magnetic memory, spintronic devices, and even quantum systems.

“It’s like discovering that static on a radio isn’t just interference—it’s someone whispering valuable information,” said Ph.D. candidate Am Shalom. “We’re now using light to ‘listen’ to these hidden messages from electrons.”

Looking Ahead: A New Window into Spin and Magnetism

The technique offers a non-invasive, highly sensitive tool for exploring magnetism in metals—without the need for massive magnets or cryogenic conditions. Its simplicity and precision could help engineers build faster processors, more energy-efficient systems, and sensors with unprecedented accuracy.

“This research turns a nearly 150-year-old scientific problem into a new opportunity,” said Prof. Capua.

“Interestingly, even Edwin Hall, the greatest scientists of all, who discovered the Hall effect, attempted to measure his effect using a beam of light with no success. He summarizes in the closing sentence of his notable paper from 1881: “I think that, if the action of silver had been one tenth as strong as that of iron, the effect would have been detected. No such effect was observed.” (E. Hall, 1881).”

“By tuning in to the right frequency—and knowing where to look—we’ve found a way to measure what was once thought invisible.”

Co-authored by Renata Ivanek, a professor in the department of population medicine and diagnostic sciences, and Martin Wiedmann, professor in food safety, the paper outlines interventions likely to make a concrete difference in the safety of the nation’s romaine.

“This study supports that interventions should focus on reducing produce contamination via contaminated irrigation water, on assuring that produce washes applied during processing consistently deliver reasonably high reductions of bacterial numbers, and on improving temperature control during distribution,” Wiedmann said.

“We tried to describe the system as holistically as possible to account for different risk factors and how they could have interactions,” Ivanek said. “There’s not just one intervention that will save us all. We spent a lot of time trying to understand the preharvest component, especially the irrigation water piece and how much risk can be explained by that.”

Study results suggested that much contamination originates from irrigation with untreated surface water applied through overhead spray irrigation systems. They found that risk from irrigation was reduced either through water treatments or by switching to furrow or drip irrigation.

“While not the most common system, spray irrigation is used in a number of fields for its benefits during germination, its cooling effect on plants and other reasons. But drip or furrow irrigation reduces the probability that water directly touches the leaves,” Ivanek said, acknowledging that switching to these other irrigation systems introduces significant potential additional costs to grower.

Ivanek and her co-authors also explored the importance of maintaining proper cold storage temperatures along the entire supply chain to romaine’s final destination.

“Time and temperature play a role in food safety, and also in food quality and shelf life,” she said, describing a “perfect storm” if contamination happens at the farm or processing level and then improper transportation temperatures allow bacteria to grow.

The comprehensive practices and interventions explored in this study intend to aid decision-makers in establishing and enhancing food safety best management practices, Ivanek said.

“The big message is the American food supply chain is extremely safe compared to other countries,” she said. “We’re exploring how can we make it even safer and where we should put additional effort.”