Cornell University researchers have found a way to produce magnetic fields at the nanoscale without using any external magnets or magnetic materials. The method uses trapped light and a specially engineered surface to generate a static magnetic field, an approach that could have implications for spintronics, quantum computing, photonic computing, and data storage.
The research, published in Advanced Science and reported by Phys.org, was led by Ph.D. candidate Shivaksh Rawat and postdoctoral researcher Samyobrata Mukherjee, working under Gennady Shvets, the J. Preston Levis Professor of Engineering at Cornell's School of Applied and Engineering Physics.
The core of the technique involves what researchers call a time interface. When light crosses a spatial boundary, such as moving from air into water, some of it reflects and some passes through. A time interface works differently. Instead of a physical boundary, it involves a sudden change in the optical properties of the material the light is traveling through, such as a rapid shift in the refractive index. Like a spatial interface, it produces reflected and transmitted waves. But a time interface can also freeze part of the light's rapidly oscillating magnetic field, converting it into a stationary magnetic field.
To create the time interface, the team built a two-dimensional metasurface: a precisely engineered rectangular array of germanium nanostructures designed to trap mid-infrared light inside the structure. While the mid-infrared light was still trapped, they hit the surface with a short, intense burst of near-infrared light.
That burst of near-infrared light released electrons from the germanium atoms, creating what researchers call electron holes and large numbers of free electrons. The rapid generation of these electron-hole pairs caused a sudden change in the refractive index of the material, creating the time interface for the trapped mid-infrared light. Some of the trapped light's energy shifted to new, longer-wavelength light waves. The rest converted into the kinetic energy of circulating free electrons, which formed current loops that sustained a persistent magnetic field in concentrated hot spots on the surface.
"We used an approach known as localized free carrier generation, which has advantages over other methods of nanoscale magnetization," Rawat said. "One of the important contributions of our work is that our approach is material agnostic. Any nonmetallic surface will work."
Without energy losses, the magnetic field would persist indefinitely. Under real conditions, the field decays over time, but the researchers say the approach still represents a meaningful step toward generating controllable nanoscale magnetic fields using light alone. The material-agnostic nature of the method means it is not limited to germanium and could potentially be applied across a wide range of surfaces and devices.
