Cornell University researchers have demonstrated a way to generate strong, localized static magnetic fields using only light, without external magnets or magnetic materials, an approach they say could advance spintronics, magnetic data storage, and photonic and quantum computing.
Illustration of the metasurface designed to control infrared light. Laser pulses can selectively activate regions of the material, allowing researchers to dynamically tune its optical properties. Image credit: Cornell University
The technique relies on what the team calls a "time interface." Just as a light wave partially reflects and partially transmits when it crosses a spatial boundary, such as the surface between air and water, it does something analogous when it encounters a sudden change in the optical properties of the medium it's traveling through over time, rather than in space. A sufficiently abrupt time interface can also excite a static, zero-frequency mode of the system, effectively pulling part of the light's rapidly oscillating magnetic field out of circulation and freezing it into a stationary magnetic field pattern. Unlike more established light-driven magnetization methods such as the inverse Faraday effect, which tend to rely on the specific nonlinear optical properties of the material used, the team's approach works with the geometry of an engineered surface rather than the material itself. "We used an approach known as localized free carrier generation, which has advantages over other methods of nanoscale magnetization," author Shivaksh Rawat said. "One of the important contributions of our work is that our approach is material agnostic. Any non-metallic surface will work."
To create the time interface, the researchers built a two-dimensional metasurface out of an array of germanium nanostructures engineered to trap mid-infrared light through a high-quality-factor electric dipolar resonance, chosen because germanium is essentially lossless and highly damage-resistant at these wavelengths. Illuminating the metasurface with an intense, roughly 8-femtosecond burst of near-infrared light drives tunneling photoionization in the germanium, rapidly freeing electrons and creating localized "hot spots" of free carriers within each nanostructure. Because this happens on a timescale comparable to a single optical cycle of the trapped mid-infrared light, it produces a sharp time interface: the metasurface's resonance shifts, red-shifting and partially reflecting the trapped light in time, while free electrons generated at rest are accelerated by the light's electromagnetic fields into circulating currents. Those currents, sustained after the driving light has moved on, are what generate the persistent magnetic field.
In simulations, the researchers found the trapped mid-infrared wave could reach a peak magnetic field of about 2.4 tesla, of which roughly 53% was converted into a rectified, static field with a peak amplitude of about 1.27 tesla. Rather than lasting indefinitely, as it would in a lossless system, the rectified field decayed with a time constant of around 300 femtoseconds, or about 20 optical cycles of the light that generated it, limited by electron-scattering losses in the germanium at room temperature; the team's modeling suggests that operating at cryogenic temperatures, where electron mobility in germanium increases sharply, could extend that persistence to several picoseconds or more.
"Our work demonstrates a fundamentally new method for creating strong magnetic fields using light rather than conventional magnets," Rawat said. "By rapidly changing the optical properties of an engineered metasurface, we were able to convert part of a passing light wave into a localized magnetic field that remained after the light had passed. We used a more efficient and highly localized approach than many existing methods, and our work helps explain how energy is redistributed in rapidly changing optical materials."
The team stated that this result may open new research directions at the intersection of photonics and magnetism, with potential relevance to spintronics, magnetic data storage, and photonic and quantum computing, alongside applications in precise, localized control of magnetic environments.