Trapped infrared light at Cornell becomes a 300-femtosecond magnetic field with no magnet involved
A germanium metasurface designed by Shivaksh Rawat, Samyobrata Mukherjee and Gennady Shvets at Cornell University traps mid-infrared light. When the array is lit by a short near-infrared pulse, electron-hole pairs appear, the refractive index changes abruptly and part of the stored optical energy passes into a static magnetic field. Neither a magnet nor a magnetic material takes part. The field lasts about 300 femtoseconds, roughly 20 cycles of the mid-infrared wave.
Science··Morning
A magnetic field stays on the Cornell metasurface after the light has passed
Shivaksh Rawat is a doctoral candidate in Gennady Shvets's group at Cornell University, and Samyobrata Mukherjee is a postdoctoral researcher in the same group. In their paper in Advanced Science they report that an engineered metasurface trapping light can give strong static magnetic fields with no external magnet and no magnetic material. The array is a rectangular lattice of two-dimensional germanium nanostructures designed to hold mid-infrared light. While that light is still inside the structure, a short pulse of higher-energy near-infrared photons arrives. In the team's modelling the near-infrared light frees electrons from the germanium atoms, leaving electron holes and many free electrons behind.[1], [2]
The time interface moves the wave's energy into a zero-frequency mode
The abrupt change in refractive index sets up what the team calls a time interface. When a light wave meets a spatial interface, as between air and water, part of it reflects and part passes through; when the optical properties of the medium change suddenly, reflected and transmitted waves appear as well. What sets a time interface apart is that it can also excite the system's static zero-frequency mode: part of the light's rapidly oscillating magnetic field stops and turns into a magnetic pattern that stays in place. In the Advanced Science paper part of the initial energy shifts into new red-shifted waves, while the rest is held as the kinetic energy of free electrons circulating in the hot spots, and the current loops those electrons form keep the magnetic field standing. The properties of the medium change faster than the trapped light can leave, so the energy it carries is converted rather than radiated away.[1], [2]
Losses set the lifetime, and the approach works on non-metallic surfaces
Without losses the magnetization would persist indefinitely; under ordinary conditions the field lasts about 300 femtoseconds, which comes to roughly 20 cycles of the mid-infrared wave. Rawat reports that the localized free-carrier generation they used is material-agnostic and works on any non-metallic surface, the claim that carries weight for any prospect of putting it in a device. The Cornell team writes that the work could open new directions for spintronics, magnetic data storage, photonic and quantum computing, and precise control of magnetic environments. The research was supported by the University of Dayton Research Institute, the Office of Naval Research and the Army Research Office, and part of the computation used resources of the Cornell University Laboratory of Plasma Studies.[1], [2]
