Using an engineered metasurface that traps light, Cornell researchers have demonstrated a new way to generate strong static magnetic fields without using external magnets or magnetic materials – an approach that could advance spintronics, quantum and photonic computing, and data storage.
Illustration of a Cornell-developed metasurface designed to control infrared light. Laser pulses can selectively activate regions of the material, allowing researchers to dynamically tune its optical properties.
In an article published in Advanced Science, Shivaksh Rawat, a Ph.D. candidate working with Gennady Shvets, the J. Preston Levis Professor of Engineering in the School of Applied and Engineering Physics, and Samyobrata Mukherjee, a postdoc in the same group, described how a new technique for light manipulation – the so-called “time interface” – can be used to convert part of an optical wave’s energy into static magnetization. When a light wave experiences a spatial interface – for example, when it travels through air and then water – some portion of the light gets reflected off the surface while the rest is transmitted through it. Similarly, when a light wave experiences a time interface – a sudden change in the optical properties of the propagation medium, such as an increase or decrease in the refractive index – it also produces reflected and transmitted waves. However, a time interface can also excite a static zero-frequency mode of the system; in other words, stopping part of the light’s rapidly varying magnetic field and converting it into a stationary magnetic field pattern that remains in place instead of continuing to oscillate.To create a time interface, the researchers utilized a 2D metasurface – a carefully engineered rectangular array of germanium nanostructures designed for trapping mid-infrared light. The researchers wondered what would happen if the metasurface is illuminated with an intense, short burst of higher-energy (near-infrared) photons while the mid-infrared light is still inside the structure. Their modeling revealed that the near-infrared light released electrons from the germanium atoms, leaving “electron holes” and many free electrons. The rapid generation of electron-hole pairs by the near-infrared light can create a time-dependent change in the light’s refractive index that acts as a time interface for the trapped mid-infrared light.“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 non-metallic surface will work.”
