An optical metamaterial from Aalto University has the potential to enable applications that would otherwise need a strong external magnetic field to work. The 3D metamaterial demonstrates an isotropic and resonant nonreciprocal magnetoelectric (NME) response in the visible frequency range. The NME effect, also known as the Tellegen effect, has both electric and magnetic properties. In materials exhibiting the NME effect, magnetization can be induced by the electric component of light, and polarization can be generated by the magnetic component. The NME effect is negligible in natural materials. However, due to its technological potential, there have been various attempts by scientists to enhance the NME effect using metamaterials and metasurfaces. A 3D metamaterial developed at Aalto University demonstrates an isotropic and resonant nonreciprocal magnetoelectric (NME) response in the visible frequency range, enabling technologies such as true one-way glass. Courtesy of Ihar Faniayeu/University of Gothenburg. According to the researchers, glass that is currently marketed as one-way is actually semi-transparent. It is only when the brightness of the light differs between the two sides of the glass — for example, when the light is brighter outside a window than it is inside — that this semi-transparent glass acts like one-way glass. A true one-way glass based on the NME metamaterial would allow light to go through in one direction only, even when there is no difference in brightness between the two sides. “Just imagine having a window with that glass in your house, office, or car," researcher Shadi Safaei Jazi said. “Regardless of the brightness outside, people wouldn’t be able to see anything inside, while you would enjoy a perfect view from your window.” To form the NME metamaterial, the researchers embedded randomly oriented nanocylinders in a host medium such as water or a polymer. Each nanocylinder consisted of a ferromagnet in a single-domain magnetic state and a high-permittivity dielectric nanodisk that supported magnetic Mie-type resonance. The random orientation of the nanocylinders in the 3D metamaterial preserves the bulk NME effect, which is enhanced by the Lorenz-Mie-type resonances in the dielectric parts of the nanocylinders. The metamaterial operates on the spontaneous magnetization of the ferromagnetic nanocylinders — no external magnetic bias is required. The bias-free, optical NME metamaterial offers both qualitative advancements, such as tunability in space, time, and frequency, and quantitative advancements, such as resonant enhancement. These features could help materials scientists achieve magnetoelectric phenomena in real-world materials that could be used to build practical applications. “So far, the NME effect has not led to realistic industrial applications,” Jazi said. “Most of the proposed approaches would only work for microwaves and not visible light, and they also couldn’t be fabricated with available technology.” The optical NME metamaterial designed by the Aalto team can be created with existing technology and nanofabrication techniques. Using conventional materials, such as cobalt and silicon, the researchers achieved an improvement in the magnetoelectric effect of two orders of magnitude, compared to other known natural materials at room temperatures. The researchers further demonstrated that, by leveraging an emerging class of magnetic Weyl semimetals as the component of the meta-atoms in the metamaterial, they could enhance the magnetoelectric effect by almost four orders of magnitude compared to natural materials, and at the same time achieve an effective magnetoelectric response with values on par with those of the permittivity and permeability of the metamaterial. Such a massive magnetoelectric response, realized experimentally, could lead to development and applications that, so far, have been purely theoretical. One potential application, the researchers said, could be the development of a type of glass that lets light in one way only, regardless of the degree of brightness on either side of the glass. One-way glass made from an NME metamaterial could also improve the amount of energy that is captured by solar cells by blocking the thermal emissions that the cells radiate back to the sun. The research was published in Nature Communications (www.doi.org/10.1038/s41467-024-45225-y).
