Episode 13: Computational model shows 3D metamaterial that exhibits magnetoelectric effect

In this podcast episode, MRS Bulletin’s Laura Leay interviews Alexandre Dmitriev from the University of Gothenburg, Sweden about his group’s computational model of a three-dimensional metamaterial exhibiting a magnetoelectric effect—known as the Tellegen effect—when exposed to light. The building blocks of the metamaterial are comprised of disks of silicon, 150 nm in diameter, supporting a cylinder of cobalt. Silicon is chosen for its high refractive index and cobalt for its magnetic properties. These building blocks are randomly distributed in a host medium such as water or a polymer. The metamaterial has applications in areas such as improving the efficiency of solar cells, creating one-way glass, or improving lasers. It also has the potential to revolutionize how the universe is understood and could hold the key to studying dark matter. This work was published in a recent issue of Nature Communications.

Transcript

LAURA LEAY: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on the hot topics in materials research. My name is Laura Leay. The study of condensed matter can yield insights into fundamental physics. Metamaterials, constructed using discrete, engineered building blocks can open up study of some interesting phenomena. One type of metamaterial has the potential to revolutionize how we understand the universe and could hold the key to studying dark matter. Professor Alexandre Dmitriev from the University of Gothenburg in Sweden has been studying metamaterials for decades. In collaborative work, his team has devised a computational model of a metamaterial that exhibits the Tellegen effect, also known as the magnetoelectric effect, when exposed to visible light.

ALEXANDRE DMITRIEV: What’s the magnetoelectric effect? The closest relative is of course chirality; when you think of matter that is, sort of, structurally non-symmetric but of course the time symmetry holds. You know, you go back and forth – it’s the same chiral effect that you experience when light is transmitted through the medium. So now imagine that you not only break the symmetry – the parity – but also the time symmetry: that is the reciprocity.  So you know, going here and going there creates a different effect. That actually is also close to what people experience when you look at these magneto-optical materials. It’s like basically, in isolation, the light passes through a medium, let’s say the polarization rotates a certain way but when it passes backwards the polarization rotates further; you do not cancel the effect but you make it stronger by going back and forth. That is the non-reciprocity in the transmission. So then imagine these magnetoelectrical materials, they combine both somehow. And that of course leads to some very cool and non-intuitive consequences.

LAURA LEAY: Prof. Dmitriev’s team built computational models of the metamaterial. The building blocks of the metamaterial are comprised of disks of silicon, 150 nanometers in diameter, supporting a cylinder of cobalt. These building blocks are randomly distributed in space. The size and properties of these nanocylinders is important for promoting the Tellegen effect. When silicon is in layers just a few hundred nanometers thick, light can effectively get trapped causing resonance. While silicon was chosen because of its high refractive index, cobalt was selected because it can spontaneously magnetize.

ALEXANDRE DMITRIEV: We pick up the electric component of light and then by manipulating it inside this unit we create a magnetic dipole which is also in-phase with the incoming electrical dipole. I mean, the whole concept of a Tellegen material as it was imagined by Bernard Tellegen – it’s actually an electrical dipole that is glued physically to a magnetic dipole – to a magnet. So there are two dipoles: they are co-linear and in-phase. So that is what we realized now for the visible light but of course you need this initial static magnetic moment. So it’s either you magnetize every other of these units – just going around and magnetize them one by one and you can do that with microwaves but what we wanted was a fully isotropic matter where everything is mixed up. It’s absolutely crucial to have that. We do not apply an external bias; we just make materials out of cobalt and then it spontaneously magnetizes.

LAURA LEAY: The structure is surprisingly simple but the challenge was in extracting the Tellegen parameter from the simulations. This dimensionless parameter describes the magnetoelectric effect which is a coupling between electric and magnetic fields. In initial simulations the Tellegen parameter was small but alterations to the structure led to an increase in its value, enough to give the proposed metamaterial very real applications once it is produced experimentally. The metamaterial has applications in areas such as improving the efficiency of solar cells, creating one-way glass, or improving lasers; but by developing a metamaterial with a design founded on fundamental physics, Prof. Dmitriev’s team will effectively be able to replicate astral bodies in the lab. 

ALEXANDRE DMITRIEV: The equations that describe Tellegen material are the same in quantum field theory that describe the axioms and dark matter. It’s a bit cumbersome to do experiments in space so lots of time people were trying to simulate what could happen there just on the optical table by making materials that would behave a bit similarly to astral bodies. Here, the mathematics and the physics holds. That was a prime motivator for having these Tellegen materials; because they are described by the same electromagnetic equations as the dark matter. So, it becomes straightforwardly available; experiments which are completely unthinkable in the realm of the real universe. 

LAURA LEAY: This work was published in a recent issue of Nature Communications. My name is Laura Leay from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on twitter, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.