Metamaterials are artificial structures exhibiting optical properties impossible to achieve with natural materials. Physicists from Fudan University, Shanghai1 now propose a metamaterial where optical properties can be tuned by a magnetic field.

Essential to the function of metamaterials are basic building blocks, which are much smaller than the wavelength of light. In that sense these tiny metallic structures in metamaterials are not unlike atoms in a crystal that determine its color even though we can’t see the atoms individually. Similarly, a light beam passing through the material will not notice the individual building blocks even though they determine the properties of the beam.

Fig. 1: Tunable refraction. A, a wedge-shaped metamaterial sample composed of thousands of ferrite rods shows negative refraction as both beams are on the same side with respect to the central axis perpendicular to the straight surface. B, if regular refraction occurs the beam is slightly deflected downwards. The refractive behavior can be changed from a, to b, through changes in magnetic field.

Metamaterials can show many interesting effects, including the cloaking of objects. Also, metamaterials can exhibit negative refraction. In normal refraction, light passing through a material gets refracted away from the axis perpendicular to the surface. In negative refraction, the light beam stays on the same side of the axis (Fig. 1). This effect can be used to make a perfect lens, which is different from regular lenses as it has no limits on the resolution that can be achieved.

One of the recently suggested designs for such negatively refractive metamaterials are periodic assemblies of a large number of metal-like pillars that scatter light in a way that leads to negative refraction. The researchers now suggest the use of ferrite yttrium-iron-garnet as the material for these pillars. Being magnetic, the magnetic properties of the ferrite are susceptible to external magnetic fields. Theoretical computations demonstrate that these changes also bring dramatic changes to key optical parameters.

Based on these findings the scientists designed a metamaterial structure composed of a regular array of more than 3600 rods. The resonance frequency of this metamaterial was strongly coupled to any changes in the magnetic properties of the ferrite, so that negative refraction could be switched on and off by application of a moderate magnetic field (Fig. 1).

However, this may be only the first step towards the active control of light propagation. Parameters such as the distance between the rods can be adjusted to provide a perfect match of properties for incoming light beams. “We are now attempting to design waveguides that work only in one way and show other effects such as wave focusing and perfect absorption,” says Shiyang Liu from the research team. It will be exciting to see the experimental realization of these new ideas.