Breaking Metamaterial Symmetry With Reflected Light

This gold metamaterial nanostructure exhibits large specular optical activity for oblique incidence illumination with light (Source: E. Plum et al)

This gold metamaterial nanostructure exhibits large specular optical activity for oblique incidence illumination with light (Source: E. Plum et al)

Optical activity as a rotation of the polari­zation of light is well known to occur within materials that differ from their mirror image. But what happens if this symmetry is broken by the direction of illumination rather than the material itself? Curiosity about this question has led to the discovery of a new type of optical activity. As a group of University of Southampton researchers reports about breaking the symmetry of meta­materials with reflected light. It will enable novel appli­cations because it causes optical activity of unpre­cedented magnitude – far exceeding previously known specular or mirror-like optical activity.

At the heart of the group’s work are meta­materials – materials constructed with unique shapes and symmetries that generate properties which don’t occur in their natural counter­parts. “Natural materials derive their properties from the atoms, ions, or molecules they consist of. Similarly, the basic concept behind meta­materials is to assemble artificial materials from meta­molecules, which are manmade elementary building blocks,” explained Eric Plum, a research lecturer at the University of Southampton’s Opto­electronics Research Centre and Centre for Photonic Meta­materials.

“This provides a huge techno­logical oppor­tunity,” Plum pointed out. “Instead of being limited by available natural materials, we can design materials with the properties we want. This has already led to the demon­stration of various enhanced and novel material properties and func­tionalities.” Meta­materials appear homogenous to electro­magnetic waves because their artificial structure is of sub wavelength size meta­materials for light are structured on the nanoscale, while those for microwaves are structured on the scale of millimeters or centimeters.

The group is interested in the chiral structures found within many natural and artificial materials because they come with the ability to rotate the polari­zation state of transmitted light. While the optical activity for light reflected by natural materials is negligible, the researchers found that the same isn’t at all true for meta­materials. “Our meta­material exhibits huge optical activity for reflected electro­magnetic waves,” Plum said. “This is particularly remarkable considering that our artificial structure is extremely thin, 30 times thinner than the wavelength of the electro­magnetic radiation it manipulates.”

Perhaps just as surprising, the optically active material involved isn’t actually chiral. “Instead, optical activity arises from a chiral experimental arrangement associated with the mutual orientation of the direction of the il­lumination and the structure of the metamaterial, which lacks two-fold rotational symmetry,” he elaborated.

The group’s discovery paves the way for “a whole new class of extremely thin and light devices for controlling and detecting the polari­zation of light, such as polari­zation rotating and circularly polarizing beam splitters and mirrors, as well as optical isolators for circularly polarized light,” Plum said.

In terms of more fundamental implications, the group’s observed effect mimics the longitudinal magneto-optical Kerr effect – in which the light reflected from a magnetized surface can change in both reflected intensity and polarity – without a magnetized medium. “This has significant implications for Kerr microscopy, because it could be mistaken for magnetization,” he added.

Plum and colleagues are now busy developing practical solutions to enable dynamic control of specular optical activity for appli­cations such as active polari­zation modulation. “It would also be interesting to study the effect in natural materials and to explore the conse­quences of similar types of symmetry breaking of other physical systems,” Plum said. (Source: AIP)

Reference: E. Plum et al.: Specular optical activity of chiral metasurfaces, Apl. Phys. Let. 108, 141905, DOI: 10.1063/1.4944775

Link: Nanophotonics & Metamaterials (N. Zheludev), University of Southampton, United Kingdom

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