Zhang, W. et al. J. Opt. A 8, 878-890 (2006)

In recent times, a new type of chiral material has attracted the attention of the optics and materials communities. Planar chiral metamaterials (PCMs) are thin-film layers that can be engineered to manipulate the polarization of light. Until now, research into these materials has concentrated on metallic chiral films fabricated on substrates of silicon that are only transparent for wavelengths longer than 1.2 µm. Subsequently, studies of PCMs at visible wavelengths have been restricted to reflection measurements. This ultimately disables the chiral material’s opposite handedness — a property whereby opposite polarization changes are induced for light transmitted through the structure in opposite directions.

Zhang and co-workers at the University of Southampton have now moved away from metallic materials to develop the first all-dielectric PCM that is transparent at visible wavelengths1. The researchers etched a microscopic array of cross-shaped holes into a thin silicon nitride film deposited on a fused silica substrate. These structures were found to modify the polarization of light transmitted at a wavelength of 632 nm. The magnitude of this modification was controlled by fabricating structures with different degrees of chirality or different film thickness.

Previous studies of metallic structures have largely attributed the polarization changes to surface plasmons, induced currents or surface-plasmon polaritons. However, the observation of a similar phenomenon in a non-metallic material has led the team to suggest that Fabry–Perot cavity effects could have a much larger role to play. Such giant optical activities in dielectrics could make these planar chiral metamaterials particularly promising candidates for integration with detectors, for example, to develop solid-state polarization-sensitive devices.