If you had asked me about metamaterials a few months ago, the range of available answers would have been limited: option 1 – admitting my sheer ignorance, option 2 – trying to make a joke about how the minds behind ‘Star Trek’ knew about these structures before the scientists who study them now. Then I stumbled upon an article published on the Physical Review Letters, and a whole new world at the nanoscale opened up…
Nanostructures interacting with electromagnetic radiation are a promising platform for a wide range of applications, from controlled drug delivery to integrated photonics. Particularly, the ability to design ‘optical shields’ that absorb and scatter light is an open research area.
Ali Mirzaei and collaborators from the Australian National University have devised a strategy to arrange specifically designed nanowires into arrays capable of shielding a given region of space from electromagnetic radiation with frequencies across the visible range. The peculiarity of these nanostructures, termed optical metacages, lies in the large gaps (comparable to the radii of the individual structures) between neighboring nanowires. This flexible arrangement allows the shields to take on a variety of geometrical shapes – even that of Australia. In fact, the spacing between the ‘bars’ of the optical cages is a key parameter: separations smaller than the radii of the nanowires can increase the bandwidth of the shielding effect, while larger gaps are useful if the requirement is to block out electromagnetic radiation but let in liquids or gases. The choice of materials for the nanowires is also crucial, with multilayer nanostructures giving access to wider spectral ranges.
A semianalytical study of the interaction between an incoming radiation and a single nanowire identified the presence of boundaries, called separatrices, between the region where the metamaterial absorbs the incident light and the area where the radiation propagates around the nanowire. By overlapping the separatrices in an array of nanowires, the researchers were able to prevent light from penetrating through the structure. Depending on the design parameters of the metacages, numerical simulations suggested that these can be reshaped into various geometries and exhibit additional properties such as suppressed backward scattering.
Phys. Rev. Lett. 115, 215501 (2015)