Novel optical applications, such as perfect lenses, optical cloaking and quantum levitation, have fuelled a search for materials that exhibit magnetism at optical frequencies. However, fabricating the required nanostructures in three dimensions is a significant challenge. Almost all three-dimensional photonic metamaterials are currently made from stacks of separate, two-dimensional functional layers. Researchers from Universität Karlsruhe, Germany, have now identified how to create these nanostructures in three-dimensions [Rill et al., Nat. Mater. (2008) doi:10.1038/nmat2197].
Most two-dimensional photonic metamaterials are made using electron-beam lithography and the evaporation of metal films. One option for fabricating the structures in three-dimensions is to replace these two-dimensional technologies with two three-dimensional analogues - direct laser writing and chemical vapor deposition (CVP). To minimize losses at optical frequencies, the metal deposited by CVP would need to be Ag.
Michael S. Rill and colleagues believe that this approach could be used to create three-dimensional arrangements of magnetic split ring resonators (SRRs). As a first step towards this goal, they have demonstrated the technique's feasibility on a much simpler arrangement: a one-dimensional lattice of elongated SRRs – essentially, a corrugated surface.
The researchers made polymeric templates of the planar structure with varying groove heights (580 nm, 640 nm, and 740 nm). Each template was coated with a protective layer of glass (50nm thick), and then metallized with multiple Ag CVP cycles to produce an estimated Ag thickness of 50 nm.
Electron micrographs demonstrated the uniform nature of the Ag coating. Connectivity between all metal parts was confirmed, and optical characterization of the planar structure agreed well with theory. Retrieval of the effective optical parameters showed that the goal of fabricating a magnetic metamaterial had been accomplished.
The team says they now face a theoretical bottleneck owing to a lack of blueprints for three-dimensional metamaterials that are compatible with their approach to fabrication. “We hope that such theoretical progress will be stimulated by our work, as it is becoming increasingly clear that truly three-dimensional photonic metamaterials of the future will very likely not just be miniaturized versions of their microwave components,” they note.