Mussel power.

nature materials | VOL 7 | JANUARY 2008 | www.nature.com/naturematerials over them to make the structure planar and to separate the layers. These steps are repeated as many times as the number of desired layers. However, many challenges need to be overcome before some of the spectacular applications of metamaterials demonstrated at microwave frequencies, such as perfect lensing and optical cloaking, are realized. Furthermore, it will be a challenge to extend the optical resonance of these structures from the present infrared region into nearinfrared and visible parts of the spectrum. Simply scaling down the size of SRRs and therefore reducing the resonance wavelength does not seem to be viable from both fundamental and fabrication standpoints and, therefore, new geometries may be needed. Another challenge is to combine the electric and magnetic responses in such a way that negative ε and μ are obtained at the same optical frequency range, resulting in a true negative-index material. Although some two-layer structures in the near-infrared indeed have such NIM properties7, they tend not to show strong metamaterials properties and are difficult to fabricate in more than two layers. The fabrication technique developed by Na Liu and colleagues offers the freedom to fabricate multilayer metamaterials that, for example, could be used to construct some of the recently designed subwavelength NIM structures9. Regardless of what shape the future designs of nanoengineered metamaterials are going to take, this powerful fabrication technique is likely to influence the way three-dimensional metamaterials are going to be produced.