Helical assemblies of gold nanoparticles.

The development of techniques for making helical and semiconductor metallic nanoand microstructures represents the extension of the works of Da Vinci and Archimedes with modern scientifi c tools. It brings forth interesting application concepts along with tough challenges in optics, biology, and mechanics. Making helical metallic objects with submicrometer pitch lengths comparable to the wavelengths of visible light represents an important technological niche for new optical materials. In this case these objects are expected to have complex 3D topologies and to be small enough to effectively interact with visible light. These features make them diffi cult to produce with traditional lithographic means. At the same time, they are large enough to exceed the capabilities of the synthetic nanotechnology. For instance, helical assemblies of metal nanoparticles (NPs) could be made by using DNA and similar templates, however they typically have the pitch length in range of few nanometers. [ 1–5 ] Other templates based on mostly oxide matrixes can give the helices with typical pitch in the range of tenth of nanometers but not in hundreds of nanometers. [ 6–8 ] Lithographic techniques are exceptionally convenient for fl at and layered structures but are complicated, expensive, and restrictive for objects with complex 3D topology. The preparation of metal helices with micrometer (and sometimes submicrometer) dimensions is possible, but they are very time-consuming and require unorthodox solutions. [ 9 ] A bottom-up approach for making helical metal systems with pitch lengths in 200–900 nm range would be most desirable from both fundamental and practical points of view. Besides geometrical elegance, helical structures from plasmonic materials, such as gold or silver, are important research targets because they provide a viable pathway to negative refractive index materials (NIMs). The ability of NIMs to refract light on the opposite side of the normal to the optical interface attracted a lot of attention due to the possibility