Designer lights from the physics lab
With the molecular “origami”technique, the researchers can program the basic structure of their material at will by taking advantage of the structure of DNA. DNA molecules are built up of four types of subunits (A, C, G, T), which can pair with each other (A with T and G with C). By mixing DNA strands that pair with one another at specific sites, one can produce three-dimensional shapes of any required form. Liedl and colleagues formed an 85 nm long cylinder containing localized binding sites for 10-nanometer gold particles, on which the particles can be arrayed like beads on a string that wraps helically around the cylinder.
“The precision and yield of the expected structure is extremely high, and the degree of control achieved is greater than in any previous attempt to pattern metallic nanoparticles in a defined geometry,” says coauthor Professor Friedrich Simmel, a physicist at the Technical University of Munich. By varying the size, disposition and composition of the nanoparticles or altering other features of the water-soluble structures, the investigators can customize the impact of the material on light rays passing through it.
For instance, by arranging the gold particles in right or left-handed helices, the properties of the transmitted light are altered accordingly. The magnitude of the optical response was found to be markedly dependent on the size of the particles, and their precise chemical nature also has a significant influence on how they interact with incident light. Thus, if the gold particles are coated with silver, the position of the optical resonance is shifted from the red to the blue range of the spectrum.
The phenomenon of circular dichroism can be used to characterize the optical activity of materials. It is measured using two light beams of defined wavelength which are circularly polarized in opposite senses and modulated in different ways by passage through the material. When the response of a given sample of the metamaterial was determined at various wavelengths, the results were found to agree with calculations based on a theoretical model. Hence, the model can be used to design materials that modulate light in specific ways. “We will now test whether we can also alter the refractive index of our materials,” says Liedl. “Materials with a negative index of refraction could, for instance, serve as the basis for novel optical lens systems.”(NIM/suwe)
DNA-based Self-Assembly of Chiral Plasmonic Nanostructures with Tailored Optical Response.
Anton Kuzyk, Robert Schreiber, Zhiyuan Fan, Günther Pardatscher, Eva-Maria Roller, Alexander Högele, Friedrich C. Simmel, Alexander O. Govorov und Tim Liedl.
Nature Volume 482, 7389, pp 311-314 DOI: 10.1038/nature10889
Prof. Tim Liedl
Department of Physics, LMU Munich
Phone: +49 89 2180 3725
Fax: +49 89 2180 3182