ENGINEERING & NANOTECHNOLOGY
Photonics Golden atoms
Similarities between the electronic states of molecules and the optical properties of gold nanostructures aid the development of new photonic devices
Surface plasmon resonance — the collective vibration of electrons on the surface of metallic nanostructures in response to excitation with light or charge — has recently gained much attention from the scientific community because of its wide range of possible applications, particularly in photonics. Mohsen Rahmani and co-workers from the A*STAR Data Storage Institute1
, working in collaboration with the
Department of Electrical and Computer Engineering, National University of Singapore, have now expanded the potential uses of this phenomenon with their discovery that the surface plasmonics of assemblies of nanoparticles closely resembles the energetic interactions among atoms in two-dimensional molecules. The resonance properties of surface plasmons are determined
by the precise makeup up of the nanostructure, including the metal, the substrate and even the shape of the structure itself. Interactions among nanostructures when brought close
together can also significantly alter the plasmon resonance of the system. This approach has been studied for application in the design of very sharp plasmon resonances that are highly sensitive to the external environment, with uses, for example, in gas sensing. The researchers studied complex two-dimensional designs
based on assemblies of four gold nanodiscs (see image). With the gap between discs set at just 18 nanometers, strong interac- tions occurred among the plasmonic modes of the individual discs. Conceptually, this interaction among the optical states of the gold nanodiscs is similar to the electronic interaction between atoms in a molecule. “The trigonal planar molecules mimicked in our work
are among the few naturally occurring planar molecules,” comments Rahmani. As in a molecule, the interactions among nanodiscs in a plas-
monic system lead to multiple plasmon resonance peaks instead of the sole peak produced by a single isolated disc. “Based on such direct analogies, plasmonic nanostructures could therefore be a convenient tool to study the properties of more complex molecules,” explains Rahmani. In the future, the researchers
84
200 nm
Plasmonic model systems. Arrangements of four gold nanodiscs into a trigonal structure resemble the structure of planar molecules, so that the optical properties of these devices are in close analogy to the electronic states of actual molecules.
plan to design and fabricate three-dimensional structures, which would allow them to study a wider range of molecules. Such studies could lead to a better understanding of molecular orbital theory in planar trigonal molecules, and of the behavior of carbon atoms within graphene sheets. The analogy between molecules and plasmonic structures could also be used to advance the development of a number of photonic devices. “The analogy could benefit applications in nanolithography,
optical switching and nonlinear spectroscopy,” says Rahmani. Given the vast number of molecules available as blueprints
for the design of specific properties, the potential applica- tions of such plasmonic systems are expected to be numerous and far-reaching.
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1. Rahmani, M. et al. Influence of plasmon destructive interferences on optical properties of gold planar quadrumers. Nanotechnology 22, 245204 (2011).
A*STAR RESEARCH OCTOBER 2011– MARCH 2012
© 2011 IOP
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