One-dimensional metallic structures are ideal as basic components in nanoscale optical devices that overcome the diffraction limit of light. This is because they can confine light to subwavelength dimensions in the form of surface plasmon polaritons (SPPS). SPPs are collective oscillations of conduction band electrons at the surface of metals. Metallic nanowires and stripes that are less than 200 nm in diameter are particularly interesting for subwavelength waveguiding and for various plasmonics applications, like optical interconnects and routers in plasmonic circuits, as plasmonic logic gates and as Fabry-Perot resonators.

The problem, however, is that optical frequency SPPs do not travel very far – at best several tens of microns – in metallic nanowires because they are scattered or lost as wasted heat. Researchers would like to better understand these losses and quantify them to develop low-loss nanoscale devices based on these nanostructures.

"Collective modes" depend on symmetry of nanowire cross-sectional shape

A team led by Eugene Zubarev, Peter Nordlander and Stephan Link at Rice has now studied how SPPs propagate in gold nanowires that have pentagonal and five-point star-shaped cross sections. Although the diameter of a nanowire is important, its cross sectional shape matters too, says Link. SPPs are localized at the sides and corners of nanowires with larger diameters and cross-sectional shapes, but as the diameter of the wires becomes smaller, side modes disappear and corner modes begin to “hybridize” into so-called collective modes that depend on the symmetry of a nanowire’s cross sectional shape.

The researchers looked at how the SPPs moved in gold nanowires using a far-field fluorescence propagation technique called bleach-imaged plasmon propagation. They began by illuminating one end of a nanowire – covered with a fluorescent dye – using a 785 nm laser. Over time, propagating SPPs bleach the dye, leaving a permanent map of the SPP near field that is proportional to the amount of bleached (that is, lost) florescence.

Comparing experiments and models

The Rice scientists then compared their experimental observations with electromagnetic simulations in which they modelled nanowires with star-shaped and pentagonal-shaped cross sections. The models assumed that the nanowires were in the same dielectric environment as those employed in the experiments. “We also used the finite difference time domain (FDTD) method to solve Maxwell’s equations to simulate SPP propagation in the nanowires,” explained Link. Maxwell’s equations are a group of partial differential equations that have been used as the basis for developing virtually all modern electrical circuits and optics devices.

The team found that SPPs appear to travel shorter distances in metallic nanowires with a five-point star-shaped cross section than in nanowires with a pentagon-shaped cross section. The shorter propagation length is related to the sharp ridges on these nanowires, says Link.

“Our result provides new insights into the different SPP loss mechanisms in metallic nanowires with different tip shapes,” he told nanotechweb.org. “This finding may help us design better nano-optics and plasmonics devices and, in particular, shows how we can create lower-loss plasmonic nanowires by tuning their cross sectional shape.”

The team is now planning to look at other ways of compensating SPP losses by incorporating a gain medium, pumped with a second laser, into the nanowires.

The current work is reported in ACS Nano DOI: 10.1021/nn405183r.

Further reading

A ‘Fano switch’ for colour displays (Sep 2012)
Test structure highlights SPP response (Feb 2010)
Short- and long-range SPP waveguides detect xylene (May 2013)