Iceland, Cologne: Advance in plasmonics brings practical applications closer

Research lead

Researchers at the University of Iceland, University of Cologne and the Fraunhofer Institute Jena have demonstrated net optical amplification in a plasmonic waveguide, opening up the ability to make light travel over sizable distances when it is bound in a plasmonic mode.

Achieving such a macroscopic propagation of surface plasma waves is critical for many applications of emerging plasmonics technology, which range from compact communication devices and optical computing, to the detection and characterisation of cells, virus particles and single molecules.

Research on plasmonics, a relatively new branch of optics, has received an increasing level of attention over the last decade, driven mainly by the fact that surface plasmons, travelling along the interface between a metal and a dielectric, make it possible to confine optical energy to volumes significantly smaller than with conventional dielectric waveguiding structures such as optical fibres.

Such tightly focused optical energy can be used as a ‘nano-probe’ for making measurements in fields including solid-state physics, chemistry and life sciences. The tight confinement of the optical field also promises to lay the foundation for optical devices with reduced dimensions.

Under normal circumstances, optical energy travels over very short distances in plasmonic waveguides, before it is absorbed due to ohmic loss in the metal. Although clever design can somewhat increase the useful length of plasmonic waveguides, it is widely accepted that the only way to completely overcome this problem is to add a mechanism that continuously amplifies the light as it travels along the plasmonic waveguide.

The researchers have now developed a structure that provides sufficient amplification to overcome the intrinsic absorption of a plasmonic waveguide. The optical amplification is sufficient to provide a net gain of the plasmon-bound light as it travels along the waveguide. The researchers used a structure consisting of an ultra-thin gold film embedded in a highly fluorescent polymer, optically pumped by an ultrafast laser source. The structure is designed to channel the light generated by the fluorescent polymer to the plasmonic waveguide. As the plasmonic wave travels along the waveguide, its intensity is increased by stimulated emission of the optical energy stored in the fluorescent polymer.

“For many years the propagation loss issue in plasmonic waveguides has been a major hurdle for the development of devices that make use of surface plasmon effects,” says one of the researchers, Klaus Meerholz. The key to the success was finding a way to embed the plasmonic waveguides into an amplifying fluorescent polymer without affecting the properties of the waveguide.