Interaction between light and matter can induce collective electromagnetic excitations propagating along the surface of metals— so-called surface plasmons. Researchers from Sungkyunkwan University have now demonstrated that surface plasmons can even propagate across short sections of an otherwise inactive metal. This finding suggests an enhanced versatility in the design of surface-plasmon-based devices with novel functionalities, for example as biosensors.

Plasmons are collective oscillations of the electrons in a material. Similar to water waves, they are particularly strong on the surface of a metal, where they can be excited by light of the appropriate frequency. These surface plasmons are of interest, as not only can they be used as nanoantennas to guide electromagnetic energy, but they also show a strong response to local electromagnetic fields, which can be used in sensing applications.

Fig. 1: Surface plasmons on gold nanostructures. The optical response (red curve) of gold nanostructures interrupted by layers of nickel (green) show almost the same response as pure gold structures of the same length, demonstrating a coupling of surface plasmons across the otherwise inactive nickel layers. The inset shows a microscope image of the actual materials.

Here, Sungho Park and colleagues now demonstrate that the propagation of surface plasmons across one-dimensional nanostructures proceeds even if the structure is interrupted by an inactive material.1 The researchers use one-dimensional gold nanostructures with lengths between 180 and 800 nm. At certain positions, a slice of nickel is included in the structure (Fig. 1). Importantly, the surface plasmons of nickel have a different frequency to those of gold. Therefore, the nickel layer should inhibit the propagation of the gold plasmons across the structure. However, the authors observe an optical response equivalent to the length of the whole structure, in contrast to the response that would be expected if the smaller gold sections were independent of each other. “These results show that optically less active materials do participate in the surface plasmon coupling of the active materials,” says Park.

These findings are of relevance in the design of enhanced surface-plasmon-based devices. Park suggests that “such composite structures may be used for applications such as chemical and biological sensors. For example, simple changes in the pH value may induce etching of the Ni blocks, which in turn affects the optical response of the structure.”

Indeed, advanced structures are expected to show an even more complex coupling behaviour, and may therefore lead to sophisticated sensing applications.