The capability of surface plasmons to locally enhance electromagnetic fields on the surface of metal nanostructures has applications ranging from biosensing to photonic devices. However, the strong dependence of plasmonic properties on the smallest variations in nanostructure shape and form has made their systematic investigation difficult. Researchers from the Chinese University of Hong Kong and Peking University in Beijing have now developed a method that has made it possible to conduct detailed studies of plasmonic effects in gold nanoparticles1.

Plasmonic effects are being studied for the enhancement of light emission from the fluorescent dyes used to track certain organic molecules, such as proteins. However, as the optical fields created by surface plasmons are highly nonlinear, the fluorescence intensity from the dye molecules depends on parameters such as their distance to the metal surface. According to Jianfang Wang of the research team, this has made the systematic study of plasmonic systems difficult. “A fundamental understanding of these effects on fluorescent dyes has been lacking,” he says.

Fig. 1: Plasmon–dye interactions. Gold nanorods are coated with silica shells that contain fluorescent dye molecules. Surface plasmons are then excited in the nanorods using a laser beam. The surface plasmon resonance is strongest when the laser light is polarized parallel to the long axis of the nanorods (0°).

The researchers fabricated nanorods coated with a silica shell containing the dye molecules. Surface plasmons were then excited by a laser at a wavelength matching the plasmon frequency. The fluorescence emitted from the dye molecules then provided information about the plasmon resonance. In particular, a strong dependence on the polarization of the laser beam was observed (Fig. 1). The excitation of surface plasmons was strongest for laser beams polarized in the direction of the long axis of the nanorods.

While the present setup allows the surface plasmons to be coupled to dye molecules as a function of laser parameters such as wavelength and polarization, further studies are needed to clarify the strong spatial variations of the optical fields close to the nanorod surface. “We will have to be able to control the exact position of each dye molecule relative to the nanorod surface if we want to understand the relationship between the local field enhancement and the enhanced fluorescence,” says Wang. The realization of such precise control over plasmon–dye interaction on the scale of a few nanometers will lead to more reliable imaging and sensing applications.