A deeper understanding of light propagation along noble metal nanostructures would enable the use of such structures in integrated nanoscale photonic devices.

Padmanabhan Pramod and George Thomas from the National Institute for Interdisciplinary Science and Technology in Trivandrum, India1 have now experimentally shown that the angle between the nanorods in noble metal waveguides has a significant influence on propagation properties.

Light propagation along the surface of metals is based on plasmons—collective oscillations of the electrons that carry the electromagnetic waves. Contrary to the limitations of classical optics, plasmonic structures allow propagation at length scales that are much shorter than the wavelength of light, making plasmonic structures an ideal candidate for photonic devices on the nanoscale.

Fig. 1: Microscopy image of gold nanorod dimers.

The researchers studied the plasmonic properties of gold nanorod dimers, where two nanorods were linked together through small linker molecules (Fig. 1). Such nanorods can be synthesized through bottom-up techniques and stabilized in water. Their integration into the final device structure can then be guided, for example through electrostatic techniques. “Our work focuses on the design of Au nanorod dimers and the study of how the plasmon coupling across the dimers depends on their orientation,” says Thomas.

The propagation of the surface plasmons across the dimers depends strongly on the orientation of the nanorods, as different orientations alter the overlap in electromagnetic fields at the gap between the nanorods. In order to study this dependency in detail, different linker molecules were used to bind the nanorods together. Rigid linker molecules tend to straighten the dimers, whereas more flexible linkers tend to lead to right angles between the nanorods.

For rigidly linked dimers, the plasmon coupling along the nanorods is strong and the plasmon resonance was determined to be at 840 nm. In flexibly linked dimers on the other hand the plasmonic coupling across the nanorods is weak and the resonance occurred at 732 nm.

In the future, “gold nanorod chains could be used as plasmonic waveguides where the transmission of light waves in principle can be controlled through the alignment of the nanorods,” says Thomas.