Nano hole scatters captured light wave
The interaction of light that is 'stuck' to the surface of a metal – such as a type of light wave known as surface plasmon polaritons – with holes that are much smaller than its wavelength can lead to amazing applications such as invisibility cloaks or extraordinary transmission based sensors. What makes these waves special is that they arise due to the coupling of the motion of electrons in a metal with light, and hence they are confined to the surface of a metal, usually to heights of just millionths of a millimeter. And while many of the consequences of the interaction of the waves with arrangements of sub-wavelength holes are well known, knowledge of the fundamental physics of a surface wave interacting with an individual hole is lacking.
Now, researchers at AMOLF have overcome the difficulties inherent in studying light-matter interactions just nanometers above a surface, and have imaged the scattering of surface waves from single sub-wavelength holes in real-time. They used a state-of-the-art near-field optical microscope that is capable of capturing a small amount of the light that would otherwise have been trapped on the metal’s surface. From these images they were able to determine when, how much, and in which direction light flowed as it scattered from the holes. Interestingly, it was found that the scattered wave could be significantly delayed with respect to the unscattered wave. The results of their research are recently published in Physical Review Letters.
To explain this behavior the researchers modeled the interaction of the electrons in the metal surrounding the hole, with the impinging light. It was found that the uniform radiation of light in all directions away from the hole, as was observed in the experiment, arose when the incident light caused the electrons to oscillate out of the plane of the film. Likewise, this model predicts how much of the light will remain stuck to the film, and how much will either flow through the hole or radiate away from the surface.
This research provides a first glimpse of these nanoscale interactions and paves the way towards the intelligent design of arrangement of sub-wavelength features. Correctly done, this optical control can result in the focusing of electromagnetic energy to highly confined volumes. Hence this research has important implications to fields such as imaging and sensing, where one is often interested in detecting very small objects such as cells, bacteria, or even single molecules.
Reference
N. Rotenberg, M. Spasenovic, T. L. Krijger, B. le Feber, F. J. Garcia de Abajo, and L. Kuipers, Plasmon scattering from single sub-wavelength holes, Phys. Rev. Lett. 108, 127402 (2012)
Contact
Prof.dr. Kobus Kuipers 020-7547100