Surface plasmon polaritons (SPPs) are quasiparticles and are quantum phenomena that arise from the interaction of light with a metal's conduction electrons. Plasmon polaritons are part light and part collective electron wave and are strongly confined at the surface of a metal. The SPPs are excited when they interact with light, but the problem is that they are then free to propagate in many different directions along the metal surface – and either forwards or backwards – and cannot be easily controlled. This limits the range of applications possible for these structures.

Now, a team led by Shuang Zhang of the University of Birmingham in the UK has shown that SPPs can indeed be excited along a single direction on a metal surface provided that the surface is suitably structured first – in this case, with nanoscale rectangular holes carefully orientated in a certain way. The metal film is then known as a metasurface. More importantly, and for the first time, the researchers have confirmed that the direction in which the SPPs travel along the metal surface can be switched by simply flipping the helicity (or circular polarization direction) of the incoming light – from left circularly polarized to right circularly polarized, and vice versa.

SPP excitation symmetry breaking

The nanoapertures on the metal surface locally excite the SPPs with a certain phase delay, explains Zhang, and this delay depends on the orientation of the apertures. “When the apertures were pointing in a certain direction, we found that we could create a phase gradient for circularly polarized light incident on the metal surface,” he told nanotechweb.org. “This phase gradient breaks the symmetry of the SPPs’ excitation along two opposite directions, which meant that we could then excite the SPPs along a single direction at a specific wavelength.

“More interestingly still, we found that we could reverse the phase gradient as we flipped the circular polarization state of the input light, and thus reverse the direction in which the SPPs propagated,” he said.

Because the researchers could simply control the direction of the SPP propagation by varying the polarization state of the incident light, they had the bright idea of dotting the metal surface with so-called polarization modulators to construct a compact, electrically controllable plasmonic circuit. Happily, the polarization state of light can easily be controlled using well established electro-optical techniques.

The team would now like to optimize the structures it has made and improve coupling between the SPPs and incoming light. “We will also look at how we can electrically control SPP excitation by incorporating liquid crystal devices onto our metasurface,” revealed team member Thomas Zentgraf of Paderborn University in Germany. “In this way, we should be able to design more complex plasmonic circuits with enhanced functionalities.”

Such circuits could find use in a wide range of application areas, from nanophotonics to biosensing.

The current work is described in Light: Science and Applications.