Researchers in the UK, China and Germany have come up with a new and useful way to couple light to the surface of a metamaterial. The technique is the first to ensure that the coupling occurs in a single direction and could lead to integrated plasmonic circuits that could be controlled using an electric current.

The coupling involves surface plasmon polaritons (SPPs), which are particle-like quantum phenomena that arise from the interaction of light with a metal's conduction electrons. These quasiparticles are part light and part collective electron wave and are strongly confined at the surface of a metal. 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 cannot be controlled easily. This limits the range of applications possible for these structures.

Now, a team led by Shuang Zhang of the University of Birmingham 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, they used a "metasurface", which is a metal film with nanometre-sized rectangular holes carefully orientated in a certain way. 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 to right and vice versa.

SPP-excitation symmetry breaking

The tiny holes in 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 says.

"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 says.

Bright idea

Because the researchers can control the direction of the SPP propagation by simply 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," reveals team member Thomas Zentgraf of the University of Paderborn. "In this way, we should be able to design more complex plasmonic circuits with enhanced functionalities."

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