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One-dimensional light on graphene

Marko Spasenovic

Nanostructured graphene illuminated with light holds potential for a wide range of applications in photonics and optoelectronics, including infrared and terahertz photodetectors, sensors, reflect arrays or modulators. Development of graphene nanopatterning technology has in recent years enabled the construction of such devices that hold promise for a quick transfer from scientific labs to the marketplace. Now scientists have carefully mapped, with nanoresolution, the structure of light on graphene nanoresonators, observing light that is confined to extremely small volumes at the edges of the nanostructures. The nearly 1D form of this light is expected to lead to novel device applications, for example to efficient control of quantum emitters, a sort of “bit” in future quantum computers.

Image: Plasmons confined to graphene nanoresonators (copyright Nature Publishing Group).

Starting from Graphenea's CVD graphene, researchers from several institutions in Spain used electron beam lithography do define circles and rectangles from graphene, with typical size of a few hundred nanometers. These graphene nanostructures are known to support tightly confined optical waves, known as surface plasmon polaritons, or simply plasmons. Plasmons occur when laser light is confined to the surface of a conductor. The confinement that the plasmons exhibit is useful for propagating light on a chip in optical computers, and for addressing tiny optical processors, such as for example quantum emitters. Graphene has shown to be an excellent support for surface plasmons, because as opposed to plasmons on metal films, plasmons on graphene can be tuned and switched with electronic gates. Tuning and switching are necessary prerequisites for optical computation.

To map out the plasmons, the researchers positioned the tip of a near-field microscope near the nanostructures. Near-field microscopes work by transferring light from a tiny tip, typically about ten nanometers wide at the apex, to the structure under study. In the reverse process, light that is already confined to the structure scatters from the tip into free space, allowing for detection and mapping of the confined waves. In this study, published in the journal Nature Photonics, the tip was used both for excitation and detection of graphene plasmons.

When they looked closely at the plasmon intensity maps, the scientists found two classes of plasmon modes: sheet and edge. Whereas the sheet modes propagate along the surface and are confined to two dimensions, the edge modes are tightly bound to the edges of the nanostructure and hence are practically one-dimensional. To be certain that they have correctly interpreted their results, the researchers compared the findings to numerical calculations, with which they showed excellent agreement. It was confirmed that edge plasmons have a shorter wavelength than their sheet counterparts, leading to much better confinement. The result is that the edge plasmons reside in a volume more than 100 million times smaller than the free space wavelength cubed, a result unprecedented in any material. The extreme confinement of plasmons in graphene brings within reach targeted addressing of single molecules or quantum dots on chip. It also opens new avenues in integrated nonlinear optics, an important area of science and technology.

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