Digital Plasmonics

Bergin Gjonaj

Authors:
Bergin Gjonaj¹, Jochen Aulbach¹, Patrick M. Johnson¹, L. Kuipers¹, Ad Lagendijk¹ & Allard P. Mosk<sup>2

1</sup>Center for Nanophotonics, FOM Institute AMOLF, Amsterdam, The Netherlands

²Complex Photonic Systems, MESA+ Institute for Nanotechnology, University of Twente, The Netherlands

Researchers from the FOM Institute AMOLF in Amsterdam and the University of Twente have developed a way to digitally control tiny electrical waves, so-called surface plasmons. These waves play an important role in nanophotonic research, which studies materials at a very small scale. The new knowledge could lead to an interface that makes nanophotonic devices more user-friendly (comparable to Windows on a computer). The advance online publication of their work appeared in the leading journal Nature Photonics on May 22nd.

When light reflects off gold nanostructures under the right conditions, it creates tiny electrical waves on the surface called surface plasmon polaritons (SPPs). The researchers have now achieved SPP control in a remarkably simple and universal manner. They did this by changing the shape of the incident light with a computer-controlled megapixel device called a phase plate (similar to a liquid crystal display).

Figure 1. Controlling the surface plasmons via a pixilated phase plate device. Light from a laser impinges on the phase plate. Each pixel of the device can be programmed to alter the amplitude and phase of the outgoing light. Light from each pixel will later be directed toward the sample surface using a lens system, as shown in figure for three different pixels. The sample is a nanohole grating engraved on a very thin gold film. When light from a pixel of the phase plates impinges on the sample it generates surface plasmon waves. These tiny waves propagate along the surface as indicated by the blue arrows. By proper tuning of the phase plate (pixel by pixel) it is possible to control the blue arrows.

Figure 2. Flexible control of the plasmonic waves using a pixelated phase plate. (a) Light from each pixel of the phase plate acts as a source of plasmons on the gold film. The computer tunes the relative phases of the pixels, and thus of the plasmonic sources, to achieve constructive interference at a chosen spot. This creates a sharp plasmonic focus. (b) Relocation of the focus to a newly chosen spot. The position of the plasmonic focus is fully selectable from the computer.

Unlike previous approaches, which rely on fixed prefabricated surface structures to control SPPs, phase plate control is cheap and highly flexible. The researchers demonstrated this flexibility by creating a sharply focused SPP spot and scanning it across a gold surface. Such a scanned focused spot could be used to create the first super-resolution SPP microscope. In addition, the method could result in new interfaces for nanophotonic devices. This would make them more accessible for industry.

Reference:
[1] Bergin Gjonaj, Jochen Aulbach, Patrick M. Johnson, Allard P. Mosk, L. Kuipers & Ad Lagendijk, "Active spatial control of plasmonic fields,” Nature Photonics, 6, 360 (May, 2011). Abstract.