Golden taper funnels light beam onto nanowire
Researchers at the FOM Institute for Atomic and Molecular Physics (AMOLF) made light waves propagate along extremely small wires. Their experiments show how a ‘funnel’ made of gold helps to compress a light beam to make it fit onto a gold nanowire with a diameter as small as 60 nanometres (0,00006 mm). The demonstrated nanoconcentration of light is possible by making clever use of surface plasmons, which are light waves trapped on the gold surface. The taper adiabatically transforms a surface plasmon mode to the specific mode that is able to propagate along the nanowire. Because the effect turns out to be very efficient, the research forms an important step in the development of ultra small light circuits, comparable with electrical circuits on computer chips. Moreover, this strong light concentration could also help to make solar cells more efficient. This week, the AMOLF researchers publish their findings in the prestigious journal Physical Review Letters (May 22, 2009).
Reference:
Nanowire plasmon excitation by adiabatic mode transformation
Physical Review Letters
Ewold Verhagen, Marko Spasenović, Albert Polman and Kobus Kuipers.
For more information, please contact Ewold Verhagen, AMOLF,
Phone (020) 608 1234
Figure 1. A golden taper squeezes a surface plasmon wave onto a nanowire. In this microscopy image, the plasmon wave propagates from the upper left to the lower right. At the end of the nanowire, the wave continues to travel in a second taper. The different colours depict opposite directions of the measured electrical field of the wave. The inset is an image of the structure, made with an electron microscope.
Figure 2. Calculation of the plasmon waves on a taper. The calculations show that a specific ‘large’ plasmon wave can be transformed by the taper into the compressed plasmon wave on the nanowire. The plasmons on the nanowire are guided within a much smaller cross sectional area than a usual light beam. The vertical axis in the graph is inversely proportional to the velocity of the plasmons, compared to that of light (the dotted line). For small widths the plasmon velocity on the nanowire strongly deviates from that of light. The wave that has to be supplied at the end of the funnel is easily excited on the interface between the gold strip and the glass substrate.
Figure 3. Excitation of plasmons on a nanowire. The used structure fabricated in a thin layer of gold on glass (left) and the measured plasmon strength in that structure (right). The plasmons are first excited by a light beam impinging on a grid of holes in the gold (at the top of the figure). They then concentrate in the taper, and propagate further along the nanowire.