Light through a blocked hole? Plasmonics is the answer

How would you react if a tiny hole in a piece of foil let through more light after you had covered it – or painted the foil a different colour?

With surprise, probably, like the physicists who discovered that this is just what happens with some very small holes. Both findings could lead to light-based transistors and other components for high-speed optical computers.

Conventional optics forbids light from passing through holes that are much smaller than its wavelength, which for visible light means less than around 400 nanometres wide. But in 1998, Thomas Ebbesen at the University of Strasbourg, France, reported that some wavelengths of visible light stream through holes in gold foil that are less than 300 nanometres wide.

It turns out that this is due to ripples known as plasmons that are found on the surface of metals and formed by the oscillation of electrons.

Total eclipse

If the frequency of light hitting the surface of a metal happens to match the oscillation of that metal's surface electrons, the plasmons grab the photons, guide them through the holes and release them on the other side. The plasmons on gold surfaces, for example, are particularly adept at interacting with visible light.

Now a team led by Hiromi Okamoto at the Institute for Molecular Science in Okazaki, Japan, have found another way to coax photons through tiny holes – paradoxically, by obscuring the hole with a gold disc.

The team was shining light down an optical fibre that tapered to a 100-nanometre-wide aperture. At first, barely any light made it through the aperture; instead, it was reflected back up the fibre. But when the researchers placed a small gold disc very close to the aperture, so that it completely eclipsed the hole without actually touching it, the light started streaming through (see graphic, right).

They suspect that plasmons from the gold disc are leaping up through the hole, grabbing the photons stuck inside the fibre and dragging them through. These photons then stream around the edges of the disc.

Dye enhancement

Okamoto's team found that if the disc touched the hole, the effect did not work; widening the disc, however, caused still more light to come through. "When we observed that the larger disc gives higher transmission, we were really surprised," Okamoto says.

This ability to open or block a hole to light could be useful when building components for optical computers, which transmit signals using light instead of electrons.

"The novelty is in controlling this transmission with various 'caps'," saysDmitry Skryabin, a nanophotonics researcher at Bath University in the UK.

Light transmission through a tiny hole can also be controlled with dyes, as Ebbesen and his colleague James Hutchison, also at the University of Strasbourg, recently found.

Plasmon passing

Normally, when white light is shone onto a piece of gold foil pierced with tiny holes, only the wavelengths of green light pass through. But Ebbesen and Hutchison found that coating the foil with a thin layer of green dye allowed red light to pass through as well; indeed, more red than green started to come through.

This was a shock, as green dye should absorb all light except green. "One certainly doesn't expect a sample to become transparent at the wavelengths where the molecule absorbs," says Hutchison.

The researchers suspect that the dye molecules absorb the red light but then "pass" it to the plasmons underneath the dye, which are not of the right frequency to interact with red light directly.

Hutchison says that holes painted with various dyes could also be useful in optical computing components.

Journal references: Nano Letters, DOI: 10.1021/nl103408h; Angewandte Chemie, DOI: 10.1002/anie.201006019

Source:http://www.newscientist.com/article/dn20116-light-through-a-blocked-hole-plasmonics-is-the-answer.html?DCMP=OTC-rss&nsref=online-news