Single atom optical modulator

Swiss researchers have made an electro-optic modulator from a single atom.

EHT Zurich single atom light switch

Plates of silver (pink) and platinum (mint) almost meet over an optical waveguide (blue). Single silver atoms can bridge the gap

The modulator is significantly smaller than the wavelength of light used in the system, and under most circumstances an optical device can not be smaller than the wavelength it needs to process. “Until recently, even I thought it was impossible for us to undercut this limit,” said Professor Jürg Leuthold of university ETH Zurich.

And he should know, because six months ago his group made a modulator 10um across, and the new one is smaller. “The footprint has been reduced by a factor of 1,000 if you include the switch together with the light guides. However, the switch itself is even smaller, with a size measured on the atomic scale,” said the university.

The trick to sub-wavelength switching is to use surface plasmons – the reversible process where photons become patterns of charge on a metal surface. This can allow photons to ‘pass’ through holes smaller than a wavelength.

Made by scientist Alexandros Emboras, the modulator consists of two pads, one silver and the other platinum, on top of a silicon optical waveguide (see diagram).

The two pads are alongside each other separated by a few nanometres, with a bulge on the silver pad protruding into the gap and almost touching the platinum pad.

Light entering from an optical fibre is guided to the gap by the optical waveguide, and above the metallic surface the light turns into a surface plasmon as its energy transfers energy to electrons in the outer atomic layer of the metal surface causing the electrons to oscillate at the frequency of the incident light.

“These electron oscillations have a far smaller diameter than the ray of light itself. This allows them to enter the gap and pass through the bottleneck. On the other side of the gap, the electron oscillations can be converted back into optical signals,” said the university.

When a voltage is applied to the silver pad, one or a few silver atoms move to the bulge tip and short to the platinum allowing current to flow between them and closes the plasmon path.

When the voltage is reduced below a threshold, the circuit is broken as the silver atom moves from the gap – a process which can be repeated millions of times, according to ETH.

Simulation by ETH Professor Mathieu Luisier confirms the short circuit at the silver point is brought about by a single atom.

The plasmon has no other options than to pass through the bottleneck either completely or not at all, creating a digital switch, said Leuthold. “We have been looking for a solution like this for a long time.” He and his team are aiming to produce a switch within the next few years that could be made commercially.

The single atom switch works at room temperature, and can switch at frequencies “in the megahertz range or below”, said ETH. “The researchers want to fine-tune it for frequencies in the gigahertz to terahertz range.”

The lithography method used to construct the switch was developed by Emboras from scratch, and succeeds in making a switch one in six attempts at the moment.

The work is published in Nano Letters as ‘Atomic Scale Plasmonic Switch‘.

Diagram courtesy of Alexandros Emboras, ETH Zurich

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