Swiss researchers claim to have created the world’s smallest integrated optical switch. Applying a small voltage causes an atom to relocate, turning the switch on or off.
The goal of the research was to reduce the size of modulators, which convert electrical signals into optical signals by switching a laser signal on or off at the frequency of the incoming electrical signals. A typical modulator measures a few centimetres across so they take up a great deal of space when used in large numbers.
The team, led by Jürg Leuthold, Professor of Photonics and Communications at ETH Zurich, had already successfully made a micromodulator just 10 microns across. They have now reduced that size by another factor of 1000 where it is operating at the atomic level
Leuthold says the modulator is actually significantly smaller than the wavelength of light used in the system. In telecommunications, optical signals are transmitted using laser light with a wavelength of 1.55 microns. Normally, an optical device cannot be smaller than the wavelength it should process.
“Until recently, even I thought it was impossible for us to undercut this limit,” said Leuthold.
However, by reconfiguring the construction of a modulator, the team were able to penetrate the order of magnitude of individual atoms, even though the researchers were using light with a standard wavelength.
The modulator they developed consists of two tiny pads, one made of silver and the other of platinum, on top of an optical waveguide made of silicon. The two pads are arranged alongside each other at a distance of just a few nanometres, with a small bulge on the silver pad protruding into the gap and almost touching the platinum pad.
The light entering from an optical fibre is guided to the entrance of the gap by the optical waveguide. Above the metallic surface, the light turns into a surface plasmon, which occurs when light transfers energy to electrons in the outermost 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, allowing 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.
If a voltage is applied to the silver pad, a single silver atom or, at most, a few silver atoms move towards the tip of the point and position themselves at the end of it. This creates a short circuit between the silver and platinum pads, so that electrical current flows between them. This closes the loophole for the plasmon; the switch flips and the state changes from on to off or vice versa. As soon as the voltage falls below a certain threshold again, a silver atom moves back. The gap opens, the plasmon flows, and the switch is on again.
Leuthold says more work still needs to be done to develop a commercially available solution, but he is confident they will be able to present a practicable solution within the next few years.
The set-up used to test the optical switches. Photo: Peter Rüegg/ETH Zurich