Optical Nanomodulator Produces Truly Digital Signals

As data rates and volumes continue to rise, an optical modulator that operates at the individual-atom level could both reduce the size of and increase the efficiency of communication networks.

The modulator is based on the voltage-induced displacement of one or more silver atoms in the narrow gap between a silver and a platinum plate. Courtesy of Alexandros Emboras/ETH Zurich.

Within these networks, components called modulators convert electrical signals into optical signals; effectively, these are fast electrical switches that turn a laser signal on or off at the frequency of the incoming electrical signal. Though they measure just a few centimeters across, data centers can contain thousands of modulators, resulting in a relatively large collective footprint.

Researchers from the Swiss Federal Institute of Technology in Zurich (ETH Zurich) previously demonstrated a 10-μm micromodulator, which they said was 10,000 times smaller than commercially available modulators. Now the team has developed an optical modulator with a footprint reduced in size by a factor of 1,000 including switches and lightguides. The switch itself measures on the atomic scale, the researchers said.

The modulator is 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 μm, and normally, an optical device cannot be smaller than the wavelength it’s intended to process.

The Zurich team’s reconfigured construction technique made it possible made to penetrate the order of magnitude of individual atoms. The modulator, constructed by researcher Alexandros Emboras, consisted 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 were arranged alongside each other at a distance of just a few nanometers, with a small bulge on the silver pad protruding into the gap and almost touching the platinum pad.

Light entering the modulator from an optical fiber was guided to the entrance of the gap by the optical waveguide. Above the metallic surface, the light turned into a surface plasmon, which occurs 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 had a far smaller diameter than the ray of light itself, which allowed them to enter the gap and pass through the bottleneck. On the other side of the gap, the electron oscillations were be converted back into optical signals.

When voltage was applied to the silver pad, a single silver atom or, at most, a few silver atoms moved towards the tip of the point and positioned themselves at the end of it, creating a short circuit between the silver and platinum pads, so that electrical current flowed between them. This closed the loophole for the plasmon; the switch fliped and the state changed from "on" to "off," or vice versa. As soon as the voltage fell below a certain threshold again, a silver atom moved back. The gap opened, the plasmon flowed, and the switch was "on" again. The process could be repeated millions of times.

As the plasmon had no other option than to pass through the bottleneck either completely or not at all, it produced a truly digital signal: a one or a zero.

"This allows us to create a digital switch, as with a transistor. We have been looking for a solution like this for a long time," researcher Jü Leuthold.

To confirm that the short circuit at the tip of the silver point was brought about by a single atom, the researchers simulated the system using a high-performance computer.

The modulator is not ready for series production due to its speed of operation. So far it only works for switching frequencies in the megahertz range or below. The ETH researchers want to fine-tune it for frequencies in the gigahertz to terahertz range.

However, unlike other devices that work using quantum effects at this order of magnitude, the device does operate at room temperature.

The researchers also want to further improve their lithography method, which was redeveloped by Emboras from scratch to build the parts, so that components like this can be produced reliably in future. At present, fabrication is only successful in one out of every six attempts.

The latest research was published in Nano Letters (doi: 10.1021/acs.nanolett.5b04537).