Research Team Develops World’s Smallest Integrated Optical Switch

Juerg Leuthold, Professor of Photonics and Communications, and his team have developed the world's smallest integrated optical switch. An atom is shifted and the switch turns on and off when a small voltage is applied.

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

The amount of globally exchanged data using communication networks has increased massively. Currently, the data volume for mobile and wired communications has been experiencing an increase by 57% and 23%, respectively. It is not possible to predict when this increase will end, so in the future, all network components will have to be more efficient.

These components include modulators, which convert the information in electrical form into optical signals. Modulators are electrical switches that rapidly turn on or off a laser signal, based on the frequency of the received electrical signals. An increasing number of modulators are installed in data centers, but they have a disadvantage of being quite large. The modulators occupy a lot of space when used in large numbers as they measure a few centimeters across.

From Micromodulators to Nanomodulators

Leuthold and his team have successfully proven that it is possible to make the technology a lot more energy-efficient and smaller. The team presented a micromodulator measuring only 10 µm across, 10,000 times smaller than commercially available modulators.

The research team have now gone to the next level and created the smallest optical modulator ever developed. This modulator is so small that the component functions at the level of individual atoms. The foot print is reduced by a factor of 1,000 when the switch is incorporated along with the light guides. However, this switch is much smaller, with the sized measured on the atomic scale. A report on the team’s latest achievement has been published in Nano Letters.

The modulator is very small compared to the wavelength of light utilized in the system. Laser light with a wavelength measuring 1.55 micrometres is used to transmit optical signals in telecommunications. An optical device is can normally not to be smaller than the wavelength to be processed.

Until recently, even I thought it was impossible for us to undercut this limit.

Juerg Leuthold, Professor of Photonics and Communications, ETH Zurich

New Structure

Leuthold’s senior scientist Alexandros Emboras successfully reconfigured the development of a modulator, and allowed the magnitude of individual atoms to be penetrated, even though the team used light with a conventional wavelength.

The modulator constructed by Emboras is made up of two small pads made of platinum and silver. These pads are placed on top of a silicon optical waveguide, and are positioned next to each other with a distance measuring only a few nanometers. The silver pad has a small bulge that projects into the gap and almost touches the platinum pad.

Short Circuit Thanks to a Silver Atom

The modulator operates when the light entering from an optical fiber is directed to the gaps entrance by the optical waveguide. Above the metallic surface, the light is converted into a surface Plasmon. A Plasmon appears when electrons present in the outer atomic layer of the metal surface receive energy transmitted by the light. The diameter of the electron oscillations is smaller than the ray of light. This enables the electrons to enter the gap and go through the bottleneck. The electron oscillations, on the gap’s other side, can be changed back into optical signals.

When voltage is introduced to the silver pad a few silver atoms - or at least one single atom - move towards the end of the point and take position at the end point. A short circuit is developed between platinum and silver pads, allowing the flow of electricity between them. As a result of this the loop hole closes for the Plasmon, and the switch moves from an “on” to “off” state or vice versa. A silver atom is sent back instantly when the voltage decreases. This action opens the gap, allowing the Plasmon to flow, and the switch is “on” again. This entire method can be repeated millions of times.

Mathieu Luisier, ETH Professor and a participant in this research, used a high-performance computer to simulate the system at the CSCS in Lugano. This enabled Luisier to verify that the short circuit at the silver point’s tip is carried out by a single atom.

A Truly Digital Signal

A completely digital signal is produced, as the only option for the Plasmon is to either not pass through the bottleneck, or completely pass through the bottleneck.

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.

Juerg Leuthold, Professor of Photonics and Communications, ETH Zurich

At this point, the modulator is not fit for series production, and it is still considered to be slow in comparison to other devices that operate with quantum effects at this magnitude. The modulator is only suitable to switch frequencies at the megahertz range or below. The ETH researchers aim at fine-tuning the modulator for frequencies ranging in gigahertz to terahertz.

Improving the Lithography Process

The researchers are focusing on improving the lithography method, developed by Emboras. They aim to produce components in a reliable manner. Currently the successful ratio of fabrication is very small, and researchers assume this to be a success, as lithography processes on the atomic scale remain an unexplored territory.

Leuthold has expanded his team as he plans to continue his study on nanomodulators. He also highlights that an increasing number of resources will be needed to create a solution that will be commercially available. Leuthold is also confident that his entire team will deliver a successful solution in the near future.


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