Researchers use light to reprogram a photonic device

Australian National University

A new technique that allows optical components to be reprogrammed by laser light could open the way for a new generation of telecommunications and signal processing technology.

Until recently photonic devices were static – if you wanted different behaviour you would have to build a new component, but by rotating liquid crystals with a laser, a team of physicists has literally found a lever and created a tunable device.

“With light shining on the liquid crystal, you have the chance to tune the function after fabrication - this function is no longer permanently built into the material,” said Ziwei Yang, a visiting student in the electronic Material Engineering Department and the ARC Centre for Transformative Meta-Optical Systems (TMOS).

“You can even alter the function of an optical component while it is operating.”

Mr Yang is part of an international team of researchers led by TMOS at ANU and including Nottingham Trent University in the UK and Friedrich Schiller University Jena in Germany, that published their work in Science Advances.

In electronic displays, that movement is normally controlled using electricity and electrodes. The researchers instead wanted to find out whether light itself could rotate the molecules and alter the device.

“Our question was whether light could directly tune or program the function of the metasurface, without needing contact electrodes,” Yang says.

Using light – photonics – instead of electronics for signal processing can lead to much faster and more energy-efficient operation. In a recent review TMOS Director Professor Dragomir Neshev mapped the path to the second quantum revolution, noting that tunable optical components are crucial to make this happen.

To create tunability, the team combined liquid crystals, the material used in many electronic displays with a metasurface, a thin material covered with structures smaller than the width of a human hair.

When a laser was shone on the device, it rotated the liquid crystal molecules and changed the way the surface interacted with light.

To make their metasurface adjustable, the researchers surrounded its tiny silicon structures with liquid crystals. Liquid crystals are made from molecules that can change direction in response to an outside force.

These tiny structures on a metasurface are designed to control the properties of light – such as colour, strength and direction. It can be imagined as a miniature road system for light, determining where it can travel and what happens to it along the way.

However, for most metasurfaces, these ‘roads’ are permanently fixed once the device is manufactured

When the researchers shone a laser on the device, this produced a tiny twisting force, known as optical torque, which rotated the liquid crystal molecules.

This changed the optical conditions around the silicon structures and altered how the metasurface responded – the laser temporarily rearranged the road system.

Experiments using light to control liquid crystals have been documented since the late 1990s, Yang says, but the change was small and difficult to observe. Adding the metasurface made the effect much clearer.

“The metasurface works as a platform, or even as an amplifier,” Yang says.

“It makes these changes much easier to observe. We combined the metasurface and the liquid crystal, and that is where it became interesting.”

The team first showed that the laser could change the metasurface’s resonance – the wavelength of light with which it interacts most strongly.

Resonance is what happens when a material interacts strongly with a specific wavelength of light. It can be compared to pushing someone on a swing – when each push matches the swing’s natural rhythm, the effect becomes stronger. In a metasurface, changing the resonance changes which wavelengths of light receive that stronger response.

While this was an important first step, Yang says the most interesting result emerged when the team moved into the world of nonlinear optics – when very intense light interacts with a material and produces effects that do not occur under ordinary lighting conditions.

The team used the device to convert invisible infrared light into visible green-yellow light through a process called third-harmonic generation.

As the laser rotated the liquid crystals, it shifted the metasurface’s resonance. Depending on the wavelength used, this could increase or reduce the amount of visible light produced.

The laser was therefore doing two things at once – producing the new light and changing the way the device produced it.

One possible application for the technology is in optical computing, where light could carry the information and perform some of the calculations which are currently handled by electrical signals.

While future optical computers would still need electricity to run lasers and other equipment, allowing light to perform more of the processing could eventually make some tasks faster or more energy efficient.

Yang says the team’s device may be particularly useful for optical neural networks – the computing systems behind many forms of AI.

A neural network contains layers that process information. Its nonlinear layers allow it to recognise complicated patterns, make distinctions and learn from data. Without those layers, even a large neural network would have difficulty solving complex problems.

“At the moment, the nonlinear layer is mostly controlled by electronics,” Yang says.

“If we can somehow use light to directly control this nonlinear layer directly, it could be useful for optical neural networks in the future.”

The same principle could eventually be used in imaging, telecommunications and systems that steer beams of light without moving mechanical parts.

The technology remains at an early stage, with the researchers now wanting to reduce the amount of laser power needed to reconfigure the device.

The next stage will require “more experiments and rigorous theory” says Yang, to refine the device and test whether it could work in a practical system.