Researchers Use Light To Reprogram A Tiny Optical Device

International research team including the University Jena controls a metasurface contact-free using laser light

A tiny optical device that can be reprogrammed using a laser could lead to adaptable components for optical computing, imaging and telecommunications. An international team of researchers led by the ARC Centre of Excellence for Transformative Meta-Optical Systems (TMOS) at The Australian National University has combined an ultrathin metasurface with liquid crystals. Researchers from Friedrich Schiller University Jena were also involved in the work. When a laser was shone on the device, it rotated the liquid crystal molecules and changed the way the metasurface interacted with light.

Most devices are built to perform a particular job. If you want them to do something different, you generally need to replace a component, rewire the system or manufacture a new one. The same applies to information processing: every time you ask a large language model such as ChatGPT or Claude a question, many electrical signals race through computer chips, carrying information and performing calculations. This takes energy.

Scientists are therefore searching for faster and potentially more energy-efficient ways to process information. One possible solution is to replace some electrical signals with light. The international team from TMOS at The Australian National University, Nottingham Trent University in the UK and Friedrich Schiller University Jena combined an ultrathin optical surface with liquid crystals, the material used in many electronic displays.

When a laser was shone on the structure, it rotated the liquid crystal molecules. This changed the way the metasurface interacted with light. Unlike a static metasurface, whose function is largely fixed after fabrication, its optical response could therefore be tuned afterwards.

A road system for light
The device is a metasurface, which is a thin material covered with structures smaller than the width of a human hair. These tiny structures are designed to control the properties of light—such as colour, intensity 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. 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.

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. When the researchers shone a laser on the structure, 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. However, the changes were small and difficult to observe. Combining the liquid crystals with a resonant metasurface made the effect much stronger and considerably easier to measure.

Jena expertise in active metasurfaces
Several researchers from Friedrich Schiller University Jena contributed to the publication. First author Ziwei Yang carried out the work as part of his Dual-Award PhD in the International Research Training Group »META-ACTIVE«, in which doctoral researchers earn doctoral degrees from both The Australian National University and Friedrich Schiller University Jena. Dr Katsuya Tanaka and Muyi Yang from the Jena research group of Professor Isabelle Staude were also involved in the experimental work in the laboratory.

The collaboration is closely linked to research within the International Research Training Group »META-ACTIVE«, in which the University of Jena and The Australian National University jointly investigate programmable and active metasurfaces whose optical properties can be deliberately changed after fabrication.

Changing light
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.

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.

»All tunable metasurfaces are trying to move beyond static devices. With light shining on the liquid crystal, you have the chance to tune the function after fabrication,« says first author Ziwei Yang.

More than a switch
One possible application for the technology is in optical computing, where light could carry information and perform some of the calculations that 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.

According to the research team, the technology may be particularly useful for optical neural networks. Their nonlinear layers are essential for recognising complex patterns and processing information. At present, these nonlinear functions are mostly controlled by electronics. The results point to a possible way of controlling such functions directly with light instead.

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. The researchers now want to reduce the amount of laser power needed to reconfigure the device and use further experiments and theoretical work to investigate how the concept could be transferred to practical systems.