A new technique for controlling optical components with light could usher in the next generation of high efficiency signal processing technology, that could drive futuristic technology such as hologram videos.

The team of ANU Physicists has developed an innovative way to take static devices and make them dynamically tunable with light, an important capability for developing flexible and powerful light-based technology - photonics.

They did this by embedding a metasurface – conventionally static – in liquid crystals mixed with a photoresponsive dye, said leader of the project Dr Yana Izdebskaya.

“We can change the optical response of the metasurface optically - without any electrodes, magnetic components or heating elements, just using light,” said Dr Izdebskaya, who is in the Department of Fundamental and Theoretical Physics, and also a member of the ARC Centre of Excellence for Transformative Meta-Optical Systems (TMOS).

“It’s entirely reversible, which has been a challenge until now.”

Computer chips based on photonics promise faster signal processing that is more energy efficient and lower noise than electronic chips. However, a number of signal processing functions were lacking: this new technique could be used for some of the missing functionality in photonic chips, such as optical switching and dynamic beam shaping. 

The work is published in Nanoscale.

Metasurfaces can generate surprising optical effects that natural materials are not capable of. They are built from surface arrays of nanoscopic shapes smaller than the wavelength of light. These effects depend on the difference in refractive index between the metasurface itself (in this case a dielectric material, hydrogenated amorphous silicon) and the surrounding medium – for example air.

The refractive index of air is not easily changed, but if the metasurface is covered by a liquid crystal, whose refractive index can be changed, then the behaviour of the surface can be dynamically controlled. 

The TMOS team had also previously pioneered the use of magnetic fields to control the metasurface filled with liquid crystals.

But their ultimate goal was to move away from electromagnetic methods and develop optical control. 

A visit from Dr Andrey Iljin from the University of Münster, Germany, provided the missing piece of the puzzle: Dr Iljin is a dye specialist, collaborated with the team to add the photoresponsive dye methyl red into a liquid crystal.

Methyl red is a large dye molecule which can switch between a straight geometry and a bent geometry when illuminated by green light (wavelength 532 nm) – depending on the polarisation of the light.

The change in geometry of the methyl red molecules in turn shifts the orientation of the liquid crystals – long skinny molecules known as nematic liquid crystals – and changes their polarisation response at the operating wavelength of the metasurface, in the infrared.

The team chose a static metasurface made from elliptical silicon nanocylinders that was designed to create resonances via an effect known as a bound states in the continuum (BIC). This supports resonances in both the electric and magnetic dipoles. To effect control they use liquid crystals doped with one percent methyl red dye.

With their new-found control they were able to change the relative strength of the two different dipole resonances in the infrared, by changing the polarisation of the green light.

Better yet, the strength of the response could be dialled up by increasing the strength of the green light, and when turned off, returned to its initial state, in less than a second; a great enhancement over previous liquid crystals that needed to be heated to reset the orientation of the crystals – a much slower process.

“We were happy with this fast response in our initial experiments, but I think if we optimise the concentration or try other dyes we can further improve this control,” Dr Izdebskaya said.

As well as improving response time, Dr Izdebskaya and colleagues in the team led by Prof Ilya Shadrivov are now working towards making tiny pixel-sized versions of these controlled metasurfaces.

“It’s important because this approach can be useful for many different devices – metalenses, imaging systems and holography.”

“In the future these very small nanoscale components could replace bulky devices such as lenses in smartphones and cameras,” Dr Izdebskaya said.