Nanoparticles control light like road signs directing traffic

Wednesday 22 June 2022

Physicists at the Research School of Physics have developed tiny translucent slides capable of producing two very different images depending on the direction in which light travels through them.  

As light passes through the slide, an image of Australia can be seen, but when you flip the slide and look again, an image of the Sydney Opera House is visible. The pair of images created is just one example of an untapped number of possibilities. 

“Asymmetric flow is a very counter-intuitive effect – light does not normally behave this way,” said Dr Kruk, the lead author on paper reporting the research in Nature Photonics.

The breakthrough puts another tool in the toolbox for light-based technology, photonics. Photonics has already revolutionised information technology, thanks to the optical fibres that power today’s global internet by carrying high-bandwidth signals over huge distances at high speeds. 

However traffic control of signals, which is ubiquitous in electronics, has proven challenging with light: enabling flow in only one direction but not in reverse is easy with electrical currents (with a diode), but much more challenging with beams of light, said lead researcher Dr Sergey Kruk, from the Nonlinear Physics Centre in the ANU Research School of Physics.

To achieve the asymmetric behaviour, the team, from ANU, Germany, China and Singapore, leveraged the power of nonlinear optics – surprising effects that come into play in the interactions of intense beams of light, such as lasers, with particular materials. 

They explored nonlinear optical effects in nanostructured materials, in contrast with much of the research which has been conducted in materials much bigger than the wavelength of light. 

The team’s computer modelling allowed them to find nanostructures that allowed light to flow from left to right, right to left, in both directions or neither.

These surprising properties come from the ability of structures smaller than the wavelength of the light – in this case rods – to generate effects not seen in bulk materials. In this case the team were able to find structural patterns that exhibit the magnetic dipole response of the device, a rare property in pure, unstructured materials.

To demonstrate their modelling, they created an array of dielectric nanoparticles arranged into a pixelated slide about 100 x 100 micrometres in dimension, 

Each pixel was formed by a resonator, spaced 900 nanometres apart, which could block or allow the light transmission independently in either direction. 

The resonators were made from rods made of 480 nanometres of silicon joined to 360 nanometres of silicon nitride. 

Such an array can be created by standard nanofabrication techniques.

“These are essentially the same techniques that industry uses to manufacture modern computer chips,” Dr Kruk said.

“This technology can be scaled up for mass-production, making individual components incredibly compact and cheap.”

This first demonstration device is based on third harmonic generation -- a nonlinear effect that changes the frequency of the light – in this case from infrared light at 1425 nm wavelength to visible light in blue-green region at 475 nm wavelength. The team are now working on a design which will transmit light asymmetrically with its frequency unchanged. The devices with such functionality are called optical isolators.

While optical isolators are available on the market, the current technology isn't without its drawbacks.

"Optical isolators are indispensable in many high-end light-based technologies such as powerful lasers and ultra-fast optical communication networks, but they are quite large in size and are also expensive, which prohibits a wider deployment of these components," Dr Kruk said.

"By contrast, our devices are created using nanofabrication technology. This allows us to drastically reduce the size of a component to less than one thousandth of a millimetre and reduce production costs to just a fraction of an Australian dollar.

According to Dr Kruk, the development of many technologies of tomorrow will rely heavily on our ability to control light at a tiny scale.  

"A wide deployment of tiny components that can control the flow of light could potentially bring technological and social changes similar to transformations brought about in the past by the development of tiny components that control the flow of electricity, which are known as diodes and transistors," he said. 

"Control over the flow of electricity at the nanoscale is what ultimately brought us modern computers and smartphones. It is therefore exciting to envision the potential of our emerging technology for controlling flow of light." 

This research was a collaboration between the Nonlinear Physics Centre at the ANU Research School of Physics, Paderborn University in Germany, Southeast University in China and A*STAR Singapore. The research is published in Nature Photonics.

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