Imagine a window with an image etched on its surface, but when you walk around to the other side, the image is entirely different.
Though it sounds impossible, that's essentially what researchers from the Australian National University (ANU) have achieved, with tiny translucent slides that can show two distinct images, at the same time, when viewed from opposite sides.
In one experiment, for instance, the scientists created a slide that shows the continent of Australia on one side, and the Sydney Opera House on the other.
The advance in the field known as "nonlinear optics" could have applications in photonic computing — using visible light or infrared instead of electrical current to perform digital computations.
These new light-based devices could eventually lead to faster and cheaper internet, the researchers said.
Their research was published in Nature Photonics today.
How does it work?
As you may have noticed, light usually travels along the same path forward and backward through a material like glass or water.
To change this, the researchers created tiny glass slides covered in cylinder-shaped nanoparticles, each particle so small that 12,000 of them could fit within the cross-section of a human hair.
Each cylinder controlled the flow of light like road signs directing traffic, said Sergey Kruk, an ANU physicist and co-author of the paper.
"We were able to introduce asymmetry in the way light propagates," he said.
"So when light propagates forward and when it propagates backwards, we get completely different results."
The technical name for these "road signs" is "nonlinear dielectric resonators".
The cylinders were made of two layers of silicon and silicon nitride. Each layer had a different refractive index -- a measure of how fast light travels through a medium, and therefore of the material's light-bending ability.
The different refractive indices of air and water, for example, is why a spoon in a glass of water looks like it's bent.
These cylinders could be positioned to be "bright" or "dark" for only the backward or forward directions, or "bright" or "dark" for both forward and backward.
By arranging these four types of cylinders in patterns, Dr Kruk and his colleagues from China, Germany, and Singapore were able to form images.
"Basically the slides consist of individual pixels," Dr Kruk said.
"And we can assemble these pixels in any patterns you like."
Light-based computing
Benjamin Eggleton, director of the Sydney Nano Institute, described the research as "significant" and a "fundamental result".
"It's a heroic fundamental advance," Professor Eggleton, who was not involved with the research, said.
The most obvious application, he said, was "nano-photonic components" for computing.
A key component of electronic computing and the complex architecture of microchips is the diode that allows electrical current to flow in only one direction.
In photonics, or light-based computing, a diode is called an isolator.
The current crop of isolators are relatively bulky and complicated, but the ANU research could lead to much smaller and simpler designs, Professor Eggleton said.
Photonic circuits, or optical computing, have been dubbed the future of computing, as they can be made smaller than electronic ones, operate at higher speeds, use less energy, and generate less heat.
"Many of the leading companies commercialising quantum computer technology rely on photonic circuits," Professor Eggleton said.
"And on those circuits, you will need these isolators."
Faster internet?
Dr Kruk also saw applications in photonic circuits.
This could ultimately lead to faster and cheaper internet, he said.
Two years ago, for instance, researchers built a photonic circuit that clocked 44.2 terabits per second across 76 kilometres of optical fibres installed between two university campuses in Melbourne.
By comparison, that's about 1 million times faster than the average broadband download speed in Australia.
Physicists are just starting to understand how intense light interacts with materials' structure at the nanoscale, Dr Kruk said.
"At this stage of technological development, we've gotten incredibly good at controlling electrical currents, and we are not as good at controlling beams of light.
"This [research] may be perhaps a first convincing step towards establishing a very sophisticated traffic control of beams of light.
"[This is] similar to very sophisticated traffic control of electrical currents, which we started to establish perhaps in the middle of the 20th century."
