The inventors of nano-isolators – the optical equivalent of a diode – say the device could start a technology revolution.

In the 1950s, a technological revolution in electronics began, spurred by the miniaturization of diodes and transistors. The first electronic devices, simple radios and TVs, soon evolved into today’s smartphones, tablets and laptops, containing billions of diodes and transistors.

Scientists Aditya Tripathi and Dr Sergey Kruk from the Nonlinear Physics Centre, believe a similar revolution could be imminent in the field of optics. Their team has developed an optical counterpart of the diode, called an isolator, that is only nanometres in size - millions of times smaller than existing technology.

Such radical miniaturization was what allowed diodes and transistors to end up installed in smartphones by the billion, when only a handful of the original vacuum-tube units could be fitted inside a fifties radio or TV.

The new optical technology offers the same potential, said Dr Kruk.

"While we can fit billions of electronic diodes and transistors inside a smartphone, we cannot do the same with conventional optical isolators. With current technology, a billion optical isolators would occupy at least an area of a football stadium and would cost the total annual earnings of all Australians.

"Small and affordable optical isolators can open up a new era of optical engineering," said Dr Kruk.

Optical isolators allow light to pass through in only one direction, just as diodes permit electrical current to flow in only one direction. But light is highly reciprocal: if it can travel in one direction it can almost always retrace its path. To avoid this, current optical isolators use crystals and magnets which makes them bulky and expensive.

But with the new nanoscale isolators, many components could be fitted in consumer devices, which unleashes their true power, said Dr Kruk, pointing out that in electronics the effectiveness of diodes is far greater when combined than as individual components.

“In the past, by including more and more electrical diodes and transistors into systems, we could progress from creating memory cells to performing mathematical operations, to developing computers and smartphones, to artificial intelligence.

"The ability to integrate large quantities of small optical isolators into systems could lead to incredible innovations. We might see advancements in optical signal processing, machine vision, or perhaps entirely new technologies we haven't even imagined yet."

To break reciprocity in the propagation of light, the team’s optical isolator, which is reported in an article published in Nature Communications, incorporates vanadium dioxide, which has a key property. When heated, it undergoes a phase change which turns the material from a transparent dielectric-like phase to a non-transparent metal-like phase, in less than a billionth of a second.

The challenge the team had to solve was how to make this transition happen for light passing through in one direction, but not the other.

To do this they leveraged their skill in designing optical components with state-of-the-art nanotechnology. Such components, often termed metasurfaces, are made of arrays of structures much smaller than the wavelength of light, thousands of times smaller than the width of a human hair. The choice of the size and shape of these structures allows the unlocking of new properties of the interaction between the light and the material.

The team introduced asymmetry into the design, which they reasoned they could make one direction of travel favour the light absorption. In this direction the vanadium dioxide would be heated, triggering its phase change, and cut out transmission for that direction.

Choosing a light wavelength commonly used for fast internet communications (around 1.5 microns), the team ran tens of thousands of computer simulations to find an optimal design that also was possible for modern nanofabrication. The modelling settled on an array of cylinders of silicon with radius and height both of 540 nanometres, arranged in a square lattice 820 nanometres apart, sitting on a layer vanadium dioxide 35 nanometres thick.

To make a prototype, the team turned to colleagues in Tennessee, United States, at Vanderbilt University and Oak Ridge National Laboratory. On delivery back to ANU, the prototype successfully allowed light to pass through from one side, while the reverse direction was indeed opaque.

The simplicity of the device could enable its widespread adoption, said lead author, PhD student Aditya Tripathi.

“Besides vanadium dioxide, this work uses some of the most basic elements in nanophotonic structures: silicon nanoresonators, which are convenient elements for modern nanofabrication.

“The underpinning concepts are also relatively straightforward to implement in contemporary nanophotonics. Other researchers can tailor the design of an optical nano-isolator based on our methodology,” Mr Tripathi said.

“We believe that this work will lead to the development of all-optical steering and routing of light as well as multiplexing optical devices at the nanoscale.

“The development of such devices will ultimately be a huge leap in the field of integrated photonics feeding further into quantum photonics.”