Spacecraft could soon carry sophisticated polarisation-sensitive optics, thanks to innovative design that miniaturises optical technology to a tiny fraction of its current size.

These new components, based on metasurfaces, would be ideal for nanosatellites, cheap, small satellites only tens of centimetres. Current polarisation optics are typically of that scale, and so are prohibitively large and heavy for this spacecraft.

The compact dimensions of the metasurface-based design are a great advantage, said Professor Rob Sharp, Head of Instrumentation at the ANU Research School of Astronomy and Astrophysics.

“This approach makes the application of powerful observational techniques practical for small satellite missions,” he said.

Another benefit of the new technology is its ability to analyse and filter different polarisations. Unlike current polarisation devices, which only detect one polarisation at a time, the new design’s capabilities will assist with suppressing glint reflections from images of water, detecting organic aerosols and identifying camouflaged human-made structures.

“Our metasurfaces can combine multiple complex optical components into a single unit,” said Sarah Dean, PhD student in the Department of Electronic Materials Engineering and the ARC Centre for Transformative Meta-Optical Systems.

Many of the light sources imaged from space contain a range of polarisations, Ms Dean said.

“Even the large unit on the very latest NASA satellite can only do linear polarisation, whereas we hope to extend the capabilities to include circular polarisations, using components mere millimetres in size.”

This research is in published in Nanoscale Advances.

Metasurfaces’ behaviour varies significantly from bulk materials due to surface structures smaller than the wavelength of light, in this case made of dielectric material, which have the lowest losses.

In this research the shape of the structures was determined by a process known as topology optimisation. This process begins with a randomly generated metasurface, whose simulated output is compared to the desired output. Iterations comparing simulations working both forward from the initial design, and backwards from the desired output are performed. Because light transmission is reciprocal the two simulation approaches can be combined to develop a metasurface pattern that gives the desired output.

The design process simultaneously optimises a number of diffraction orders: four channels enable selection of different polarisations, with additional ones providing redundancy. This redundancy enables error monitoring and subsequent recalibration - a major advantage for satellite equipment, which operates in the hostile and inaccessible environment of space.

“This method results in designs you would not expect – there’s no way to intuit what will come out,” Ms Dean said

Metasurfaces often feature regular arrays of geometrical shapes, but the topological optimisation method came up with a design featuring a “combination of blobs,” Ms Dean said.

However, some correction is needed at the end of the computer optimisation to smooth out sharp corners and narrow features and gaps that are difficult to manufacture.

A prototype has been manufactured at the Melbourne Centre for Nanofabrication, part of the Australian National Fabrication Facility, and is in testing.

The design process is flexible and allows the metasurface to be optimised to satisfy particular operating requirements, such as wavelength, resolution or specific polarisation characteristics.

Because the metasurface analyses polarisation from raw light – in contrast to current technology which requires filters to separate the different polarisations – it can operate effectively in lower light levels.

As well as space applications, the small size and weight of metasurface-based componentry makes it versatile, Ms Dean said.

“You could put it on a drone, or even a terrestrial robot – it’s a quite a futuristic technology.”