Microscopic structures that can bend light are helping researchers lay the foundations of an invisibility cloak.
Two research teams have made structures that could help conceal objects from daylight – taking the next step towards making the visible, invisible. Recent progress draws on advances in so-called metamaterials, which are microscopic structures that bend light in unnatural directions. Metamaterials have already managed to reroute microwaves, infrared radiation and, given the right circumstances, visible colours, so that they go around metal obstacles and living creatures.
“These experiments have demonstrated the underlying physics of a cloaking device,” said Professor Costas Soukoulis from the Foundation for Research and Technology in Heraklion, Greece, who is also working to develop this technology through a research project called Photometa, funded by the EU’s European Research Council (ERC). But Prof. Soukoulis acknowledged that existing invisibility cloaks still fall short of the standards set by Perseus or Harry Potter and said that “most metamaterials still struggle to bend light that is visible to the naked eye”.
An additional shortcoming is the tendency of metamaterials to absorb part of the light that shines through them, which casts a recognisable shadow. Most are also cumbersome to carry and impractical to manufacture. However, Dr. Patrice Genevet from the CRHEA research centre in Valbonne, France, is hoping to address these challenges by using light materials and visual techniques from the electronic display industry.
As part of his ERC-funded research, Flatlight, Dr. Genevet is coating flat lenses with skin-thin layers of gallium nitride, the material that emits blue light in LED displays. The gallium nitride is then carved into pillars that are small enough to create delays in how light waves flow through them. Having studied how different-shaped pillars distort light, Dr. Genevet can now design lenses that force light in any direction, looping it sideways or backwards on demand.
All metamaterials can pull off similar feats, but electronic materials like gallium nitride are unusual in that they do so with visible light. The material’s properties provide the potential of someday developing a real-life cloaking device. “If you want to bend light around sharp angles, you have to use materials that cannot be found in nature,” said Dr. Genevet. While conventional metamaterials tend to be ill-suited for moving around unnoticed, Dr. Genevet fashions his pillars in thin layers that could, in principle, be deposited on flexible surfaces and incorporated into stealth suits.
EU’s future cyber-farms to utilise drones, robots and sensors
Bee-based maths is helping teach swarms of drones to find weeds, while robotic mowers keep hedgerows in shape.
“We observe the behaviour of bees. We gain knowledge of how the bees solve problems and with this we obtain rules of interaction that can be adapted to tell us how the robot swarms should work together,” said Vito Trianni at the Institute of Cognitive Sciences and Technologies of the Italian National Research Council. Honeybees, for example, run on an algorithm to allow them to choose the best nest site, even though no bee knows the full picture.
Trianni runs an EU-funded research project known as SAGA, which is using the power of robotic groupthink to keep crops weed free. “We can use low-cost robots and low-cost cameras. They can even be prone to error, but thanks to the cooperation they will be able to generate precise maps at centimetre scales,” said Trianni. “They will initially spread over the field to inspect it at low resolution, but will then decide on areas that require more focus,” said Trianni. “They can gather together in small groups closer to the ground.” Importantly the drones make these decisions themselves, as a group. Next spring, a swarm of the quadcopters will be released over a sugar beet field. They will stay in radio contact with each other and use algorithms learnt from the bees to cooperate and put together a map of weeds. This will then allow for targeted spraying of weeds or their mechanical removal on organic farms.
Today the most common way to control weeds is to spray entire fields with herbicide chemicals. Smarter spraying will save farmers money, but it will also lower the risk of resistance developing to the agrichemicals. And there will be an environmental benefit from spraying less herbicides.
Plants light trick to look blue to bees
Plants use a trick of the light to make themselves look blue to bees, UK scientists have found.
Unlike humans, which dress up with the aim of attracting others, plants need to pretty themselves up to pull in pollinators. Their aim is to swap genes with other members of their species so that they can successfully set seeds. Many have developed attractive blooms laced with a sugary treat intended to appeal to insects like bees. But the quandary for Cambridge plant scientist Beverley Glover and her colleagues was that insects, like bees, see best at the blue end of the spectrum. And yet blue is a very costly colour for many plants to make, which is why blue flowers tend to be rare.
Then the team noticed, when they looked at several unrelated plant species, that some plant petals have an iridescent effect, like a reflection from the surface of a CD, when light falls on them. “And then we realised that they also scatter blue light off the petal surfaces to produce a blue halo around the flowers,” says Glover. “And when we investigated we found that cells on the surfaces of the petals have a finely wrinkled surface. The wrinkles are the same size as the waves of blue light, which is why the blue colour gets scattered off the petal surface.”
Wondering whether the observation was a one-off in the couple of species they’d looked at, the Cambridge team then worked their way through the diverse collection of plant species at the Cambridge University Botanic Gardens, and also looked at specimens from Kew. Many of the flowers they studied were also endowed with cells in their petals capable of scattering blue light in this way. But although the end result was always the same – the appearance of a subtle blue halo around the flower – the individual shapes and structures of the cells responsible were different. “That told us that all these different plant species had independently evolved this trait, because they were all doing it slightly differently.”
The question is why. Glover and her colleagues suspected that, because blue is hard for a flower to make, perhaps they were using this trick of the light from their petals to make their flower look blue to a bee and thereby boost their pollination prospects. “So then we did the experiment,” she explains. They made mock flowers with surfaces etched at the nanoscale in an identical pattern to the wrinkles in the petal surfaces so that they produced the same blue halo seen around real blooms. These fake flowers were charged either with a sweet sugary treat, or a bitter quinine-laced nectar. Bees unleashed on the phony flowers learned to avoid the ones laced with quinine, or to target only the sugar-loaded examples, proving that they could clearly see the blue effect. They were also much more time efficient at visiting flowers when they were marked up with the blue halo effect.