A flexible film represents a big step toward the “invisibility cloak”, made famous by Harry Potter, said scientists in the UK. (photo, from bbcimg.co.uk)
Together, the tiny structures inside the film form a “metamaterial” that can, among other tricks, manipulate light in order to render objects invisible.
Although flexible metamaterials have been made before, they only work for light of a colour far beyond that which we see.
According to physicists, the approach is a “huge step forward”.
The New Journal of Physics reported the bendy approach for visible lights.
Metamaterials interrupt and channel the flow of light at a fundamental level. They can be seen as bouncing light waves around in a prescribed fashion to achieve a particular result.
But the laws of optics say that only structures that are about as large as the waves’ length can manipulate light waves.
So far the most striking demonstrations of invisibility have occurred for light waves with a much longer wavelength (a far redder colour) than what the human eye can see, simply because it is easier to construct metamaterials with relatively large structures.
Even flexible metamaterial films have been shown off for this high-wavelength range.
‘The right step’
For the waves that we can see, the far shorter ones, a metamaterial requires structures so tiny, nanostructures, that they push the boundaries of manufacturing.
“The first step is imagining first of all that this could be done,” said Andrea Di Falco of St Andrews University, the author of the paper.
“All the typical results have been reached in flat and rigid surfaces because this is the legacy of the procedures used to create nanostructures.”
Therefore instead of building the typical stacks of the “fishnet” structures on hard, brittle silicon, Dr Di Falco used a thin polymer film.
“Typically what you do is stack several layers of fishnet structures and this all together will give you a metamaterial,” Dr Di Falco explained.
“What I’ve done here is fabricate a single layer – I lift it off so that at the end I am left with a self-standing membrane – and show that it has the properties required to create a 3D flexible metamaterial.”
The work was called “a huge step forward in very many ways” by Ortwin Hess, a physicist who recently took up the Leverhulme Chair in Metamaterials at Imperial College London.
“It clearly isn’t an invisibility cloak yet – but it’s the right step toward that,” he told BBC News.
He also said that the next step would be to characterise the way that the material’s optical properties change as it is bent and folded.
If they were sensitive to the movement, delicate manipulations of the films may make them useful for next-generation lenses in handheld cameras for example.
But if on the contrary they were impervious to bending and motion, the films might still be useful, but in contact lenses.
And although the invisibility cloak could be that much closer, Professor Hess added that is still some way off.
“Harry Potter has to wait still – that’s the huge goal,” he said.
“So far he’s had to live in a house and now he can live in something like a tent; it’s not the cloak that adjusts to his shape, but it’s a bit more flexible. Now we have to take the next step forward.”