While the idea of an invisibility cloak may seem more science fiction than actual science, a group of American researchers are well on their way.
Researchers from Towson University and the University of Maryland used 25,000 individual cloaks 30 micrometres in diameter to slow down light to the point where it is stopped, causing a ‘trapped rainbow’ effect.
Placed on a gold sheet only 25mm per side, these ‘invisibility cloaks’ have the power to stop light, causing it to separate into different colour components.
Their findings, published in the New Journal of Physics, are by no means like a Harry Potter invisibility cloak that can make any object underneath vanish.
Nevertheless, researchers found that these tiny invisibility cloaks altered around 20 per cent of any given surface area.
Lead author of the study, Dr Vera Smolyaninova said: ‘In our array, light is stopped at the boundary of each of the cloaks, meaning we observe the trapped rainbow at the edge of each cloak.
‘The benefit of a biochip array is that you have a large number of small sensors, meaning you can perform many tests at once.
‘For example,’ she said, ‘you could test for multiple genetic conditions in a person’s DNA in just one go.’
Earlier this year, researchers cloaked a three-dimensional object standing in free space – bringing the idea of an ‘invisibility cloak’ a step closer.
The researchers used a hi-tech technique to ‘hide’ an 18-centimetre cylinder – and made it totally invisible.
So far, the researchers can use a hi-tech coating to make an object disappear – but can only hide it from microwave scanners.
It’s still visible to the naked eye.
The researchers say that the technique could be used to hide objects from visible light – but that it would only work on objects on the millimetre scale, ie ones close to invisible anyway.
Previous studies have either been theoretical in nature or limited to the cloaking of two-dimensional objects, this study shows how ordinary objects can be cloaked in their natural environment in all directions and from all of an observer’s positions.
Published today, 26 January, in the Institute of Physics and German Physical Society’s New Journal of Physics, the researchers used a method known as “plasmonic cloaking” to hide an 18-centimetre cylindrical tube from microwaves.
Some of the most recent breakthroughs in the field of invisibility cloaking have focussed on using transformation-based metamaterials — inhomogeneous, man-made materials that have the ability to bend light around objects .
This new approach uses a different type of artificial material — plasmonic metamaterials.
When light strikes an object, it rebounds off its surface towards another direction, just like throwing a tennis ball against a wall. The reason we see objects is because light rays bounce off materials towards our eyes and our eyes are able to process the information.
Due to their unique properties, plasmonic metamaterials have the opposite scattering effect to everyday materials.
‘When the scattered fields from the cloak and the object interfere, they cancel each other out and the overall effect is transparency and invisibility at all angles of observation.’
‘One of the advantages of the plasmonic cloaking technique is its robustness and moderately broad bandwidth of operation, superior to conventional cloaks based on transformation metamaterials. This made our experiment more robust to possible imperfections, which is particularly important when cloaking a 3D object in free-space,’ said study co-author Professor Andrea Alu.
In this instance, the cylindrical tube was cloaked with a shell of plasmonic metamaterial to make it appear invisible.
The system was tested by directing microwaves towards the cloaked cylinder and mapping the resulting scattering both around the object and in the far-field.
The researchers, from the University of Texas at Austin, have shown in previous studies that the shape of the object is irrelevant; oddly shaped and asymmetric objects can both be cloaked using this technique.
Moving forward, one of the key challenges for the researchers will be to demonstrate the cloaking of a 3D object using visible light.
‘In principle, this technique could be used to cloak light; in fact, some plasmonic materials are naturally available at optical frequencies.
However, the size of the objects that can be efficiently cloaked with this method scales with the wavelength of operation, so when applied to optical frequencies we may be able to efficiently stop the scattering of micrometre-sized objects.
‘Still, cloaking small objects may be exciting for a variety of applications. For instance, we are currently investigating the application of these concepts to cloak a microscope tip at optical frequencies. This may greatly benefit biomedical and optical near-field measurements,” continued Professor Alu.
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