It’s hard to resist the idea of owning a cloak that could make you invisible. But is this really possible?
Harry Potter-like invisibility cloaks have always eluded scientists despite the fact that over the last decade, there have been many attempts to achieve this with the help of artificial materials called meta-materials that can manipulate electromagnetic waves including radio waves, microwaves, infrared and visible light, and make objects invisible to the human eye.
Scientists at the Queen Mary University of London (QMUL), for instance, said on Friday that they have made an object disappear by using a composite material with nano-size particles that can enhance specific properties on the object’s surface.
The researchers coated a curved surface with a nanocomposite medium, which has seven distinct layers (called graded index nanocomposite) where the electric property of each layer varies depending on the position. The effect is to ‘cloak’ the object: such a structure can hide an object that would ordinarily have caused the wave to be scattered.
The underlying design approach has much wider applications, ranging from microwave to optics for the control of any kind of electromagnetic surface waves, the researchers said in a 15 July note.
Acknowledging that the research might not quite yet lead to the invisibility cloak made famous in the Harry Potter novels, the researchers said that this “practical demonstration could result in a step-change in how antennas are tethered to their platform”. It could allow for antennas in different shapes and sizes to be attached in awkward places and a wide variety of materials.
The broad theory behind invisibility cloaks is to manipulate light by controlling and bending it around an object to make the latter seem invisible to the human eye.
It is the scattering of light—visible, infrared, X-ray, etc. — that interacts with matter to help us detect and observe objects. However, the rules that govern these interactions in natural materials can be circumvented in metamaterials whose optical properties arise from their physical structure rather than their chemical composition.
However, making an invisibility cloak for bigger objects like humans is easier said than done and there are “stringent” constraints for that.
On 5 July, researchers in the Cockrell School of Engineering at The University of Texas in Austin indicated that while it is possible to use cloaks to perfectly hide an object for a specific wavelength, hiding an object from an illumination containing different wavelengths becomes more challenging as the size of the object increases.
Andrea Alù, an electrical and computer engineering professor and a leading researcher in the area of cloaking technology, along with graduate student Francesco Monticone, created a quantitative framework that can help researchers calculate the expected optimal performance of invisibility devices before designing and developing a specific cloak for an object of interest. The researchers published their work in the journal Optica.
“The question is, ‘Can we make a passive cloak that makes human-scale objects invisible?’ ” Alù said. “It turns out that there are stringent constraints in coating an object with a passive material and making it look as if the object were not there, for an arbitrary incoming wave and observation point.”
Alù’s lab is working on the design of active cloaks that use metamaterials plugged to an external energy source to achieve broader transparency bandwidths.
But this has not stopped multiple attempts to make an invisibility cloak. For instance, this March, researchers from Columbia University published a paper suggesting “A Cloaking Device for Transiting Planets”. “We suggest that advanced civilizations could cloak their presence, or deliberately broadcast it, through controlled laser emission,” the researchers said in their paper.
Similarly, the cover story in the March 2015 edition of the journal Advanced Optical Materials explained how Debashis Chanda at the University of Central Florida and his fellow optical and nanotech experts were able to develop a larger swathe of multi-layer three-dimensional (3D) metamaterial operating in the visible spectral range.
Metamaterials are not natural but are engineered in labs to produce properties that don’t occur naturally. Light is electromagnetic radiation, made up of perpendicular vibrations of electric and magnetic fields.
Natural materials usually only affect the electric component–this is what is behind the optics that we’re all familiar with such as ordinary refraction. But metamaterials can affect the magnetic component too, expanding the range of interactions that are possible.
The metamaterials used in attempts to make invisibility cloaks are made up of a lattice with the spacing between elements less than the wavelength of the light we wish to ‘bend’
Chanda and his team hope to be able to create larger pieces of the material with engineered optical properties, which would make it practical to produce for real-life device applications.
In April, 2015, a group of researchers from the Karlsruhe Institute of Technology (KIT) in Karlsruhe, Germany, said they have developed a portable invisibility cloak that can be taken into classrooms and used for demonstrations. It can’t hide a human, but it can make small objects disappear from sight without specialized equipment.
Scientists hoping to divert light around an object to render it invisible must find a way to offset the increased distance by a higher speed limit. To address this challenge, the KIT team constructed their cloak from a light-scattering material. By scattering light, the material slows down the effective propagation speed of the light waves through the medium. Then the light can be sped up again to make up for the longer path length around the hidden object. In this cloak, the object to be concealed is placed inside a hollow metal cylinder coated with acrylic paint, which diffusely reflects light. The tube is embedded within a block of polydimethylsiloxane, a commonly used organic polymer, doped with titanium dioxide nanoparticles that make it scatter light.
There have been similar attempts. On 17 September, 2015, scientists at the US department of energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California Berkeley said they devised an ultra-thin invisibility “skin cloak” that can conform to the shape of an object and conceal it from detection with visible light. Working with brick-like blocks of gold nano-antennas, the Berkeley researchers fashioned a “skin cloak” barely 80 nanometers in thickness. The surface of the “skin cloak” was meta-engineered to reroute reflected light waves so that the object was rendered invisible to optical detection when the cloak is activated.
A month later, on 21 September, scientists at the Nanyang Technological University (NTU) in Singapore said in a statement that they have developed a thermal cloak that can render an object thermally invisible by actively redirecting incident heat.
To construct the cloak, the researchers deployed 24 small thermoelectric modules, which are semiconductor heat pumps controlled by an external input voltage. The modules operate via the Peltier effect, whereby a current running through the junction between two conductors can remove or generate heat. When many modules are attached in series, they can redirect heat flow.
The researchers also found that their active thermal cloaking was not limited by the shape of the object being hidden. When applied to a rectangular air hole, the thermoelectric devices redistributed heat just as effectively as in the circular one. Baile Zhang and his team plan to apply the thermal cloaks in electronic systems.
So will we ever have a cloak that will be invisible even when dealing with bigger objects and multi-spectrums?
The above-cited researchers from Columbia University acknowledge that while “…Einstein’s theory of relativity fundamentally limits the ultimate performance for invisibility…with new concepts and designs, such as active and nonlinear metamaterials, it is possible to move forward in the quest for transparency and invisibility.”
