• The spectral cloak works by transferring energy from certain colours to others
• It makes objects appear invisible by matching the colour needed to mask it
• It would turn green light into blue light to mask the presence of a green object 
• It then flips it back to the original wavelength afterwards to make it undetectable
• With some more work it could be used to hide 3D objects in normal daylight

It may seem like pure fantasy, but a Harry Potter-style invisibility cloak is now a step closer to reality.

Scientists in Canada have created a cloaking device capable of making an object 'fully' invisible for the first time, even in natural light.

Up until now, similar efforts at making a cloaking device have failed because they don't work in all wave lengths of light 

The 'spectral cloaking' device is different because it works by cancelling out the imprint an object leaves in a light wave that passes through it. 

The energies of certain colours are shifted by a sheet of optical fibre as they hit the object.

As a result, the wave which reaches the observer is almost exactly the same as the wave on the other side, causing the object to appear 'invisible' in daylight.

As well as hiding people, researchers claim the breakthrough could pave the way for radar-proof aircraft and hack-proof internet cables.  

Invisibility powers have long been a staple of popular culture, from HG Wells' The Invisible Man to Star Trek, and physicists have been trying to replicate the results. 

Existing techniques use metamaterials, which are structures that bend the path of light around the object they are trying to hide.

This technique struggles to cope with a range of different frequencies, such as the different colours of light.

It also distorts the wave so the end result doesn't appear convincing.  

Researchers from the National Institute of Scientific Research in Canada sought to overcome this.

Broadband illumination, the light we experience in day-to-day life, spans from red to blue and encompasses a large range of wavelengths. 

All visible objects reflect a small amount of these wavelengths, and that dictates what colour they appear. 

The spectral cloak works by selectively transferring energy from certain colours of the spectrum to others.

By matching the colour needed to mask reflection from the object, the device makes it appear invisible. 

For example, if the object was green, it would turn any wavelengths of green light into blue light to mask the presence of the green object. 

Once the light has passed past the object, the device then restores the light to its original state, so that the interference is not detected. 

Professor Jose Azana, who worked on the project, said: 'Our work represents a breakthrough in the quest for invisibility cloaking.

'We have made a target object fully invisible to observation under realistic broadband illumination by propagating the illumination wave through the object with no detectable distortion, exactly as if the object and cloak were not present.' 

When viewing an object, what you are really seeing is the way in which the object modifies the energy of the light waves that interact with it. 

Most solutions for invisibility cloaking involve altering the paths that light follows, so that waves spread around, rather than through, an object.

Other approaches, called 'temporal cloaking', tamper with the speed at which the light spreadsm so that the object is temporarily concealed as it passes through the light beam during a prescribed length of time.

In either approach, different colors of an incoming light wave must follow different paths as they travel through the cloaking device, thus taking different amounts of time to reach their destination. 

This alteration of the wave's profile can make it apparent to observers that something is not as it should be.

'Conventional cloaking solutions rely on altering the propagation path of the illumination around the object to be concealed; this way, different colours take different amounts of time to traverse the cloak, resulting in easily detectable distortion that gives away the presence of the cloak,' added Luis Romero Cortés, who was also involved in the research.

'Our proposed solution avoids this problem by allowing the wave to propagate through the target object, rather than around it, while still avoiding any interaction between the wave and the object.'    

The team proved their concept by concealing an optical filter - a device that selectively transmits different wavelengths of light - after illuminating it with a laser.

The cloaking device was constructed from two pairs of commercially available components called dispersive optical fibre and a temporal phase modulator, placed in front of the filter and behind it.

Dispersive optical fibre forces a beam of light, which is actually a wave, to travel at different speeds, splitting it into its constituent colours, much like a prism.

A temporal phase modulator modifies the optical frequency of light, depending on when the wave passes through the device, allowing it to change one colour to another.

The spectral cloak was able to transform the light waves in the range of frequencies that would have been absorbed by the optical filter, then completely reverse the process as the light wave exited the filter on the other side.

This made it look as though the laser pulse had hit a non-absorbing medium, that is, it appeared to pass through the object without any interference.

The device could also find applications beyond invisibility cloaking. 

For example, selectively removing and reinstating colours in the broadband waves could allow more telecommunication data to be transmitted, reducing logjams.

It could also prevent an eavesdropper from gathering information by probing a fibre optic network with broadband light.

The full findings of the study were published in the journal Optica.

 
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