Over the last decade, scientists have tried many different ways of concealing objects by manipulating light. There has been some success in highly controlled situations, but no one has been able to create an invisibility cloak that can function in natural light.
Now, a team of scientists at the National Institute of Scientific Research (INRS) in Montreal have developed a new technique dubbed “spectral cloaking” that is a completely new approach to invisibility and could offer a new path towards developing a real invisibility cloak.
The challenge with natural light is that although it appears white, it is actually made up of a range of colours of different wavelengths, like you see in a rainbow or when you shine a light through a prism. When this broad spectrum ‘white’ light from the sun or a flashlight hits an object, some wavelengths are absorbed and some are reflected. A flower might look blue because the blue wavelengths are reflected, bouncing off the flower for us to see, while other wavelengths are absorbed.
José Azaña and his team have found a way to manipulate light waves so this interaction doesn’t happen, making an object invisible to the human eye. Still in its early stages, the researchers used a single light source and a semi-transparent object, for the first time demonstrating that an object illuminated by a broad spectrum light source can be made invisible without the light source being affected in any way.
To achieve this, the researchers surround the object in a device in two stages, the first between the light source and the object removes the wavelengths that would interact with the object for us to see, and the second between the object and the observer replaces these wavelengths, reconstructing the light as if the object was never there, Azana told Global News.
The device has two main components: an optical fibre that separates the colours of the incoming light by forcing each colour of the spectrum to travel at a different speed, and a device known as a temporal phase modulator, which is used to change the frequency of a given colour depending on when it reaches the device, turning it into another colour at will. By programming the device to modify and remove specific colours in the light source, any object could be rendered invisible.
The research, published in Optica, shows that this completely new concept works with light coming from one direction in a highly controlled situation. But in theory, the concept could be extended to conceal any object as long as the device knows the exact colours to filter out.
“It’s a very promising concept, but we’ll need to test and develop it further to determine how far we can go in optimizing and adapting the concept to different practical applications,” explained Azaña.
Even so, spectral cloaking is already attracting attention for telecommunication systems that use broadband light waves as data signals to transfer and process information. The filters could provide a way of reorganizing and ‘cloaking’ sensitive information that could later be decoded, and may also allow engineers to improve the quality of data transmission.
Although there is a long way to go before we can conceal objects in the real world and we all have Harry Potter style invisibility cloaks, this is a major breakthrough in the quest for invisibility cloaking, made possible by an entirely new conceptual approach to invisibility.
