10 February 2012
A device that can make objects invisible under visible light has been created, and unlike past cloaking devices, it doesn’t need a reflective surface, so is completely portable.
A study published in Nature’s Scientific Reports journal describes the creation of a sample cloaking device that looks like a hexagon when viewed from above, and can make an isolated cylindrical object invisible in six directions. The sample is based on a theoretical design of a portable device that can render objects invisible in all directions, which brings the goal of a fully movable invisibility cloak one step closer to reality.
“Based on previous theory of transformation optics, we showed a scheme to design an isolated polygonal cloak for visible light using simple electromagnetic parameters,” said co-author of the study, Hongsheng Chen from the Electromagnetics Academy at Zhejiang University in Hongzhou, China. “In the visible light frequencies, it is the first design of a moveable directional cloaking device.”
Previous invisibility devices
Previous studies have designed and created devices that work as invisibility cloaks thanks to a mathematical approach called transformation optics, which describes what material properties are needed to guide light.
In order to render an object invisible, light is shone on the device and passed around the object inside to go back onto its original path. This makes the light look like it travelled in a straight line through the device as if nothing was in the middle. These devices have commonly made objects invisible under microwaves, but researchers have wanted to create a device that works under visible light so they can be used in real world applications.
‘Carpet cloaking’ has been a popular technique for invisibility devices because it allows the use of visible light. But it requires a reflective surface like a mirror. Such devices are able to conceal larger objects but are restricted in their portability.
Chen’s team created a simplified hexagonal cloak based on their theoretical model. It is a small box with six sides and six directions of invisibility, made from a particular type of natural material that does not behave the same way in all directions. The device concealed a macroscopic cylindrical item with a maximum diameter of 3mm.
The materials required for this sample device are called anisotropic birefringent materials, which are extremely hard to come by, but according to the researchers are the ideal for creating a device that could make objects invisible in all directions.
“[Birefringent] material properties are helpful to guide the rays with a specific polarisation in the right way – flowing around the cloaked object and appearing on the other side without any deviation. In such a way, an observer cannot tell that the light flowed around the hidden object or not,” said Chen. “Anisotropy provides an ability to bend the light in an unusual way at the interface of two medium and thus can be very helpful to achieve an omni-directional cloak with a large cloaking area.”
From theoretical to practical
Creating a real onmi-directional cloaking device will depend on obtaining these rare materials, so the launch of a device that can conceal large objects from all directions is not in the near future. However, as research into this field increases, Chen said that it will not be that long before a directional cloak is created that will be capable of concealing larger items such as a tennis ball or a human being from several directions.
Physicist David Powell from the Nonlinear Physics Centre at the Australia National University in Canberra, who was not involved in the study, said that this experiment is “quite visually impressive, in the sense that you can actually see the cloaking effect in action, and the cloaked object is quite large.” He said that the work is an advance over previous studies, but not a huge leap.
Powell pointed out that there are major obstacles to creating a “fully 3D cloak”, in that this device only works for light of a certain polarisation and the cloak itself reflects light, so it can be seen even if the object inside cannot.