Invisibility Cloak

An afocal system (derived from greek deprivation prefix "a-" and the adjective "focal") is an optical system for which a beam of light coming from infinity exits with an infinite focal length. In other words, it is an optical system that effectively doesn't focus a beam of light. In the case of a two lens system, it can easily be shown that an afocal system occurs when the distance d between the elements is equal to:

d = f₁ + f₂  

where f_1, f₂ are their focal lengths.

Dr Eva Philippaki, 6CCP3380 Optics module

On this very simple principle it is based how to collimate a laser beam or how to create a telescope. The beam of light though doesn't pass the system remaining "intact". If f₂>f₁  then our beam will expand with a magnification M:

M=f_2/f₁

and respectively shrink in the case where f_2< f_1.

Now, hold the above information for the moment. In optical research as well as in fiction for sure, a lot of discussion is done around how to create real-life invisibility cloaks. The answer is most likely to be given by the use of metamaterials, an area in which our Photonics and Nanotechnology Group is specialised. Meanwhile, researchers from the University of Rochester have suggested an optical setup that can work as an invisibility cloak in one dimension.

In summer 2017, we decided to make a similar to the Rochester Cloak at King's College London, with PhD student Michela Picardi and the support of Physics undergrad students. Photos below show our first results at the Wheatstone Lab.

Dr Eva Philippaki, KCL Wheatstone Lab, July 2017

As shown in diagram below, the basis of this optical cloak is a symmetrical double telescopic system, consisting of two types of lenses: (a) internal lenses are of a high dioptric power and (b) outer lenses of a low power. The highlighted in yellow areas act as "blind areas" where you can hide objects!

Optical cloaking device diagram

We had the chance to demonstrate our device at the London Light Launch Event as part of the celebrations for the International Day of Light. We are now happy to demonstrate our device this summer at the Royal Society Summer Science Exhibition Lates event, July 3rd along with other researchers from KCL P&N Group. Event is open to the public (adults only) and admission free. Come along to interact yourself and explore how you can control light with simple geometrical optics!