Optical invisibility: Science Fiction Movie Plot May Come True in the Future
The Principle of Invisibility
Have you ever fantasized about being invisible, going anywhere and taking anything you want, like in the movies? Have you ever longed for a Harry Potter invisibility cloak? None of this seemed possible before, and not even the U.S. military's stealth fighter could do the optical stealth of marvel's QuinJet. Because of its importance in the military field, scientists from all over the world have been making continuous attempts in relevant fields, and some achievements have been made in recent years. There are several simple and effective solutions for directional cloaking.360-degree stealth is theoretically possible, but no one has been able to do it yet.
So first, let's look at how invisible materials work. In a nutshell, there are two principles, one is absorbing waves, and the other is the special shape reduces reflection. At present, the solutions are as follows: first, the strong absorbing material as the shell, so that the radar can not detect; Second, a metamaterial with a negative refractive index, and a transformation optical structure based on it, bend the light.
Development of Invisible Materials
Negative refractive index materials were first proposed theoretically in 1967 by The Soviet physicist Victor Veselago. The calculation formula of refractive index is n=sinθ/sinθ‘. Normally, refraction in glass, water, etc., corresponds to a positive index of refraction. At that time Victor Veselago said that the index of refraction did not have to be positive and could be negative. His guess was not accepted by physicists at the time. It wasn't until 2001 that the Department of Physics at UCSD published a paper in Science reporting that their experimental scattering data at microwave wavelengths showed that their two-dimensional structure, on a macro level, had a left-handed, negative refractive index. Although it's only in certain microwaves, not in visible light, but it's definitely an epoch-making discovery in stealth technology. It was only after this that negative refractive index metamaterials began to become a craze. Gradually, in 2006, John Pendry of Imperial College, London, and David Smith, one of the authors, who had moved from UCSD to Duke university, published two papers in Science that formally proposed using negative refractive index materials for invisibility. In 2008, UC Berkeley's Xiang Zhang group published in Nature their 3-d negative refractive index material in the visible region. Visible light stealth technology is another step forward. Despite the advances, the level of negative refractive index metamaterials is actually only within the smaller physical size range, making a gold-plated surface with a small pit look flat in visible light. It is clear that the use of negative refractive index metamaterials to achieve invisibility is a long way off. But this revolutionary technology is the orthodox of stealth technology, based on which it is possible to achieve full dimensional 360-degree or even three-dimensional space Angle stealth.
The research of using strong absorbing materials to reduce the radar cross section is relatively mature. There are three common stealth methods: appearance design technology, material absorption technology, cancellation technology. The molecular composition of absorbing coating is more diverse, ferrite absorbing coating, four needle zinc oxide and so on. The thinner the coating, the wider the absorption spectrum and the greater the attenuation of the reflected wave, the better the absorption effect.
The Future of Invisible Materials
Because of its military and civilian implications, scientists are still working on stealth materials. With the development of technology, we have reason to believe that optical stealth may become a reality in the future