Researchers from the University of California, Berkeley, have recently taken a step towards the dream of "invisibility cloak".
This research group, led by Zhang Xiang, a professor in China, has successfully developed a new three-dimensional material, which can make light bend when it passes through, thus mysteriously "disappearing". For example, when running water passes a stone, it will bypass the stone and move on, as if it had never met a stone.
The emergence of metamaterials
The material developed by Zhang Xiang and his colleagues can change the direction of light propagation because it has the characteristic of "negative refraction". In contrast, all natural materials have a positive refractive index.
The refraction process can be illustrated by a classic illustration: the part of chopsticks inserted into the water seems to bend in the direction of the water surface. If the water is negatively refracted, the submerged part of chopsticks seems to jump out of the water. If we use a fish instead of chopsticks, we can see a similar effect.
Because natural materials can't achieve "negative refraction", scientists thought of artificially developing a metamaterial. Through the artificial design of material structure, extraordinary material functions beyond the inherent commonness of nature can be obtained.
The theoretical and experimental development of metamaterials directly gave birth to the research of "invisibility cloak". At the beginning of 2006, Professor John Pendry of Imperial College London, London, put forward the feasible idea of "invisibility cloak". Metamaterials can make light bypass objects, thus making them invisible. At the end of that year, Pan Derui and scientists such as David Schurig and david smith from Duke University demonstrated a metamaterial.
In recent two years, metamaterials have gradually become a hot research topic in the world.
However, the magic of metamaterials made by scientists is still very limited: they have only succeeded in single-layer two-dimensional materials, and the "negative refraction" characteristic only appears in the microwave range. For light with shorter wavelength, such as visible light adapted by human eyes, there is nothing we can do. In other words, these metamaterials cannot be made into "invisibility cloaks" that disappear in front of that kind of people.
The disappearance of visible light
The research team led by Zhang Xiang promoted the research of metamaterials and invisibility cloaks.
This research group published papers in the online edition of Nature published on August 13 and Science published on August 15, respectively, and reported two methods of synthesizing metamaterials.
In the paper Nature, the research team described a three-dimensional "fishing net" shaped metamaterial. They stacked conductive silver and non-conductive magnesium fluoride alternately, and dug up a fishing net pattern with nanometer size (the diameter of a hair is roughly equivalent to 65438+ 1 100 million nanometers) between the layers.
The three-dimensional metamaterial obtained by Berkeley researchers, the left picture shows the structure, and the right picture shows the picture under scanning electron microscope.
In this way, negative refraction occurs in the range where the wavelength does not exceed 1500 nm at most, that is, near infrared rays. The researchers explained that each pair of adjacent conductive layers will form a current loop, and alternating stacking will produce a series of loops, which are used to respond to the magnetic field generated by incident light, thus deflecting the light.
In scientific papers, researchers take another approach. This metamaterial is composed of silver nanowires embedded in porous alumina, which can make red light (visible light) with wavelength less than 660 nm negatively refract in infrared band.
This is also the first time that scientists have achieved "negative refraction" in the visible light band.
Zhang Xiang told the media: "We have made a large negative refraction metamaterial in a wide wavelength spectrum by two completely different methods, with little energy loss, which is a step towards the practical application of metamaterials."
How far is the invisibility cloak?
So, when can I put on the "invisibility cloak"?
To achieve invisibility, it is theoretically necessary to achieve negative refraction in all visible light bands, but scientists have not been able to do this at present.
Yao Jie, a member of Zhang Xiang's research group and one of the main authors of the Science paper, told Caijing that although these two technologies have been successful, there are still some technical difficulties in making visible light invisible. The metamaterials embedded silver nanowires in porous alumina, which he participated in, have no effect on other wavelengths of light, such as blue light, except red light. "The deflection of different lamps is different, which is also an important problem we will face in the next research work."
Of course, light, or electromagnetic waves, has a wide band. Even if it is invisible in all visible light bands and "disappears" in front of people, it can still be detected by other means if it cannot be invisible in other bands.
Realizing the invisibility of the required band is just one of many problems that scientists need to face.
Yao Jie, for example, said that with the current technology, "there is no way to make larger visible metamaterials". In other words, metamaterials can't be produced on a large scale at present, and they can't be made into the required shapes at will. At present, the "large-scale metamaterials" that Berkeley researchers can manufacture are only a few square millimeters at most.
In addition, this metamaterial is made of metal and is very fragile.
Therefore, it is difficult to predict when the "invisibility cloak" will become a reality.
In fact, the launch of "invisibility cloak" is not the main purpose of scientists to study metamaterials. Metamaterials are more likely to play a more direct role in nano-imaging and semiconductor industry. For example, the use of metamaterials is expected to produce smaller and more accurate semiconductor components while reducing manufacturing costs.
Perhaps the military is most interested in metamaterial research. Compared with daily life, the military needs stealth technology more urgently.
It is understood that the research of Berkeley scientists has not only been funded by the National Science Foundation of the United States, but also received project funds from the US military.