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This article is from the official account of Wechat: back to Park (ID:fanpu2019), by Xu Lei (Professor, Department of Physics, the Chinese University of Hong Kong)

Humans have imagined an ability to be invisible since ancient times, and "invisibility" still appears frequently in literary works or video games-often in the form of magic or science fiction. The invisibility cloak of the protagonist in the popular Harry Potter all over the world satisfies our fantasy about the invisibility cloak. In fact, the optical invisibility cloak that allows light to "pass through" the body has been developed. Not only that, scientists have carried out extensive and in-depth exploration of the invisibility of matter in different physical environments, thus constructing different types of invisibility cloak, which is definitely beyond your imagination.

Realizing invisibility has always been a dream of human being since ancient times: from Sun Dasheng's invisibility technique in Journey to the West to Harry's invisibility cloak in Harry Potter, this dream has attracted wide attention from people all over the world. In a narrow sense, stealth only refers to invisibility with the eyes, while in a broader sense, stealth is not limited to the eyes, but generally refers to the use of a certain detection means (such as radar, sonar) can not be detected, which is called stealth under this detection means. The realization of stealth has important military applications, such as stealth aircraft, stealth ships and so on can quietly approach the target, so stealth technology has become the research focus of various countries in the military field. With the development of science and technology, all kinds of invisibility cloaks have been successfully realized in many different physical fields, and continue to become research hotspots in different fields. What is the principle of invisibility cloak? What different invisibility cloaks do you have? How are they implemented? If you can wear an invisibility cloak one day, what kind of surprise (shock) are you going to give to your friends and relatives? If you are interested in these questions, this article is carefully prepared for you.

The working principle of invisibility cloak in order to achieve invisibility, let's first understand why we can see. A more common situation is that light reflected or scattered by an object enters our eyes so that we can see the object (as shown in the following picture):

Junior high school physics textbooks (people's Education Edition) know this principle, can not help but give rise to a bold idea: if a piece of clothing can completely absorb light without reflecting or scattering any light, can it be invisible? As we all know, black can absorb light very well, wear a super black cloak to cover the whole body (as shown in the following picture), can you do whatever you want on the street? It's far from that simple.

The stills of "Batman: the Mystery of the blur" are obvious. Although Batman's black clothes absorb all the light in the image above, the contrast with the bright background completely exposes Batman, so it is impossible to become invisible by absorbing light alone.

Since the reflection, scattering and absorption of light will expose its position, how can we achieve stealth? Harry Potter's invisibility cloak provides us with the right idea (as shown below):

In the stills of Harry Potter and the Sorcerer's Stone, Harry Potter's invisibility cloak neither reflects nor scatters nor absorbs light, but allows the light to travel unaffected. When the light around Harry is free to pass through his body without any influence, his body "disappears" out of thin air-we can clearly see the background wall behind him but not his body. Therefore, the basic principle of designing an invisibility cloak is to make the propagation of light unaffected by objects.

Although the principle is simple, implementation is difficult: because most objects are opaque, light cannot penetrate them and continue to propagate. How to make the light unaffected by the object? At present, the basic idea is to design a special structure of material to wrap the object, and this special material allows light to bypass the object and continue to travel along the original path (as shown in the following figure). The material of this special structure is the invisibility cloak.

Source: how does gfycat.com get light around an object? The common practice is to calculate the required refractive index theoretically, and then find the materials with this refractive index to achieve it experimentally. An important method of theoretical calculation is the theory of transformation optics proposed by Professor John Pendry and his collaborators at Imperial College London in 2006. The key is to transform a point with zero volume into a region with non-zero volume by means of coordinate transformation, and hide the object in this special region. Since a point with zero volume will not have any effect on light propagation, the special area it transforms will have similar properties, so as to achieve the effect of light bypassing the object. The specific refractive index parameters can be obtained from the coordinate transformation, as shown in the following figure:

Professor John Pendry

Spatial transformation: transform the point into a region, so the essence of the invisibility cloak is to create an area around which the light will bypass and hide the object in it, thus achieving invisibility. This approach can already be achieved in experiments, such as using several lenses to create an area that the light will completely bypass, which is completely undetectable when an object is placed in this area. Well, this special space can be called a peekaboo artifact, right? Do you also want to own and use it to implement some bold plans?

Source: Choi, J. S., & Howell, J. C. Optics Express (2014) the realization of different types of invisibility cloak this kind of stealth space created by lens is feasible, but it is not nearly as convenient and practical as the invisibility cloak-you can achieve invisibility with a cover over the object. To achieve an invisibility cloak like Harry Potter, we need to control the deflection of light and get it around objects at will. In general, this kind of deflection can not be realized in natural materials, and only negative refractive index materials can be achieved successfully. In order to achieve negative refractive index, we need to use a special material: superstructure material (Metamaterial).

Photo Source: Wikipedia superstructured materials are usually not natural materials, but artificial materials constructed through the design and special spatial arrangement of functional elements. It can show many novel and extraordinary physical properties, and is widely used in optics, mechanics, acoustics and many other fields, such as optical negative refractive index materials, mechanical negative Poisson's ratio materials, acoustic sound absorption materials and so on.

Left: negative refractive index superstructured material. Right: positive Refractive Index ordinary Materials Photo Source: Dolling et al., Optics Express, 2006. (1) electromagnetic wave invisibility cloak using superstructured materials, scientists first realized the electromagnetic wave invisibility cloak at microwave frequency in 2006 (see figure below). Like visible light, microwaves are electromagnetic waves, except that the wavelengths are on a longer scale (millimeters to meters). Our commonly used microwave ovens use electromagnetic waves in this band to heat food. Although the invisibility cloak does not look "invisible" at all at visible light wavelengths, it is invisible at certain microwave wavelengths (3.5cm), and its internal objects are hidden and cannot be detected by such wavelengths of 3.5cm.

(a) structural design of microwave invisibility cloak. (B) the numerical simulation of the invisibility cloak shows that the microwave continues to spread through the invisibility cloak. (C) the experimental results are consistent with the simulation results. Image source: D. Schurig, et al., Science, 314977 (2006) following the microwave invisibility cloak, the invisibility cloak for visible light has also been developed by scientists, and many different types have been developed. A common design is a carpet invisibility cloak: it hides an object under a carpet stealth device, making it invisible to nearby observers like flat ground. The design principle is essentially to reflect or refract the light that should be shining on the object through a special mirror or hidden device, bypassing the object and propagating along the original path.

Photo Source: wikimedia

Image source: IOA (2) Optical invisibility cloak using this design, scientists successfully realized the carpet optical invisibility cloak in 2009 (see image below). When there is no invisibility cloak, objects scatter light in all directions (bottom left). Once you put on the invisibility cloak, the light travels in exactly the same direction as the smooth floor (middle, bottom): distant observers will mistakenly think that only the floor has no objects (bottom right). This successfully achieved invisibility.

Source: Valentine, Jason, et al. Nature Materials 8 (7), 568-571 (2009). There is also a very important practical principle when designing invisibility cloaks, that is, the thinner the invisibility cloak, the better. If you can wear it like a thin piece of clothing, it will naturally be more convenient and practical than a set of heavy armor. According to this principle, scientists have developed an ultra-thin optical invisibility cloak: by covering the surface of an object with an 80-nanometer-thick nano-antenna, the nano-antenna can adjust not only the direction of the reflected light but also the phase of the reflected light. to reflect light like a smooth plane and disguise the object as a smooth ground (see image below). This ultra-thin nano-antenna is obviously very similar to the invisibility cloak, but it can only work on specific wavelengths of light (730nm). In order to achieve a broad-spectrum invisibility cloak that works throughout the visible light band, the current technology is not yet mature and scientists need to make further efforts.

Source: Ni, Xingjie, et al., Science 349.6254 (2015): 1310-1314. Obviously, advanced science and technology has gradually turned the optical invisibility cloak from film and television stories into reality. Is it possible to frighten your friends at any time by putting on an optical invisibility cloak? Smart readers may say: I can detect the approach of others from the sound of footsteps even if I close my eyes. In fact, sound waves and ultrasonic waves can also be used to detect objects, such as bats flying in the dark, B-ultrasound examination, sonar detection of schools of fish, submarines and so on. Accordingly, the research on the invisibility cloak of sound waves and ultrasonic waves is also in full swing.

(3) Acoustic invisibility cloak the basic principle of acoustic invisibility cloak is the same as that of optical invisibility cloak: the acoustic invisibility cloak can be realized if the object has no influence on the propagation of sound waves. As the following example shows, scientists have designed and implemented an acoustic invisibility cloak of superstructural materials composed of 16 concentric rings: these concentric rings can guide sound waves around the central object to achieve acoustic stealth. If only the object is placed in the sound field, it will have a greater impact on the sound field (top picture), while with the invisibility cloak, the sound field propagation will not be affected (bottom picture). Acoustic invisibility cloak also has a very important application prospect in many fields, such as the anti-sonar detection of submarine, the design of sound insulation facilities and so on.

Source: Zhang, S.S., Xia, C., & Fang, N. (2011). Physical review letters, 106 (2), 024301 (4) are there any other types of invisibility cloaks besides optical invisibility cloak, acoustic invisibility cloak? The answer is yes: thermal invisibility cloak is also a common invisibility cloak. As we all know, many objects, including the human body, have different temperatures from the surrounding environment, so according to this temperature, temperature measuring devices such as infrared detectors can be used to find objects. Therefore, scientists can design the corresponding thermal invisibility cloak: through a similar principle, we can design the thermal superstructure material for the heat flux propagation equation and make the corresponding thermal invisibility cloak. When wearing this thermal invisibility cloak, the temperature of the human body is consistent with the background temperature, thus achieving thermal stealth, as shown in the infrared photo below.

Image source: Adam Harvey is inspired by these invisibility cloaks, and recent research hotspots have begun to focus on stealth in the flow field. The invisibility cloak of the flow field pursues to minimize the disturbance of the object to the flow field, which makes it impossible for the outside world to detect the object from the change of the flow field. This kind of flow field invisibility cloak was first realized in the flow field of porous media in 2019, as shown in the following diagram: the flow field is flat (a) without an object, the flow field is disturbed (b), and the flow field is flattened (c) when covered with a layer of flow field invisibility cloak. This kind of flow field stealth cloak is of great significance in the fields of underwater vehicle stealth and underwater drag reduction.

Image source: J Park, JR Youn, YS Song, Physical review letters, 2019 similarly, the flow field invisibility cloak is as thin as possible. In view of this, our team (Professor Xu Lei's research team of the Chinese University of Hong Kong) has developed the thinnest shell invisibility cloak in the world. Our design skillfully combines the inner layer and hidden objects of the double-layer stealth design, further reducing the double-layer stealth design to a single-layer stealth, thus achieving the thinnest flow field invisibility cloak-its thickness is only 3/1000 of that of the hidden object in the middle. As shown in the following figure: the theoretical calculation shows that the flow field is flat (A) when there is no object, the flow field is distorted (B) after the object is placed, and the flow field becomes flat again (C) after covering our ultra-thin flow field invisibility cloak. Numerical simulations show similar results (D, E, F). The experiment further verifies that the streamline is straight (G) when there is no object, the streamline becomes curved (H) after putting into the object, and the streamline becomes straight again after putting on our ultra-thin invisibility cloak (I). This kind of ultra-thin invisibility cloak is of great significance to realize the near-field stealth of objects.

Source: Chen, M., Shen, X., & Xu, L. (2022). The Innovation, 100263. To sum up, there are different physical fields in nature, such as light field, sound field, temperature field, flow field, and so on. Various invisibility cloaks can be developed for different physical fields as an effective means of anti-detection. These invisibility cloaks can be used alone or combined to form super invisibility cloaks for multiple physical fields. With the increasing improvement of detection methods, the stealth research of anti-detection is also a rising tide. What new cool techs will be born out of it? Let's wait and see!

Thank you: I would like to thank Dr. Chen Menglian and Dr. Shen Xiangying for their materials and pictures for this article.

reference

1. Choi, J. S., & Howell, J. C. Optics Express, Vol. 22, Issue 24, pp. 29465-29478 (2014).

2. Gunnar Dolling, Martin Wegener, Stefan Linden, and Christoph Hormann, Optics Express, Vol. 14, Issue 5, pp. 1842-1849 (2006).

3. D. Schurig, et al., Science, 314,977-980 (2006).

4. Valentine, Jason, et al. Nature Materials 8 (7), 568-571 (2009).

5. Ni, Xingjie, et al., Science 349,6254, 1310-1314 (2015).

6. Zhang, S.S., Xia, C., & Fang, N, Physical review letters, 106,024301 (2011).

J Park, JR Youn, YS Song, Physical review letters, 123,074502 (2019).

8. Chen, M., Shen, X., & Xu, L. The Innovation, 3 (4), 100263 (2022).

Product: popular science China

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