Introduction

For many years, specialists from different countries have been working on creating a universal camouflage for maximum camouflage of military personnel in various geographical and weather conditions. 

The first distinctive camouflages appeared during the Second World War and their pattern was kidney-shaped spots. 

In the late 1970s, the US Army introduced a new pattern called "Dual-Tex", which used perfect colored squares to simultaneously simulate two patterns, one large and one small, effective at different distances. 

By 1990, the development of patterns on the computer began, and with it a revival of scientific research. American officer Timothy O'Neill developed a new pattern - small colored squares on camouflage that could deceive the eye looking at a soldier or truck, since they combined them with the background scene.

Today, camouflage production primarily uses pixel patterns because pixels are better at simulating fractal patterns that the human eye interprets as white noise. If you look at such “digital” camouflage, your eyes simply have nothing to fixate on. 

In the 21st century, with the development of new technologies, new ideas are emerging on how to hide objects on the battlefield and beyond. In particular, scientists are considering the possibility of using optical invisibility technologies, which may soon become widely available.

Invisibility in optics is known and has been used practically for a relatively long time. Thus, one of the first most important problems solved at the Petrograd State Optical Institute, created in 1918, was the creation of optical glass. This required, in particular, a quick determination of its refractive index without a complex traditional procedure for treating the sample surface [1].

Proposed by the Soviet physicist I.V. Obreimov's method was as follows: the glass under study was crushed to grains about 0.5 mm in size and placed in a cuvette with flat walls. If you then pour any liquid into the cuvette, the light beam passing through the cuvette will be strongly scattered due to the sharp inhomogeneity of the refractive index of the medium.

However, scattering disappears if the liquid has exactly the same refractive index as the glass. You can select such a liquid by mixing, for example, benzene with carbon disulfide. Accordingly, the refractive index of glass is determined by the concentration of the liquids being mixed (when their refractive indices are known). 

This kind of invisibility of transparency also has some limitations. Thus, due to the difference in frequency dispersion (dependence of the refractive index on the frequency or wavelength of radiation) of glass and liquid, invisibility is disrupted when the wavelength changes, and therefore the radiation - the “meter” must have a narrow spectrum. 

In nature, jellyfish are almost invisible, the refractive index of which is close to the refractive index of water (a significant part of their volume is filled by the so-called mesoglea, a gelatinous substance, highly saturated - up to 97.5% - with water). It is also important that this option requires the coincidence of the refractive index of the environment and the object throughout its entire volume, which, obviously, is not easy to implement for an arbitrary masked object.

Significant progress towards the development of invisibility technology is associated with the possibility of manufacturing metamaterials - artificial environments with predetermined characteristics. 

1. Metamaterials

Metamaterials are artificial structures that resemble a very fine pattern made up of various substances: conductors and dielectrics. They got their name because they can interact with electromagnetic waves in a way that no natural materials alone can. 

Metamaterials do not exist in nature. These are exclusively man-made objects that, due to the created heterogeneity of their structure, make it possible to control the properties of light and achieve certain effects.

The main feature of metamaterials is a negative (or left-handed) refractive index, which manifests itself when the dielectric and magnetic permeabilities are simultaneously negative.

The refractive index is equal to the ratio of the sine of the angle of incidence of light on the material to the sine of the angle of refraction of light in the material. If you take air, its refractive index is very close to unity, so it does not distort the outlines of objects. Already water has a refractive index above unity, and a number of optical glasses have even more. 

The first theoretical justification for the possibility of their existence was given by Soviet physicist Viktor Veselago in 1968. 

A pioneer in the creation of such metamaterials was the physicist at Imperial College London, Sir John Pendry. In the mid-90s, he suggested that achieving the desired refraction angle is possible not so much due to the chemical composition of the molecules, but due to their location. 

The dimensions of internal structures embedded in the metamaterial must be less than the radiation wavelength. For example, microwaves can have a wavelength on the order of 3 cm, so if we want the metamaterial to bend the path of microwaves, we must implant smaller than 3 cm implants into it. But to make the object invisible to green light (with a wavelength of 500 nm), The metamaterial must have embedded structures only about 50 nm long. And nanometers are already an atomic scale; nanotechnology is required to work with such dimensions. 

It is worth noting that due to the wave nature of light, even metamaterials will not be able to camouflage any object perfectly. This is related to a statement proven by Adrian Nachman in 1988: by measuring the amplitude and direction of propagation of light rays (using a special detector), we can completely reconstruct the spatial profile of the refractive index of the medium through which they passed.

Currently, more and more new serious discoveries are being made in this area, so it is not surprising that some physicists expect the first samples of a real invisibility shield to appear in laboratories within a few decades. Thus, scientists are confident that in the next few years they will be able to create metamaterials that can make an object completely invisible, at least in two dimensions, to visible light of any specific frequency [2]. 

2. Advances in the creation of metamaterials

2.1 Superlens

John Pendry and colleagues argue in Physical Review Letters that materials with a negative refractive index can overcome the diffraction limit of resolution of conventional optics. In the right environment, the image space of the lens is not identical to the object itself because it is formed without damping waves. In the left medium, evanescent waves do not attenuate; on the contrary, their amplitude increases as the wave moves away from the object, so the image is formed with the participation of evanescent waves, which can make it possible to obtain images with a resolution better than the diffraction limit.

The first experimentally demonstrated lens with a negative refractive index had a resolution three times better than the diffraction limit. The experiment was carried out with microwave frequencies. In the optical range, a superlens was implemented. It was a lens that did not use negative refraction, but used a thin layer of silver to amplify the evanescent waves. To create a lens, alternating layers of silver and magnesium fluoride deposited on a substrate are used, onto which a nanograting is then cut. The result was a three-dimensional composite structure with a negative refractive index in the near-infrared region. In the second case, the metamaterial was created using nanowires that were electrochemically grown on a porous aluminum oxide surface [3]. 

2.2 Anti-mirror

British physicists created a type of reflective surface that did not previously exist in nature. To the naked eye, the new mirror reflects visible light just like a regular mirror. But in reality it is fundamentally different. Alexander Schwaneke and his colleagues from the NanoPhotonics Portfolio Center at the University of Southampton report on the achievement.

The magnetic mirror created by Schwaneke operates in this way: when an electromagnetic wave is reflected, it reverses the magnetic component of the oscillations, but does not affect the electric one. So, in comparison with a regular mirror, it can be called an anti-mirror. The mirror created by scientists works with the visible range of light waves, so theoretically you can look into it. Only it is very small - it is a square with a side of 500 micrometers. But even if such a mirror were made of macroscopic size, no one would be able to see the difference by eye.

The authors of the device say that its exotic properties can be useful in many experiments with light, in the creation of new types of photo sensors or elements of communication systems. Moreover, according to them, it is possible to build the same mirror for the infrared range.

The secret of the invention lies in the fact that this mirror is actually a metamaterial, that is, an ingenious combination at the micro level of ordinary substances that gives properties that are not inherent in any of them individually.

This mirror consists of two layers of substrate (first aluminum, silicon dioxide on top) and a working layer made of aluminum, but not solid, but in the form of an ordered structure of wavy nanowires forming a “fish scale” pattern. The size of the “scales” is less than the wavelength of the incident light. A million of such elements fit on the surface of this mirror. These “scales” are responsible for the reflection of the electromagnetic wave in such an irregular way [4].

2.3 Carbon nanotubes: the mirage effect

Scientists at the University of Texas at Dallas have developed carbon nanotube technology that allows objects to be “erased.” It is based on the mirage effect, or photothermal refraction. To make an object “disappear,” experts use cylindrical carbon molecules with high thermal conductivity. By turning the current on and off, scientists heat and cool the material, causing the object behind it to appear and disappear. The main problem with the Texas invention, however, is that for it to work, the hidden object must necessarily be in a container of water [5].

2.4 Invisibility Cloak

2.4.1 Development of the Pennsylvania State University invisibility cloak

As of 2015, the laboratory at Pennsylvania State University mainly works in the field of nanophotonics and plasmonics, as well as a little in the field of optical electronics and is developing an invisibility cloak. 

According to Pennsylvania State University professor Xinjie Ni, their laboratory is creating the optical properties of materials at the nanoscale, which will influence the behavior of light interacting with this material. Scientists have created small nanostructures (nanoantennas) that look like bricks with squares on top. These antennas have a unique property: they can resonate with the light falling on them and then scatter it. Nanoantennas can change the phase of light while maintaining its intensity. This allows them to fully reconstruct the wavefront and phase of the light they come into contact with. You can cover a randomly shaped object with this material, shine light on it, and it will look completely flat and blend in with its surroundings. In this way, you can hide the object from optical registration.

This cloak works by being wrapped around a 3D object, also a breakthrough from previous research. It is made of a very thin artificial material that can precisely envelop an object, like fabric or paper. The main advantage of the invention is that the problem with phases is solved in such a way that the phase-sensitive device is not able to “see” the cloak, and in addition to this, its dimensions can be freely increased. This is a two-dimensional manufacturing technology.

The cloak consists of two layers. One layer, the backing, is something like a gap between the backplate and the cloak itself, made of magnesium fluoride, a transparent dielectric used for the optical coating of glass and other lenses. The second layer is the antennas, made of gold - they look like little bricks with squares on top, and the proportions of the square affect the phase of the light. Magnesium fluoride has a very low refractive index. As for gold, it belongs to the group of so-called plasmonic materials - metals such as gold, silver, copper. An interesting property of them is that if you shine light on them, the free electrons inside them will begin to vibrate. Due to this, the resonance can be very strong in the case of different shapes.

Since this is a completely new development and this laboratory is the first to achieve invisibility in this way, scientists still have to work on scaling: despite the fact that in theory the dimensions can be changed to the desired parameters, now this possibility is limited for technical reasons. At this point, producing a large enough piece of such material would be insanely expensive, and in addition to this, the production technology may also not stand up to the size test. 

Another problem is that these antennas cannot have a very large reception angle. This means that even though the object can be hidden, it is still there and raised above the surface, which means it blocks light and for this reason casts a shadow when viewed from the plane in which it is located. When viewed from certain angles it is not noticeable, but from certain positions the shadow is visible.

“Our raincoat still has limitations in functionality, we are still trying to overcome them and improve our invention. It will still be a long time until this device becomes large enough and accessible,” says Professor Xinjie Ni [6].

2.4.2 Development of the Duke University invisibility cloak

Another real-life implementation of the “invisibility cloak” - albeit only for microwaves and for two dimensions - was demonstrated by David Smith, David Schurig (from the Pratt School of Engineering at Duke University (Duke University, Pratt School of Engineering) and John Pendry from Imperial College London (Imperial College London).

The device is a wide (several centimeters in diameter) but very low cylinder, surrounded by a number of concentric rings made of so-called metamaterials.

When this cylinder is irradiated edge-on (which is why scientists talk about the two-dimensional nature of this invisibility technology), the metamaterials deflect the waves so much that after the cylinder the shape of their front is almost exactly restored, as if there was no cylinder at all. True, the restoration is not complete, but very close to the original.

The new American device, called the "cloak of invisibility" by its authors, is one of the most complex structures ever made from metamaterials. The creators of the “cloak” used fiberglass and copper as ingredients.

While the cylinder's microwave hiding is impressive, the researchers say it's the first baby step toward new stealth technology. A three-dimensional version of the device and a more advanced wave deflection system will require further research [7].

2.5 Bluff wall

A new variation of stealth technology has been tested in laboratory conditions. True, the small device only works in the radio range, but its authors believe that the main thing is to demonstrate the principle. In theory, it can be extended to the visible spectrum, although this will take a lot of time.

A group of physicists from the Institute of Electronics of the Chinese Academy of Sciences (Institute of Electronics), Soochow University and Hong Kong Science and Technology University (HKUST) built an “invisible gate”. The system is almost the exact opposite of the invisibility cloak, reports Physics World. If the “cloak” creates the illusion of the absence of a real object, then the “gate” creates the impression that an object (in this case a wall) exists where in fact it does not exist (that is, there is an open channel).

The installation is based on networks of capacitors and inductors. They form a free cutout between two conductive walls. Moreover, one of them is a metamaterial with a negative dielectric constant index. The combination of different metamaterials forms plasmons (electron density waves) on their surface, which do not allow electromagnetic radiation to pass into an open opening.

The observer (provided that his vision operates in the range of 45-60 MHz) at the site of this opening will see a continuation of the surrounding walls [8].

2.6 Reflectin protein

Reflectin is a protein first identified in the Hawaiian short-tail squid. Reflectins are a group of proteins rich in aromatic and sulfur-containing amino acids used by some cephalopods to change skin color [9]. 

Scientists from the University of California, Irvine have succeeded in incorporating a protein found in squid cells into human kidney cells. This method made it possible to make them invisible. 

For their experiment, scientists chose the squid Doryteuthis opalescens. Females of this squid species can turn the white stripe along their back from opaque white to almost transparent. They do this using specialized cells called leucophores, which contain particles made of reflector proteins.

Depending on how these proteins are positioned, they can change the way light is transmitted or reflected around them. And this is not a random process: squids can change the location of these highly refractive proteins in their cells using an organic chemical called acetylcholine. To harness the squid's "superpower" in human tissue, the research team genetically engineered human kidney cells to produce reflectin. Using quantitative phase microscopy, the researchers showed that these proteins changed the way light scattered through the engineered cells compared to kidney cells without reflectin. They then exposed the cells to varying levels of sodium chloride and found that they could adjust the levels of light passing through them, as the salt caused the reflected light particles to increase in size and change their location. 

The team says their success lays the groundwork for incorporating other squid features into mammalian cells. This will also allow researchers to further explore the mechanisms underlying these abilities [10].

Thanks to the principles on which the squid's skin color-changing function is based, scientists have also created a camouflage coating that works effectively in the infrared range.

The research group, through genetic engineering, has developed a special strain of bacteria Escherichia coli (Escherichia coli), which intensively synthesizes reflectin. Having produced a sufficient amount of this protein, scientists covered the surface of a special polymer material with it and forced it to increase in volume by treating it with vapors of concentrated acetic acid.

It is claimed that as a result of processing, the film of the polymer material acquired the ability to completely reflect infrared “light”. In other words, an object covered with such a film completely blends into the surrounding background and is invisible to a thermal imager, infrared camera or night vision device.

For military use, reflectin can be applied to the surface of an elastic polymer coated on the reverse side with an adhesive composition similar to adhesive tape. Such a coating can be glued both to the surface of the mold and various types of equipment, completely or partially hiding it from infrared detection means.

Before using the know-how for practical purposes, scientists will have to increase the reflection rate of infrared waves, as well as develop a technology for producing film sheets of a sufficiently large area [11].

2.7 Development of Quantum Stealth from Hyperstealth

A few years ago, a video circulated on the Internet about a new material that could make various objects “disappear.” The distributor turned out to be Hyperstealth Biotechnology Corp., which presented the new material.

Since 2012, Hyperstealth has been working with the US and Canadian militaries to develop Quantum Stealth, a plastic sheet material that bends light around an object, hiding it from view. 

Quantum Stealth works using biconvex lenses, a technology commonly used in plastic cards with a 3D effect or movie posters. In biconvex lenses, rows of cylindrical lenses refract light according to viewing angle. The company has found a way to arrange the layers of these biconvex lenses in such a way that “dead spots” are created at specific distances behind the material. When viewed from the front, the object behind the material is not visible, but the background is visible. This creates the illusion of invisibility. The company also claims that their material is capable of capturing infrared and thermal radiation. 

The developers emphasize that the object “does not disappear,” but rather “blurs,” so in any case it will not be possible to completely hide from observers. [13]. 

Conclusion

Optical stealth technologies are of interest mainly to the military industry, as these materials can hide or distort a potential target. Otherwise, the use of this technology is controversial, because there is a possibility that invisibility technology will be used for illegal purposes, for example, by robbers or for espionage purposes. In any case, while such technologies are still at the stage of scientific research, it is difficult to talk about their real practical application and commercial benefit.

According to scientists from the Cockrell School of Engineering at the University of Texas at Austin, new research now confirms that invisibility is theoretically possible, but in practice it has not yet been possible to hide objects from more than one wavelength of light at the same time. 

Speaking about the development of this technology in the Republic of Kazakhstan, it should be noted that this will require certain material investments, since we are talking about nanotechnologies. Also, transformation optics, a branch of science that has become possible thanks to recent advances in the field of metamaterials, involves theoretical, experimental, and technological research. For this purpose, appropriate laboratories with modern equipment are needed. 

The world leaders in terms of total investment in nanotechnology today are the EU countries, Japan and the USA. Recently, Russia, China, Brazil and India have significantly increased investments in this industry. 

Despite the fact that today engineering laboratories have been created at some universities in Kazakhstan in areas of scientific and technological development, including the development of nanotechnologies, the country does not yet have a strong school for the creation and development of nanotechnologies and nanoindustry. These laboratories are not sufficiently equipped with modern technological equipment for obtaining metamaterials, and there is still a shortage of qualified personnel to supply this industry. 

On the basis of the National Open Nanotechnology Laboratory of the NAO KazNU named after. al-Farabi already has electron lithography technology, which makes it possible to create the necessary nanometer-scale structures (from 80 nm) on the surface of materials. At the same time, the total surface area of ​​the created structures does not exceed 100-150 microns (test samples). 

According to Kazakh scientists, to create prototypes of camouflage coatings, such as Quantum Stealth, it is necessary to purchase or create appropriate scaling technology.