“Invisibility Caps” are still invisible

The latest in a series of "cloaks of invisibility" is the one born at the University of Rochester (1), which uses the appropriate optical system. However, skeptics call it some kind of illusionistic trick or special effect, in which a clever lens system refracts light and deceives the observer's vision.

There's some pretty advanced math behind it all—scientists need to use it to find how to set up the two lenses so that the light is refracted in such a way that they can hide the object directly behind them. This solution works not only when looking directly at the lenses - an angle of 15 degrees or another is enough.

1. "Invisibility Cap" from the University of Rochester.

It can be used in cars to eliminate blind spots in mirrors or in operating rooms, allowing surgeons to see through their hands. This is another in a long series of revelations about invisible technologythat have come to us in recent years.

In 2012, we already heard about the "Cap of Invisibility" from the American Duke University. Only the most inquisitive read then that it was about the invisibility of a small cylinder in a tiny fragment of the microwave spectrum. A year earlier, Duke officials reported on sonar stealth technology that may seem promising in some circles.

Unfortunately, it was invisibility only from a certain point of view and in a narrow scope, which made the technology of little use. In 2013, the tireless engineers at Duke proposed a 3D printed device that camouflaged an object placed inside with micro-holes in the structure (2). However, again, this happened in a limited range of waves and only from a certain point of view.

The photographs published on the Internet looked promising cape Canadian company Hyperstealth, which in 2012 was advertised under the intriguing name of Quantum Stealth (3). Unfortunately, working prototypes have never been demonstrated, nor has it been explained how it works. The company cites security issues as the reason and cryptically reports that it is preparing secret versions of the product for the military.

Front monitor, rear camera

First moderninvisibility cap» Introduced ten years ago by Japanese engineer Prof. Susumu Tachi from the University of Tokyo. He used a camera positioned behind a man wearing a coat that was also a monitor. The image from the rear camera was projected onto it. The cloaked man was "invisible". A similar trick is used by the Adaptiv vehicle camouflage device introduced in the previous decade by BAE Systems (4).

It displays an infrared image "from behind" on the tank's armor. Such a machine is simply not seen in sighting devices. The idea of ​​masking objects took shape in 2006. John Pendry of Imperial College London, David Schurig and David Smith of Duke University published the theory of "transformation optics" in the journal Science and presented how it works in the case of microwaves (longer wavelengths than visible light).

2. An "invisibility cap" printed in three dimensions.

With the help of appropriate metamaterials, an electromagnetic wave can be bent in such a way as to bypass the surrounding object and return to its current path. The parameter characterizing the general optical reaction of the medium is the refractive index, which determines how many times slower than in vacuum, light moves in this medium. We calculate it as the root of the product of relative electric and magnetic permeability.

relative electric permeability; determines how many times the electrical interaction force in a given substance is less than the interaction force in vacuum. Therefore, it is a measure of how strongly the electrical charges within a substance respond to an external electric field. Most substances have a positive permittivity, which means that the field changed by the substance still has the same meaning as the external field.

The relative magnetic permeability m determines how the magnetic field changes in a space filled with a given material, compared to the magnetic field that would exist in a vacuum with the same external magnetic field source. For all naturally occurring substances, the relative magnetic permeability is positive. For transparent media such as glass or water, all three quantities are positive.

Then light, passing from vacuum or air (air parameters are only slightly different from vacuum) into the medium, is refracted according to the law of refraction and the ratio of the sine of the angle of incidence to the sine of the angle of refraction is equal to the refractive index for this medium. The value is less than zero; and m means that the electrons inside the medium move in the opposite direction to the force created by the electric or magnetic field.

This is exactly what happens in metals, in which the free electron gas undergoes its own oscillations. If the frequency of an electromagnetic wave does not exceed the frequency of these natural oscillations of electrons, then these oscillations screen the electric field of the wave so effectively that they do not allow it to penetrate deep into the metal and even create a field directed oppositely to the external field.

As a result, the permittivity of such a material is negative. Unable to penetrate deep into the metal, electromagnetic radiation is reflected from the surface of the metal, and the metal itself acquires a characteristic luster. What if both types of permittivity were negative? This question was asked in 1967 by the Russian physicist Viktor Veselago. It turns out that the refractive index of such a medium is negative and light is refracted in a completely different way than follows from the usual law of refraction.

5. Negative refraction on the surface of a metamaterial - visualization

Then the energy of the electromagnetic wave is transferred forward, but the maxima of the electromagnetic wave move in the opposite direction to the shape of the impulse and the transferred energy. Such materials do not exist in nature (there are no substances with negative magnetic permeability). Only in the 2006 publication mentioned above and in many other publications created in subsequent years, it was possible to describe and, therefore, build artificial structures with a negative refractive index (5).

They are called metamaterials. The Greek prefix "meta" means "after", that is, these are structures made from natural materials. Metamaterials acquire the properties they need by building tiny electrical circuits that mimic the magnetic or electrical properties of the material. Many metals have a negative electrical permeability, so it is enough to leave room for elements that give a negative magnetic response.

Instead of a homogeneous metal, a lot of thin metal wires arranged in the form of a cubic grid are attached to a plate of insulating material. By changing the diameter of the wires and the distance between them, it is possible to adjust the frequency values ​​at which the structure will have a negative electrical permeability. To obtain negative magnetic permeability in the simplest case, the design consists of two broken rings made of a good conductor (for example, gold, silver or copper) and separated by a layer of another material.

Such a system is called a split ring resonator - abbreviated as SRR, from the English. Split-ring resonator (6). Due to the gaps in the rings and the distance between them, it has a certain capacitance, like a capacitor, and since the rings are made of conductive material, it also has a certain inductance, i.e. ability to generate currents.

Changes in the external magnetic field from the electromagnetic wave cause a current to flow in the rings, and this current creates a magnetic field. It turns out that with an appropriate design, the magnetic field created by the system is directed opposite to the external field. This results in a negative magnetic permeability of a material containing such elements. By setting the parameters of the metamaterial system, one can obtain a negative magnetic response in a fairly wide range of wave frequencies.

meta - building

The dream of the designers is to build a system in which the waves would ideally flow around the object (7). In 2008, scientists at the University of California, Berkeley, for the first time in history, created three-dimensional materials that have a negative refractive index for visible and near-infrared light, bending light in a direction opposite to its natural direction. They created a new metamaterial by combining silver with magnesium fluoride.

Then it is cut into a matrix consisting of miniature needles. The phenomenon of negative refraction has been observed at wavelengths of 1500 nm (near infrared). In early 2010, Tolga Ergin of Karlsruhe Institute of Technology and colleagues at Imperial College London created invisible light curtain. The researchers used materials available on the market.

They used photonic crystals laid on a surface to cover a microscopic protrusion on a gold plate. So the metamaterial was created from special lenses. The lenses opposite the hump on the plate are located in such a way that, by deflecting part of the light waves, they eliminate light scattering on the bulge. By observing the plate under a microscope, using light with a wavelength close to that of visible light, the scientists saw a flat plate.

Later, researchers from Duke University and Imperial College London were able to obtain a negative reflection of microwave radiation. To obtain this effect, individual elements of the metamaterial structure must be less than the wavelength of light. So it's a technical challenge that requires the production of very small metamaterial structures that match the wavelength of light they're supposed to refract.

Visible light (violet to red) has a wavelength of 380 to 780 nanometers (a nanometer is one billionth of a meter). Nanotechnologists from the Scottish University of St. Andrews came to the rescue. They got a single layer of extremely densely meshed metamaterial. The pages of the New Journal of Physics describe a metaflex capable of bending wavelengths of about 620 nanometers (orange-red light).

In 2012, a group of American researchers at the University of Texas at Austin came up with a completely different trick using microwaves. A cylinder with a diameter of 18 cm was coated with a negative impedance plasma material, which allows manipulation of the properties. If it has exactly the opposite optical properties of the hidden object, it creates a kind of "negative".

Thus, the two waves overlap and the object becomes invisible. As a result, the material can bend several different frequency ranges of the wave so that they flow around the object, converging on the other side of it, which may not be noticeable to an outside observer. Theoretical concepts are multiplying.

About a dozen months ago, Advanced Optical Materials published an article about a possibly groundbreaking study by scientists at the University of Central Florida. Who knows if they failed to overcome the existing restrictions on "invisible hats» Built from metamaterials. According to the information they published, the disappearance of the object in the visible light range is possible.

7. Theoretical ways of bending light on an invisible object

Debashis Chanda and his team describe the use of a metamaterial with a three-dimensional structure. It was possible to get it thanks to the so-called. nanotransfer printing (NTP), which produces metal-dielectric tapes. The refractive index can be changed by nanoengineering methods. The light propagation path must be controlled in the three-dimensional surface structure of the material using the electromagnetic resonance method.

Scientists are very cautious in their conclusions, but from the description of their technology it is quite clear that coatings of such a material are capable of deflecting electromagnetic waves to a large extent. In addition, the way the new material is obtained allows the production of large areas, which has led some to dream of fighters covered in such camouflage that would provide them with invisibility complete, from radar to daylight.

Concealment devices using metamaterials or optical techniques do not cause the actual disappearance of objects, but only their invisibility to detection tools, and soon, perhaps, to the eye. However, there are already more radical ideas. Jeng Yi Lee and Ray-Kuang Lee from the Taiwan National Tsing Hua University proposed a theoretical concept of a quantum "cloak of invisibility" capable of removing objects not only from the field of view, but also from reality as a whole.

This will work similar to what was discussed above, but the Schrödinger equation will be used instead of Maxwell's equations. The point is to stretch the object's probability field so that it is equal to zero. Theoretically, this is possible at the microscale. However, it will take a long time to wait for the technological possibilities of manufacturing such a cover. Like any "invisibility cap“Which can be said that she was really hiding something from our view.