Category Archives: Continuum

Metamaterials and the Science of Invisibility

Invisibility is just one of the many features of Kiera Cameron‘s (Rachel Nichols) City Protective Services (CPS) uniform. A cloaking device, such as the one used by CPS officers, deflects light around it rendering it invisible. Such a device would be based on metamaterials; an artificial material specifically engineered to have properties not be found in nature.

Metamaterials

Prof. Sir John Pendry, the father of metamaterials.

The idea for such a material was first proposed by English physicist, Sir John Pendry in 2006 in a paper published in Science, “Controlling Electromagnetic Fields”. In this paper, Pendry describes how electromagnetic fields can be redirected around an object thereby rendering it invisible.

The way a material affects light is largely determined by its chemical composition. A metamaterial is different because its properties are derived from its physical structure; there are repeating microscopic patterns on the surface. Scientists can engineer this structure to bend light around an object thereby rendering it invisible.

Radar Absorbing Materials

HMS Argus using dazzle camouflage in 1918

HMS Argus using dazzle camouflage in 1918

Becoming invisible has always been an important part of military strategy since the 18th century and the emergence of the long-range rifle. Soldiers would camouflage themselves by either dressing in forest green or field grey. During World War I, troops started to experiment with “dazzle camouflage”.

The striped pattern of the dazzle camouflage is a poor choice to hide something as it draws attention to a person or an object. The pattern does have one advantage–it makes it difficult to estimate an enemy’s range. This concept was soon used on ships as it made it difficult for an observer to know exactly whether the stern or the bow was or whether the ship was moving towards or away from an observer’s position.

Dazzle camouflage continued to be used until World War II but its effectiveness was severely limited with the introduction of radar. The Germans sought to hide their craft with the use of radar-absorbing materials, the earliest of which was used on submarine periscopes and consisted of a layered coating of graphite particles and other semiconducting materials embedded in a rubber matrix. This material was effective at reducing reflections in the 20 cm radar band range used by the Allies.

Metamaterials

The Horten Ho’s flying wing design made it the first stealth aircraft.

The Germans went further by incorporating carbon-impregnated plywood in the Horten Ho 229–the first ever pure flying wing powered by jet engines. It was believed that the carbon powder in the glue would shield the aircraft from detection by the British early warning ground-based radar, known as Chain Home.

Carbon is a cheap material with low conductivity. This makes it possible tailor conductivity from synthetic materials made from the element. Conductivity can be very poor (if made from insulated grains of carbon black or soot) or very high (if made from connected chains of graphite). When an electric field encounters a carbon-based absorber, it induces electrical currents in the material which are then dissipated as heat.

Structure Dependent Properties

The hint that a material’s shape and structure at the nanoscale could affect its properties came in the 1990s. The British company, Marconi Materials Technology manufactured a carbon material capable of hiding battleships from radar but had no idea how it worked. They approached John Pendry to find the answer. Pendry discovered the electrical properties that allowed the material to absorb radiation didn’t come from the carbon itself but from the shape of its long thin fibers.

This was a significant discovery. Instead of changing a material’s chemistry to alter its behavior, scientists could alter the internal structure at very fine scales–smaller than a wavelength of light–to get the same effect.

All electromagnetic radiation has two components: a magnetic field and an electric field. As an electromagnetic wave strikes a material the atoms respond like a tiny magnet–its electrons move in a circle in response to the magnetic component and back and forth in response to the electrical component.

Each split-ring is designed to respond to the electromagnetic field in a certain way. When put together in an array with other split-rings, the periodic construction of many of these cells interacts with the electromagnetic wave as if these were homogeneous materials. This is similar to how light interacts with everyday materials, e.g. glass.

Each split-ring is designed to respond to the electromagnetic field in a certain way. When put together in an array with other split-rings, the periodic construction of many of these cells interacts with the electromagnetic wave as if these were homogeneous materials. This is similar to how light interacts with everyday materials, e.g. glass.

We can also create a magnetic field by looping a current around a circle. This magnetic field is more concentrated in the center of the loop than outside. Pendry hypothesized that by creating loops of a non-magnetic material, such as copper, he could create a similar magnetic response typically found in magnetic materials. Scientists would be able to tune how electrons move by tuning the size and shape of these loops. This controls how incoming radiation is bent when it encounters an object.

Pendry wondered how far this new insight could go. Would it be possible to change the magnetic properties of a material by simply changing its fine structure alone and not its chemistry? If so, then a theoretical non-magnetic metamaterial could mimic some of the properties of a magnetic substance like iron.

A split-ring resonator array constructed using copper split-ring resonatorss and wires mounted sheets of fiberglass circuit boards. The copper rings respond to the magnetic component while the mounted wires respond to the electroc field of an EM wave.

A split-ring resonator array constructed using copper split-ring resonatorss and wires mounted sheets of fiberglass circuit boards. The copper rings respond to the magnetic component while the mounted wires respond to the electroc field of an EM wave.

Pendry thought of taking this a step further. By cutting the loops, he created what is known as a magnetic resonator that acts like a switch. This switch would allow him to change a metamaterial’s magnetic properties on command. In so doing, by combining what he learned from Marconi’s radar absorbing material he figured a way to manipulate electromagnetic radiation. This makes invisibility possible for Continuum’s CPS officers.

Bending Light around Objects

Metamaterials

An example how an object appears invisible using mirrors.

Invisibility can be considered the supreme form of camouflage as it does not reveal anything about an object to an observer. We can accomplish this using a plane mirror and two parabolic mirrors to reflect light around an object. The object becomes invisible from two sides.

Metamaterials make an object invisible by bending light around it.

Metamaterials make an object invisible by bending light around it. (Image from Pendry et.. al. Science (2006): 1780-1782).

A metamaterials invisibility cloak will work in almost the same way as the parabolic mirrors by steering radiation around an object. The many tiny elements of a metamaterial–the fine scale structures–pick up rays or light from the far side of an observer and relay that ray around the material. When the ray arrives at the side facing the observer, it is re-emitted in the direction it would have taken as if the object was not there at all. Unlike the parabolic mirror trick, a metamaterial cloak will have to do so in all directions. To do this, we need a three-dimensional array of metamaterials

The fishnet metamaterial could one day become the invisibility cloak of the future.

The fishnet metamaterial could one day become the invisibility cloak of the future.

One way of accomplishing this is to create a “fishnet”. In this case the metamaterial is made of alternating sheets of glass and silver containing rectangular holes. This design was developed in 2008 at the University of California Berkeley. As light travels through the fishnet, the alternating layers bend light in unusual ways. The research group at Berkeley hope that this array will eventually be able to guide visible light around an object.

Other uses of Metamaterials

The use of metamaterials extend beyond manipulating the electromagnetic spectrum. It can also be used to create acoustic and tactile cloaks, preventing a user from being heard or felt. The acoustic cloak is made from perforated plastic sheets arranged in a pyramid. When it is placed over an object, sound waves act as if nothing is there, as if there was only a flat surface in their path.

The sound cloak is made of perforated plastic sheets in a periodic pattern.

The sound cloak is made of perforated plastic sheets in a periodic pattern.

The acoustic cloak alters a sound’s path in the same way the invisibility cloak does. Sound doesn’t penetrate into the pyramid but is rerouted in a way to create the impression that noting is there.

While it may one day be possible to completely hide future CPS officers in the visible spectrum and from being detected by sonar, what happens when they bump into something or, per chance, someone bumps into them? Metamaterials can also be designed to create a mechanical cloak.

Metamaterials prevent the object at the bottom from being felt.

Metamaterials prevent the object at the bottom from being felt.


Like the acoustic invisibility cloak, the tactile cloak is made from a periodic polymer array, the properties of which are determined by its special structure. AN object placed under a blanket or layers of foam would still be felt under the blanket. When an object is placed under this “cloak” it redirects stress in such a way that its shape can’t be felt when the cloak is touched.

The CPS Invisibility Suit: Is it possible?

The fishnet design has the advantage that it can handle a wide range of wavelengths; previous designs could only cloak at a specific wavelength.There are some disadvantages, however. It only works on flat surfaces and not the sleek CPS uniform we see Kiera wearing.

There is another limitation to wearing an invisible CPS uniform: an officer won’t be able to see out for the same reason you can’t see in. While the suit is invisible in a certain range of wavelengths–the visible spectrum–it is visible in other areas of the spectrum.

We know that a CPS officer’s cybernetic visual implants allow them to see in other parts of the spectrum, in the infra-red and possibly ultraviolet range. In a previous article, we discuss this ability may be based on graphene contact lens, a hexagonal mesh of carbon atoms. A CPS officer can activate their visual implants to see in other parts of the spectrum when they turn invisible.

We can only imagine what tech is hidden in Kiera’s CPS uniform. Though she has revealed some of her suits abilities to us we can only imagine what more she can do.