Metamaterials: Bending light backwards

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The light that you and I see with our eyes is just a small range of frequencies on a much larger spectrum called the Electromagnetic Spectrum.

Different frequencies of light interact with matter in many different ways depending on the atomic structure of the material, and it's electromagnetic properties.

For instance Glass is clear to us because it permits light in the visible frequency range to pass straight through it, but move a little higher in the spectrum to Ultraviolet and glass is no longer "see through".

X-Rays are another example of this. In the x-ray frequency our bodies are clear, like glass almost, while our bones are not clear because they have a different structure than our skin and organs.

When light interacts with matter a couple different things can happen: You can have reflection: where the light bounces off completely. You can have refraction where the light enters the material and passes through it. or you can have a combination of the two. You can also have absorption, where the energy from the light particle (or Photon) is absorbed and converted to another form of energy. This is what happens in the photo-electric effect, which is what Einstein published his first paper on. In the photoelectric effect the energy from a photon is tranfered to an electron in the form of kinetic energy. Basically the photon comes in and smack the electron sending it flying off. These jumping electrons can then be harnessed to produce electricity. This is the underlying principle behind a photovoltaic cell, this is how solar panels convert sunlight into electricity.

We have millions of electromagnetic waves passing through our bodies at any given time. Most pass right through us with no interaction, but some of the higher energy waves can and do cause interference to our bodies sometimes harmful.

How to Make things Invisible

Now that I've given you the basic on electomagnetic waves and their interaction with matter we are ready to talk about how to make things invisible.

Scientists and researchers have made lots of important discoveries in the field of metamaterials, they have found ways to bend light so the it passed around an object as if it weren't even there. So now the hunt has been on to find a Metamaterial that will do this for the visible frequency range, so that visible light will pass around an object making it appear as if it weren't even there.

A metamaterial is a material which gains its properties from its structure rather than directly from its composition. Kind of like the way you can see through a screen becuase of the holes in it. If you get up close to the screen you can see that it is made of something but from far away you can see right through it.

The main reason researchers have investigated metamaterials is the possibility to create a structure with a negative refractive index, since this property is not found in any naturally occurring material. Almost all materials encountered in optics, such as glass or water, have positive values for both permittivity e and permeability µ. However, many metals (such as silver and gold) have negative e at visible wavelengths. A material having either (but not both) e or µ negative is opaque to electromagnetic radiation. This is due to the interaction of the surface plasmons.

In order for its structure to affect electromagnetic waves, a metamaterial must have structural features smaller than the wavelength of the electromagnetic radiation it interacts with. For instance, if a metamaterial is to behave as a homogeneous material accurately described by an effective refractive index, the feature sizes must be much smaller than the wavelength.

For visible light, which has wavelengths of less than one micrometre typically (560 nanometers for sunlight), the structures are generally half or less than half this size; i.e., less than 280 nanometres. For microwave radiation, the structures need only be on the order of one decimetre. Microwave frequency metamaterials are almost always artificial, constructed as arrays of current-conducting elements (such as loops of wire) which have suitable inductive and capacitive characteristics.

Metamaterials usually consist of periodic structures, and thus have many similarities with photonic crystals and frequency selective surfaces. However, these are usually considered to be distinct from metamaterials, as their features are of similar size to the wavelength at which they function, and thus cannot be approximated as a homogeneous material.

Left-Handed Metamaterials

Left-handed (LH) materials were first introduced theoretically by Victor Veselago in 1967.

J. B. Pendry was the first to theorize a practical way to make a left-handed metamaterial (LHM). 'Left-handed' in this context means a material in which the 'right-hand rule' is not obeyed, allowing an electromagnetic wave to convey energy (have a group velocity) in the opposite direction to its phase velocity. Pendry's initial idea was that metallic wires aligned along propagation direction could provide a metamaterial with negative permittivity (e<0). Note however that natural materials (such as ferroelectrics) were already known to exist with negative permittivity: the challenge was to construct a material which also showed negative permeability (µ<0). In 1999, Pendry demonstrated that an open ring ('C' shape) with axis along the propagation direction could provide a negative permeability. In the same paper, he showed that a periodic array of wires and ring could give rise to a negative refractive index. A related negative permeability particle, which was also proposed by Professor Pendry, is the Swiss roll.

Image Source: www.cmth.ph.ic.ac.uk/photonics/Newphotonics/pdf/pw2article.pdf

The analogy is as follows: Natural materials are made of atoms, which are dipoles. These dipoles modify the light velocity by a factor n (the refractive index). The ring and wire units play the role of atomic dipoles: the wire acts as a ferroelectric atom, while the ring acts as an inductor L and the open section as a capacitor C. The ring as a whole therefore acts as a LC circuit. When the electromagnetic field passes through the ring, an induced current is created and the generated field is perpendicular to the magnetic field of the light. The magnetic resonance results in a negative permeability; the index is negative as well. (The lens is not truly flat as the C and its nearby Cs imposes a slope for the electric induction.)

1-D, 2-D, and 3-D arrays of Split C-Ring Resonators

Through active arrays of split ring resonators meta-invisible materials can be produced in 1D, 2D, and 3D arrays. These arrays allow for invisibility in those corresponding dimensions.

However the frequencies of light which this works on is limited to how small we can build the split c-ring resonators, since the dimensions of these rings are the same as the wavelengths of light which the structures can act upon. The smallest circuits we can build are on the scale of nanometeres (10^-9) however visible frequencies of light are on the scale of (10^-11) so we will not be able to use these types of circuits to build things that are invisible to humans.

Radar Invisibility

Barium Titanate metamaterials

Barium Titanate was actually one of the first metamaterials to be discovered. Although not confirmed, it is suspected to be one of the key ingredients in Radar invisible paints which are used to paint stealth aircraft. It is also suspected to be one of the toxic materials that was burned at Area 51 illegally resulting in toxic smoke covering the town of Rachel Nevada and several law suits being filed by individuals working on the base.

----The Investigation Continues----