Metamaterial


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These unit cells are microscopically built from conventional materials such as metals and dielectrics like plastics.

Researchers develop 'acoustic metamaterial' that cancels sound

However, their exact shape, geometry, size, orientation, and arrangement can macroscopically affect light in an unconventional manner such as creating resonances or unusual values for macroscopic permittivity and permeability. Some examples of available metamaterials are negative index metamaterials, chiral metamaterials , plasmonic metamaterials , photonic metamaterials , etc.

Due to their subwavelength nature, metamaterials that operate at microwave frequencies have a typical unit cell size of a few millimetres, while metamaterials operating at the visible part of the spectrum have a typical unit cell size of a few nanometres. Metamaterials are also inherently resonant, i. For conventional materials, the electromagnetic parameters such as magnetic permeability and electric permittivity arise from the response of the atoms or molecules that make up the material to an electromagnetic wave being passed through.

In the case of metamaterials, these electromagnetic properties are not determined at an atomic or molecular level. Instead, these properties are determined by the configuration of a collection of smaller objects that make up the metamaterial. Certain experiments are made only with single layers monolayers as described above.

Three-dimensional structures 3D metamaterials can be made by stacking such arrays in the vertical direction with a period which is again well below the optical wavelength. When light impinges such nano-resonators, it can excite electromagnetic oscillations. These are particularly strong for frequencies near the resonance frequency, but the most interesting optical effects may occur somewhat above or below the resonance.

The latter can strongly deviate from 1 even if no ferro magnetic materials are used. Such couplings can occur, e. Still more simplified forms of resonators may consist only of a periodic arrangement of short wires, or of metallic pieces with other shapes. Such forms can be used at shorter wavelengths, now reaching the red end of the visible optical spectrum. There are also metamaterial structures where the whole metallic structure is electrically connected, in contrast to the isolated nano-resonators as described above.

Furthermore, there are structures with genuinely three-dimensional building blocks, such as short helically wound wires helical antennas , which are sensitive to the direction of circular polarization. Such three-dimensional structures can be fabricated, e. All three parameters are generally frequency-dependent and often also dependent on the propagation direction and polarization state of the light.

The magnitude of the refractive index is determined by the following equation:. The sign of the refractive index is chosen such that Snell's law correctly describes the direction of the refracted beam at an optical interface. Similarly, two media can have the same impedance while differing in the refractive index. This is relevant, e. The following subsections discuss photonic metamaterials with optical properties which are of particular interest. This can be described with a negative refractive index, changing the sign of the angle in Snell's law.

Structures Containing Nano-Resonators

Negative refraction has a number of consequences, leading to additional very unusual optical properties. In particular, the wavefronts of the refracted beam travel toward the interface, i. For that reason, such media are also called left-handed metamaterials LHM : a left-hand rule instead of the usual right-hand rule delivers the direction of the Poynting vector from E and B.


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A convex lens made of a negative-index material could be defocusing rather than focusing, and a concave lens could be focusing. Even a simple plate with planar and parallel end faces of a negative-index medium could act as a lens. Note also that there are superlenses based on effects other than negative refraction.

Very peculiar effects would occur if some object could be embedded in a negative-index metamaterial. Depending on its depth in the medium, it may appear strongly magnified and distorted for an observer outside the material.

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It should even be possible to make cloaking devices invisibility cloaks , which can hide objects within them against viewing with light from outside, although probably only in a very limited frequency range. As mentioned above, there are also metamaterials containing metallic helical nanowires. These have a certain handedness, which can lead to substantially different optical effects for light with different directions of the circular polarization.

It is possible to obtain metamaterials which are highly absorbing for one circular polarization direction and have a high transmission for the other polarization direction. The resonances of such a resonator can be in the mid-infrared domain with wavelengths of a few microns when its width is reduced to the order of a few hundred nanometers. Note that multiple resonances with different frequencies may occur, but the lowest-frequency resonance is often most relevant for the application. Such metal—dielectric composites can be fabricated, e. A frequently used metal is gold.

Silver is better suited for high frequencies, but is more difficult to process due to its tendency to be oxidized. Certain experiments are made only with single layers monolayers as described above. Three-dimensional structures 3D metamaterials can be made by stacking such arrays in the vertical direction with a period which is again well below the optical wavelength.

What are metamaterials and why do we need them?

When light impinges such nano-resonators, it can excite electromagnetic oscillations. These are particularly strong for frequencies near the resonance frequency, but the most interesting optical effects may occur somewhat above or below the resonance. The latter can strongly deviate from 1 even if no ferro magnetic materials are used. Such couplings can occur, e. Still more simplified forms of resonators may consist only of a periodic arrangement of short wires, or of metallic pieces with other shapes.

On The Quest To Invisibility - Metamaterials and Cloaking: Andrea Alu at TEDxAustin

Such forms can be used at shorter wavelengths, now reaching the red end of the visible optical spectrum. There are also metamaterial structures where the whole metallic structure is electrically connected, in contrast to the isolated nano-resonators as described above. Furthermore, there are structures with genuinely three-dimensional building blocks, such as short helically wound wires helical antennas , which are sensitive to the direction of circular polarization.

Photonic Metamaterials

Such three-dimensional structures can be fabricated, e. All three parameters are generally frequency-dependent and often also dependent on the propagation direction and polarization state of the light. The magnitude of the refractive index is determined by the following equation:. The sign of the refractive index is chosen such that Snell's law correctly describes the direction of the refracted beam at an optical interface.

Similarly, two media can have the same impedance while differing in the refractive index.


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This is relevant, e. The following subsections discuss photonic metamaterials with optical properties which are of particular interest. This can be described with a negative refractive index, changing the sign of the angle in Snell's law. Negative refraction has a number of consequences, leading to additional very unusual optical properties. In particular, the wavefronts of the refracted beam travel toward the interface, i.

For that reason, such media are also called left-handed metamaterials LHM : a left-hand rule instead of the usual right-hand rule delivers the direction of the Poynting vector from E and B. A convex lens made of a negative-index material could be defocusing rather than focusing, and a concave lens could be focusing.

Even a simple plate with planar and parallel end faces of a negative-index medium could act as a lens. Note also that there are superlenses based on effects other than negative refraction. Very peculiar effects would occur if some object could be embedded in a negative-index metamaterial.

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