Metamaterial

A metamaterial is a man-made structure whose permeability for electric and magnetic fields ( permittivity and permeability) is different from the usual in nature. This is achieved by specially made, usually periodic, microscopic structures ( cells, individual elements ) of electrical or magnetically active materials in its interior.

Metamaterials can have a negative real part of the complex refractive index. The transition from the vacuum in such a material will be broken over the solder wave in the negative direction also. The propagation of the waves is thus inside and outside of the material to the same side of the solder. Ordinary materials have a positive refractive index. For them, waves are deflected towards the Lot during the transition to the particular material, but not beyond. With metamaterials, the real part of the refractive index <1, applications are also conceivable, which are in principle not possible with conventional materials. So they can make objects invisible by directing them incoming waves around the objects.

The structure of metamaterials with the aid of the refractive index is designed must be significantly smaller than the wavelength of the radiation. This complicates the construction of visible light significantly. Most metamaterials realized so far are therefore designed for microwave radiation.

Definition

The definition of metamaterials is still in flux:

• The more common definition restricted on the cell size (significantly) less than a quarter of the wavelength in a vacuum. The arrangement behaves like a homogeneous medium. That is, in the first line of the cell contents is determined function.
• Some authors refer also photonic crystals with a, in which the cell size is on the order of a half wavelength. Here primarily determined the cell size the function.

The term metamaterial was coined in the late 1990s by John Pendry.

Physical Basics

The special feature of metamaterials is that their material constants and negative values ​​can accept. This means from the perspective of field theory that

• The field of the electric flux density (D ) field, and the electric field strength (E-field ) and
• The field of the magnetic flux density (B- field) and the magnetic field strength (H-field )

Each directed opposite to each other.

The different signs are contrary to any fundamental physical reasons, since the D and E and the B- and H- fields according to the Maxwell equations in their material-independent form independent " causal mechanisms " underlying:

The different sign of D - and E - fields in metamaterials come about through clever arrangements and processes, which are characterized in that the changes of the magnetic flux generate an electric field that the D- box, facing the opposite direction.

Similarly, the different signs of B- and H- fields are thereby concluded that the changes of the electric field in Metamaterials a magnetic flux (and thus a B- field) produce that in the H- field facing the opposite direction.

The wave vector, the electric and the magnetic field strength forms a left-handed metamaterials with tripod - hence the term left-handed material.

Properties

In 1968, the propagation of waves in a medium with a negative refractive index has been studied by the Soviet physicist Viktor Wesselago. That in such a material, the phase velocity and group velocity run contrary to the given by the Poynting vector flow of energy, was known by Henry Cabourn Pocklington since 1905. Wesselago now showed that the left-handedness of metamaterials, inverse Doppler effect and inverse law of refraction leads to inverse Cherenkov radiation. The inverse law of refraction at curved surfaces leads to an interchange of convergence and divergence. Unlike ordinary media bundles a concave lens made of metamaterial incident radiation.

In addition, it was shown by Ilya V. Shadrivov that the beam displacement at the Goos- Hänchen effect with metamaterials also changes sign.

Metamaterials can have a repulsive ( repulsive ) Casimir effect cause.

Production

There are in the manufacture approaches that exploit resonance (resonant approaches) and those that do not ( non-resonant approaches).

Resonant approaches

Split-Ring/Wire-Grid

When Split-Ring/Wire-Grid-Ansatz ( pictured above ) leads the wire mesh ( wire grid) to negative permittivity, as in metals below the plasmon resonance electrons behave like a plasma ( Drude model ). A resonator, often embodied as a (semi - ) ring with split (split ring ), leads to a magnetic dipole moment, and a negative effective permeability, but only in a very narrow frequency range. The properties of the resonator can be chosen so that there is a negative refractive index in the desired frequency range.

This arrangement has the property of low loss can be achieved at a low bandwidth of the resonance. In addition, the losses increase with the ohmic resistance of the metal with the frequency. Visible light absorption was so dominant that it covers an unusual effects real part of the refractive index.

Dielectric spheres

The approach on dielectric spheres of different diameters in a NaCl lattice has the advantage that as a non-metallic structure and the optical frequency range could be developed., The theoretical work on this approach shows, however, that only very small bandwidths are to be expected and in accordance with extreme demands are made of the tolerances of manufacturing technology.

Nonresonant approaches

Possible way out of Bandbreiten-/Dämpfungsproblematik, at least in the microwave range, are nonresonant concepts based on inverse line structures. This bandpass- like structures simultaneously offer high bandwidth and low losses - as long as structures can be designed to behave like discrete series and parallel resonators. Due to the derivation of the transmission line theory first such metamaterials were one-dimensional and exc (t ) s the controversy as to whether it makes sense to speak of metamaterials or of applied filter theory. Generalizations ( isotropic ) 2D/3D-Anordnungen were presented theoretically and experimentally demonstrated some.

Possible applications

The analyzed by Wesselago plan lenses are potentially advantageous because of the lack of optical axis, the demonstrated by John Pendry resolution enhancement led to particularly great attention in physics and electrical engineering. It is characterized in that a point light source has a point-like image, that is, in contrast to the conventional lens, the evanescent wave vector spectrum of the source is resonantly enhanced by the flat metamaterial lens and then in the picture ' reconstructed '. This is not to be confused with finite resolution in conventional lenses due to finite entrance pupil, the diffraction limit is not heranziehbar as a comparison criterion, because Pendrys lens is infinitely large.