Photonic MetaMaterials

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The optical multilayer films widely used as anti-reflection films etc. for the surface of television monitors, glasses and other optical components can be considered to be one dimensional photonic crystals. However, generally the term photonic crystal refers to two dimensional and three dimensional structures.

1.  Description

2.  Why

3.  How

4.  Future Trends

5.  Related Links

Useful Links: photonic crystals(1,2 )

Description 

A photonic crystal is a periodic structure of materials with differing refractive indices. A metamaterial (or meta material) is a material which gains its properties from its structure rather than directly from its composition. To distinguish metamaterials from other composite materials, the metamaterial label is usually used for a material which has unusual properties.Photonic crystal fibre is made by stacking pure silica tubes in a hexagonal array, and then heating and stretching it. This reduces the width of the structure, while retaining the geometry.The pitch of the structure is usually chosen such that it is half the wavelength of the light for which the element is designed for. Typically, photonic crystals used in the visible optical region are designed and fabricated so that the period is about 300nm. 

Why 

The periodic arrangement of ions on a lattice gives rise to the energy band structure in semiconductors. Energy bands control the motion of charge carriers through the crystal. Similarly, in a photonic crystal, the periodic arrangement of refractive index variation, controls how photons are able to move through the crystal.Traditional optical fibres have a glass core surrounded by a cladding layer. The cladding has a lower refractive index than the core. The difference in the refractive index between the two materials is slight, but it is enough to cause light within the core to be reflected along the core by total internal reflection.At high powers, the light is concentrated in a very small region of space. Raman scattering can result, causing the light signal to be corrupted and at high powers the fibre can even be damaged. Photonic crystal microcavities are more efficient than conventional semiconductor diode lasers since there are few directions in the which the photons can escape.The defect mode or microcavities formed by breaking the periodicity of the crystal amplify only those wavelengths of light that are able to pass freely through the crystal. 

How

Optical functionality that cannot be achieved by conventional materials are possible because of phenomena that occur naturally in photonic crystals. For instance, two such phenomena are:

  • Negative refraction

  • Anomalous dispersion

Such phenomena occur in the narrow region of the band edge, the wavelength range between the photonic band (transmission) and the photonic bandgap (reflection).The technology of controlling the behaviour in such a boundary area is known as photonic band edge engineering. 

  • Replacing the ions on a lattice are regions of low refractive index within a high-refractive index material or vice-versa.

  • Photons react to the refractive index contrast in an analogous manner to the way electrons react when confronted with a periodic potential of ions.

  • Each results in a range of allowed energies and a band structure characterised by an energy gap or photonic band gap.

  • A band gap forms when the electron wavelength is comparable to the inter-atomic spacing.  

Breaking the periodicity of the voids in the photonic crystal, either by enlarging or reducing the size of a few of the voids, introduces new energy levels within the photonic band gap.This is analogous to the creation of energy levels within the band gap by the addition of dopant atoms in semiconductor crystals. Narrow line width lasers, that are so important in the area of Dense Wavelength Division Multiplexing (DWDM) communication systems, can be fabricated using photonic crystals formed from the III-V semiconductors or rare-earth doped glass.The wavelength range of emission is related to the diameter of the microcavity divided by the diameter of the regular holes.The fact that when electromagnetic waves of light reach the surface of a negative refraction lens, they excite a collective movement of surface waves, such as electron oscillations, also known as surface plasmons.That results in an enhancement of the evanescent waves and is different from the way light typically behaves when it reaches a conventional lens.The first photonic crystal fibres used a hexagonal array of air holes running the entire length of the fibre. The light travelled close to the fibre axis and so the power was still limited. By removing the central tubes to leave a central hole of 15 m m in diameter.The hole acts as a waveguide, the light being trapped by the photonic crystal structure around the hole. The photonic crystal allows only some of the light to propagate along the fibre.

Future Trends

Moving a step closer to optical cloaking, researchers at the University of Stuttgart (Stuttgart, Germany) recently created a stacked split-ring metamaterial for the optical wavelength range. This breakthrough may eventually lead to superior lenses that can beat the diffraction limit, as well as optical cloaking devices capable of providing invisibility for macroscopic objects.Until now, because of the extremely intensive processing technology required to fabricate photonic crystals of two or more dimensions, and despite the wide adoption of one dimensional multilayer optical films, higher dimentional photonic crystals have not been commercialized.

Keywords

Photonic crystals, photonic crystals, metamaterials, Negative Refraction, magnetization waves.

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