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Metamaterials

Interaction between light and materials is the key to exploiting and controlling light. From this simple observation follow the major contributions of the science of optics: camera lenses, optical fibres, lasers to name only a few. To tailor these properties to requirement chemical composition can be adjusted: we might, for example, add lead to glass and raise its refractive index. More recently it has been realised that the internal microstructure of a material can be just as important as chemistry in determining optical properties. In fact by exploiting both chemistry and microstructure materials can be produced with properties never found in nature and of great potential commercial value. This new class of materials, metamaterials, achieve the effect through structure that is smaller than the wavelength of radiation at the operating frequency. They give access to materials with a hugely expanded range of electromagnetic properties.

 

 

 

 

 

Figure A on the right illustrates the concept of a metamaterial in which sub-wavelength engineered units replace atoms and molecules as the determinants of electromagnetic properties. Thus at optical frequencies these units, or metamolecules as one might call them, would need to be nanoparticles of diameter much less than the 500nm wavelength of visible light.  The concept is applicable to electromagnetic radiation of all frequencies. At mobile telephone or radar frequencies with wavelengths of a few cm the component units can be as large as a few mm across and still satisfy the sub-wavelength requirement. Not surprisingly metamaterials are easier to make for operation at these frequencies and there has been much progress already particularly in creating materials with the elusive property of negative refraction.

An early example of a metamaterial designed to give a magnetic response at around 10GHz is shown below in figure B. The copper rings are approximately 1cm in diameter. Other structures giving novel electrical responses have also been reported. One of the most important applications of metamaterials is creating a negative refractive index which requires both the magnetic and electric responses of the material to be negative.

A

 

B

Some metamaterials are designed to work in the MHz region of the spectrum with magnetic resonance imaging as the target application. Resonances at such low frequencies require high capacitance and inductance realised in the "Swiss roll" structure shown in figures C and D. The particular structure shown is designed to be chiral and comprises an insulated gold tape wound at an angle around a cylinder to form a helix. In contrast to most other chiral systems this particular design exhibits an extreme chiral response: the plane of polarisation can be rotated by 90 degrees  within one wavelength of propagation. By way of comparison even highly concentrated sugar solutions require propagation over 1000 wavelengths to achieve the same rotation.

C

D

 

 

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