Friday, 9 March 2012

How Do Terahertz Metamaterials Work and What Is Their Potential?

Metamaterials are artificial materials designed to provide control of electromagnetic waves. While natural materials are limited in their wave refracting properties due to limitations from the size and pattern of their atoms and molecules, metamaterials' cells are made from are subwavelength metallic resonators, designed to refract electromagnetic waves.

Each metamaterial needs to behave as a homogenous material, with features smaller than the wavelength being controlled. They have a negative refractive index, which is only achievable through artificial materials.

The periodic lattice structures of such metamaterials are created to interact with light's magnetic field with a customised polarization angle. This is currently only possible within a small area of the electromagnetic spectrum (terahertz included, a light wave placed between microwave and infrared on the spectrum), but promise has been shown in the system for developing refracting visible light.

Refraction using the split ring resonator (SRR)
The most common effective unit cell from which the metamaterial is constructed is the split ring resonator (SRR). Terahertz metamaterials utilise the split ring resonator, which is made from a ring with two parallel gaps. These rings and their gaps can be angled to provide the desired direction and angle of refraction, allowing the magnetic field through the gap, and inducing an electric current along the rings.

Possible uses of metamaterials
Just five years since the conception of terahertz metamaterials, research and device development in the field have come a long way, and new concepts of application are still being discovered. Instruments which generate, detect and manipulate terahertz waves can be (and are) used within public health (including non-ionizing medical imaging and tumour detection), counter terrorism and defence (through security detection of chemicals and weapons), and manufacturing quality control and fault detection. The ability to refract terahertz waves has opened up the potential for a huge range of applications in these fields and many more.

A development of photonic metamaterials may make it possible to manipulate optical light in the same way. The ability to control the visible light to move around an item rather than through it enables the possible development of metamaterial cloaking devices; a huge area of study dedicated to rendering something invisible to the human eye. In the field, this could be used for invisibility cloaking members of the Armed Forces (and their vehicles) from enemies. If the light can be diverted around its object and back to its original course, the object won't even leave a shadow.

This groundbreaking field of transformation optics has only recently opened up, but it also studies invisibility cloaking at microwave frequencies. The ability to refract microwaves around an object or person may, eventually, be able to make them invisible to radar.

All of these, and many more possibilities, have been made realistically possible thanks to research in terahertz metamaterials and terahertz wave generation, detection and refraction.

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