A new generation of sensors for detecting explosives and poisons could result from research into terahertz radiation published recently by Bath University chemist Christopher Williams, together with a team of British and Spanish researchers.
Terahertz waves lie in the far-infrared part of the electromagnetic spectrum, and their frequencies correspond to important energy level transitions in chemical and biological systems. But for the waves to be used in a sensor requires a very high detection sensitivity, and that means they must be confined closely to the sensor surface.
A simple metallic surface can be used to guide terahertz waves, but the radiation extends as a weak electromagnetic field many centimetres above the surface, thereby severely reducing the sensitivity. A metamaterial, on the other hand, can draw terahertz waves close to it, creating a strong field that extends less than a millimetre above its surface.
Metamaterials are man-made structures consisting of metallic surfaces decorated with two-dimensional arrays of sub-wavelength periodicity pits. Sensor designers can chose the size and arrangement of the pits so as to maximise the confinement of terahertz waves as they travel along the waveguide.
“Terahertz rays have the potential to revolutionise security screening for dangerous materials such as explosives,” says Imperial College London physicist and study co-author Stefan Maier. “Until now it hasn’t been possible to exert the necessary control and guidance over pulses of this kind of radiation for it to have been usable in real world applications. We have shown with our material that it is possible to tightly guide terahertz rays along a metal sheet, possibly even around corners, increasing their suitability for a wide range of situations.”
In this study the researchers measured the degree of wave confinement by detecting the decay of the radiation field from the surface. In doing so they demonstrated a reduction of the decay length of two orders of magnitude with respect to a flat surface. They also found that the vertical confinement of terahertz waves can be increased by almost an order of magnitude by filling the pits in the metamaterial with a high-index dielectric such as silicon.
The technology needs refining before metamaterials can be used in real-world sensing applications. For one thing the researchers have so far only managed to confine a small number of frequencies to their sensor surfaces. And more sophisticated designs are required in order that the whole terahertz pulse is affected by the surface structure, thereby ensuring that absorption features of molecules can be clearly identified.
“What is needed now is an in-depth investigation on how much more we can increase the confinement by varying the size and filling of the pits, and how we can engineer broad-band operation,” says Maier. “We expect a new generation to emerge within the next year.”
“Williams and co-workers have demonstrated that by structuring the surface of a metal is it possible to confine terahertz radiation tightly to the surface while still letting this radiation propagate,” says Jaime Gómez Rivas, a nanophotonics expert based at the Institute for Atomic and Molecular Physics in the Netherlands. “The results could lead to new approaches to wave-guiding, since the confinement is determined by geometrical factors such as the size of the holes on the surface and their spacing, rather than material properties.”
Maier and Bath physicist Steve Andrews designed the metamaterial together with colleagues at universities in Madrid and Zaragoza. Financial support was provided by the US Air Force and the UK’s Royal Society. The work follows theoretical predictions made by the Spanish team together with Imperial College’s John Pendry, which were published in the journal Science in 2004.
“Highly confined guiding of terahertz surface plasmon polaritons on structured metal surfaces”, Williams et al., Nature Photonics (2008).
“Metamaterials and Negative Refractive Index”, Smith et al., Science 305, 788 (2004).
An artist’s impression of a specially designed metamaterial with an array of nanoscale pits that guide terahertz waves along the material surface (source: Imperial College London).
Article first published in Nanomaterials News.