Terahertz filter could harness unused spectrum

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A "metamaterial" that selectively filters terahertz radiation could perhaps be used for short-range wireless communications.
Terahertz (THz) radiation exists between radio and infrared in the electromagnetic spectrum. It passes through clothing but is reflected by metals and explosives, making it potentially useful for security scanning.
Using terahertz waves for communications would make an unused region of the electromagnetic spectrum available. The high frequency of the waves means they could also transfer wireless data very rapidly. However, the waves are normally beyond the influence of traditional optical and electronics devices.
The new apparatus, developed by researchers at the University of Utah, US, can block out certain terahertz frequencies while allowing others to pass through. "We've essentially created a filter," explains lead researcher Ajay Nahata.
Sub-wavelength features
The device is essentially a sheet of metal foil incorporating a carefully designed pattern of holes. It is a so-called metamaterial, since it interacts with electromagnetic waves in novel ways, thanks to sub-wavelength structural features.
Working with Valy Vardeny and colleagues, Nahata devised an arrangement different sized holes – each smaller than the millimetre-size of terahertz radiation – in a "quasicrystal approximate" (partly regular) pattern.
As the waves pass through, they generate plasmon polaritons – resonating electrons – at the holes. These produce interference that blocks out some frequencies. And, by altering the size and the arrangement of the holes, it is possible to select different frequencies to block.
Multiple frequencies
"The holes are smaller than the wavelength, so they attenuate the amount of light that goes through," says Nahata.
The filter is a first step towards developing a terahertz communications system. It is important to filter multiple frequencies since it is common to use several to simultaneously transmitted information. However, it will only work over relatively short distances, like a computer network, as the waves are absorbed by moisture in the atmosphere over longer distances.
Hou-Tong Chen, a physicist at Los Alamos National Laboratories, in New Mexico, US, describes the research as "interesting", but warns that many challenges remain. Even so, Chen predicts that metamaterials will allow terahertz communication systems within the next five years.
Journal reference: Nature (vol 446, p 517