By Hamish Johnston, editor of Physics Web.

Source: PhysicsWeb.org

    Researchers in the US have used an artificially-structured ¡°metamaterial¡± to build a device that can control highly-elusive terahertz (THz) radiation. The modulator is claimed to be ten-times better at switching a THz beam than previous designs and could pave the way for the use of the radiation in a wide range of applications in chemistry, astronomy and even airport security (Nature 444 597).

    Sandwiched between the microwave and infrared regions of the electromagnetic spectrum (at about 300 GHz to 10 THz), THz radiation is notoriously difficult to work with. It is too high frequency to be manipulated electrically like microwaves and too low frequency to be controlled by optical means.

    This is unfortunate because THz radiation is particularly useful for studying the rotations and vibrations of molecules, allowing for the creation of new ¡°fingerprints¡± for the identification of chemical species. This could be combined with its ability to travel through paper, plastics and cloth to develop scanning equipment for the detection of weapons and explosives. Astronomers are also interested in THz radiation because the cosmic microwave background originated by the Big Bang includes a THz component.

    While there has been some recent success in developing THz detectors and sources, devices that can switch and filter THz radiation have remained elusive. Simple, low-cost and effective switches and filters are essential for the creation of commercial devices that can manipulate beams of THz radiation in an effective way. The best that researchers have done so far are modulators that can change the intensity of a THz beam by only a few percent ¨C and this often requires the device to be operated at very low temperatures. As a result, the development of practical THz technologies remains in its early stages, with only a handful of companies currently offer commercial spectroscopy and imaging equipment.

    Now Hou-Tong Chen and Willie Padilla of Los Alamos National Laboratory (LANL) and colleagues have created a device that can switch or modulate a THz beam with 50% efficiency. The device is based on an array of micrometre-sized structures, that each combine two inductive rings with a parallel-rod capacitor (see figure ¡°Rings and rods¡±). Similar metamaterials using larger structures have been used to control microwave radiation and the LANL team simply reduced the size of the rings and rods to make the structure respond to radiation at about 1 THz.

    When a voltage is applied across the array it absorbs THz radiation in a narrow frequency band. When the voltage is switched off the structure becomes more or less transparent to THz radiation ¨C thereby forming the basis of switch. This narrow-band operation also means that the array could be used to create filters that selectively block or transmit specific THz frequencies.

    Don Arnone, who is chief executive of the UK-based THz equipment supplier TeraView, told Physics Web that the LANL work offers a potential solution to the problem of how to manipulate THz radiation. ¡°Reducing the cost and size of THz technologies is a challenge and metamaterials could do this¡±, he said. However, he cautioned that while important, this result should be seen as one of many incremental improvements in THz technology.