Cited research Nature Photonics doi:10.1038/nphoton.2010.165 (2010)

Nestled between microwaves and infrared light on the electromagnetic spectrum, terahertz waves can penetrate plastic and fabrics and are used to identify chemicals. But unlike most electromagnetic radiation, they cannot travel far through air because they are absorbed by water vapour.

Xi-Cheng Zhang and his colleagues at Rensselaer Polytechnic Institute, New York, have now devised a way to detect the radiation from at least 10 metres away. The researchers focus two laser beams together in air to create a plasma (an ionized gas), which interacts with terahertz radiation to produce ultraviolet fluorescence that can be measured from afar. Variations in the intensity of this fluorescence betray the phase and amplitude of the original terahertz wave.

This method could be used to spot chemical explosives, which have characteristic terahertz spectra, from a safe distance.