For the first time, terahertz rays, or T-ray pulses, have been made to emit separate “packets” of terahertz radiation – rather than one continuous beam – from a quantum cascade laser. The finding could open new doors for T-rays to image natural and synthetic materials. Researchers from Denis Diderot University in Paris, the French National Centre for Scientific Research and the University of Leeds in the UK published their work online April 24, 2011, in Nature Photonics (doi: 10.1038/nphoton.2011.49). The term “T-ray” describes a band of radiation in the electromagnetic spectrum that falls between radio waves and visible light. T-rays can be used to detect impurities in chemical and biological materials, generating characteristic “spectral fingerprints” that can identify various substances. Recently, scientists have become interested in a technique known as terahertz time-domain spectroscopy, a particularly sensitive way of probing materials using pulses of T-rays. Until now, these pulses were made using laser sources that generated very little power – about one-millionth of a watt). Harnessing the power of a quantum cascade laser that is almost 10,000 times more powerful than previous versions, researchers have developed a T-ray pulse train, an advance that confirms the technique could be used for probing materials. In this latest work, the scientists harnessed the power of a quantum cascade laser (almost 10,000 times more powerful than previous versions) to create a train of T-ray pulses. They also devised a way of detecting the full pulse train, confirming that the technique could be used for probing materials. “The potential for T-rays to provide new imaging and spectroscopy techniques for a range of applications, such as chemical and atmospheric sensing or medical imaging, is immense,” said Edmund Linfield, a professor at the University of Leeds’ School of Electronic and Electrical Engineering. The research was supported by the Délégation Générale pour l’Armement, the National Agency for Research, the UK Engineering and Physical Sciences Research Council, and European Research Council programs NOTES and TOSCA.