interview technology focus The promise of chalcogenides Chalcogenide are attracting significant attention thanks to their mid- transparency and highly nonlinear properties. Nadya Anscombe talks to Dan Hewak from the University of Southampton in the UK.

■■ What are chalcogenide glasses and how ■■ What’s been holding the area back? are they different to silica? The chemistry of making chalcogenides Chalcogenides are compounds formed is simple, but the engineering is complex. primarily from elements such as Theoretically, chalcogenide fibres have sulphur, or , and are very all the right properties to enable many different from silica . Chalcogenides exciting applications, but making fibres that come in a variety of colours that depend on match theoretical parameters is another their chemical constituents, ranging from matter. There was a time when fluoride partially transparent to completely opaque. In fibres were tipped to be the next big thing; contrast with silica, which is predominantly in theory, they could out-perform silica a passive material, chalcogenides exhibit fibres and provide trans-Atlantic spans active properties and are highly nonlinear. without the need for repeaters. There was Chalcogenides are also photosensitive, which, a flurry of research activity, but no one although a potential disadvantage for some k managed to realize a fibre that could match applications, is a significant advantage for the required performance characteristics, writing structures into fibres. In addition, and the technology soon disappeared they are glassy , which gives Hewa Dan for telecommunications applications. them interesting electrical properties. Hewak: “We urgently need to find a reliable If we stick to conventional methods of and reproducible way of making high-purity manufacturing chalcogenide fibres, we ■■ What are the potential applications? chalcogenide glasses, and I believe chemical will see history repeat itself. I believe it is Chalcogenide glasses in the form of thin vapour deposition could be the answer.” possible to make chalcogenide fibres with films are already used in optical data the optical parameters that we need, but only storage and in the manufacturing of highly crucible drawing process is an alternative if we manage to develop new glass-melting efficient solar cells. Chalcogenide fibres technique that can help prevent the technology and reproducible, reliable can be used for the transmission of light formation of . However, the main fabrication techniques. at wavelengths beyond those possible with challenge is not drawing the fibre, but silica. Shortly after the invention of the rather manufacturing a glass with high ■■ Are there many start-ups in this area? , chalcogenide fibres were studied as purity. The techniques we use today to Several research groups around the world waveguides for delivering CO2 laser light manufacture novel glasses are the same as are developing the technology required to at wavelengths of 10 μm. As new those used hundreds of years ago, but they make chalcogenide fibres, and a handful of were discovered, various chalcogenide are not suitable for making chalcogenide companies have already started up in this compositions were developed to address glasses because of the high purity levels field. One example is the Canadian company individual wavelengths. Adding tellurium required. Glass purity is of the utmost CorActive, which offers both sulphide glass to the glass tends to increase transmission importance for both the scientific study (wavelengths of 2–6 μm) and selenide glass at infrared wavelengths, whereas sulphur and practical application of chalcogenide (wavelengths of 2–9 μm). The company uses aids transmission at visible wavelengths. glasses. Varying levels of trace impurities, state-of-the-art technology developed at the Chalcogenides are also useful for medical even at levels of a few parts per million, Naval Research Laboratory in the USA. The applications such as laser surgery, which can alter the spectroscopic behaviour of a processes they use are repeatable and reliable, requires a wavelength of around 3 μm. chalcogenide glass. In optical components but also time-consuming and expensive. The Gallium lanthanum sulphide fibres transmit such as fibres, impurities not only chalcogenide fibre market therefore remains well at this wavelength, and their non-toxic contribute to the optical loss through small and the fibres are expensive. This cost components and high melting temperature absorption and scattering but also serve is predominantly due to the expense of the suit minimally invasive surgery. At similar as nucleation sites for crystallization. raw materials and manufacturing processes. wavelengths, chalcogenide fibres have several We urgently need to find a reliable and As the applications of chalcogenide fibres atmospheric transmission windows for reproducible way of making high-purity become more widespread, the balance applications in spectroscopy and sensing. chalcogenide glasses, and I believe chemical between cost and manufacturing quantity vapour deposition could be the answer. will shift. It takes only a little more effort ■■ How are chalcogenide fibres made? This process is capable of producing high- to draw 1 km of fibre than 10 m — with Like silica fibres, chalcogenide fibres purity glasses at low cost because it relies increased demand the price per metre of can be drawn from a preform. However, on liquid-metal chloride precursors and chalcogenide fibre is certain to fall. unlike silica, the drawing temperature of a operates at atmospheric pressure. However, chalcogenide is close to its crystallization this technique has a slow growth rate and Nadya Anscombe temperature. Crystallization weakens a the required precursors are often dirty Nadya Anscombe is a freelance journalist fibre and alters its optical properties. The and expensive. based in the United Kingdom.

474 nature photonics | VOL 5 | AUGUST 2011 | www.nature.com/naturephotonics © 2011 Macmillan Publishers Limited. All rights reserved.