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Phase-shifting glass

A five-year project led by the University of Southampton in the UK has just begun to advance the use of chalcogenide glass, a material with unusual optical properties. Greg Blackman reports

Chalcogenide glasses are by no means uncommon – they are used widely in rewritable DVDs and infrared lenses for night vision – but the materials have potential for much greater use, so much so that the UK Engineering and Physical Sciences Research Council (EPSRC) has funded an initiative to advance their manufacture and demonstrate their applications. The Chalcogenide Advanced Manufacturing Partnership (ChAMP) was launched on 9 March at a University of Southampton open day held in conjunction with the Photonex Southampton exhibition. The five-year project is a partnership between the Universities of Southampton, Exeter, Oxford, Cambridge and Heriot-Watt, along with 15 industrial partners.

The functional properties of chalcogenides make them just as interesting to those working in electronics as to those in photonics. They react and change properties in response to light, electrical current, acoustic waves and other stimuli – they can go from polycrystalline to amorphous, for example, on exposure to a laser beam. One potential use is building acousto-optic modulators, especially at infrared wavelengths using the material’s active optical coefficients.

In electronics, people are interested in the material as a high-density multi-state memory, and trials are underway into its use as memory in mobile phones.

There are also passive users – the material transmits in the infrared and is ideal for applications for which conventional glasses no longer transmit light, for use in multispectral imaging systems, for example. ‘Here, the advantages of chalcogenides are that they are highly durable and have high refractive indexes,’ explained Dr John Lincoln, industrial liaison officer at the Optoelectronics Research Centre (ORC) at the University of Southampton. The material also has interesting properties as 2D thin films.

ORC’s chalcogenides are based on gallium, lanthanum and sulphur, and are ‘only one of a few chalcogenide materials that can be drawn into fibres’, commented Professor Dan Hewak at the ORC, who leads the ChAMP programme. The ORC is therefore able to produce an optical fibre that transmits in the mid-infrared, which is a property highly sought after for medical, aerospace and sensing applications. In one particular project, ORC reproduced a range of optical equivalents of brain functions in its chalcogenide fibres, work that the centre hopes will advance neuromorphic computing (for more on the work, see the April 2015 issue of Electro Optics).

The ORC has also enhanced the transmission of chalcogenides at 4µm by a couple of orders of magnitude, according to Hewak, through improvements in the centre’s glass melting facilities. This is a particularly good wavelength for medical applications.

The other advantage of the chalcogenide glass developed at the ORC is that it doesn’t contain arsenic. ‘One of the reasons why commercial adoption of chalcogenides hasn’t been prevalent is that the glasses contain arsenic,’ noted Mark Middleton, CEO of Crystran, a UK-based manufacturer of optical components.

Crystran is an industrial partner within ChAMP and has been working with the ORC for more than 20 years. Within the ChAMP programme, the company machines blanks from ORC chalcogenide glass boules. The blanks are polished flat to a specification of λ/10 and a scratch-dig of 10-5, or better, and then supplied back to the ORC for research into using the material.

‘The key emphasis on the ChAMP project is to show chalcogenides can be manufactured in different forms,’ commented Lincoln. ‘As part of this, we want to produce a number of demonstrators in different application spaces. The academic partners will work on different demonstrators in different areas – University of Exeter does electronics, University of Oxford non-linear demonstrators, Heriot-Watt University waveguides and non-linear media demonstrators, while Cambridge is helping us [ORC] understand the fundamental properties of these materials.’

Professor Ajoy Kar at Heriot-Watt University is designing some demonstrators using non-linearity, one being light continuum generation. ‘There are white light continuums available, but what do you use if you want a continuum in the infrared? You need a different glass to generate the continuum and you need it to be low-loss throughout its transmission range. Chalcogenides are a strong contender material for that,’ added Lincoln.

Photonics company, Qioptiq, which is owned by Excelitas Technologies, is an industrial partner within ChAMP. The company’s principal interest in chalcogenide glasses is for lenses in optical systems operating over wavebands of around 1µm to 14µm.

‘Qioptiq is investigating a number of multispectral solutions, incorporating sensors which operate over more than one waveband,’ remarked Graham Evans, business development manager at Qioptiq. ‘Chalcogenides are very useful for these systems since they transmit over a broad waveband.’

The company is currently looking at the short wave  infrared and the long wave

infrared in a combined system. However, there are several other waveband combinations of interest, noted Evans, depending on the specific product application area.

‘The chalcogenides that are currently being developed under ChAMP – specifically, variants of the [ORC’s] GLS material – have improved mechanical properties, so will be beneficial for Qioptiq as robust lens materials for application in hostile environments,’ Evans said.

He continued: ‘We don’t currently use a great amount of chalcogenide glass compared to other more conventional infrared materials, like germanium and silicon, but we are increasingly looking at chalcogenides for future systems.’

The company uses chalcogenides for colour correction and athermalisation in a lens system – i.e. control of defocus over a specific waveband and over a wide temperature range. The products Qioptiq supplies for defence and security applications generally operate from -40°C to 70°C. ‘The focal length of typical infrared lenses will change significantly with temperature,’ noted Evans. Chalcogenides can therefore be useful to minimise this thermal focus shift in the overall optical system.

Purer glasses

The EPSRC grant for ChAMP builds on success in a previous EPSRC programme at the University of Southampton called the Centre for Innovative Manufacturing in Photonics. One of the core programmes under that centre was manufacturing purer chalcogenide glasses.

‘One of the issues with chalcogenides is that they are quite reactive, so many of the glasses are full of impurities,’ said Lincoln. ‘Water is the well known one, but there are also many metals, and those impurities have meant that traditionally chalcogenides have had relatively poor transmission. Southampton has been able to significantly reduce moisture levels and metallic impurities in these glasses, improving transmission by over two orders of magnitude.’

The glasses transmit much better with lower levels of impurities. ‘The base level loss of the glass is much lower, plus you remove any spurious peaks you may have around the water absorption areas at 3µm. When you’ve done that, you also open up the applications,’ Lincoln continued. ‘The work at ChAMP is to build on that work and say, “now we know how to make the glasses, let’s make them in different geometries, for example, thin films, bulk optics, fibres and nanoparticles, and make them usable for different applications and demonstrate how well they work”.’

Mouldable optics

Qioptiq is also interested in the associated manufacturing processes of chalcogenides as well as the material properties. ‘We’re very interested in understanding the moulding capability of the GLS variant chalcogenides for low-cost, high-volume applications,’ noted Evans.

The material’s relatively low melting point means it can be forged or pressed into lenses, which makes it easy to manufacture. Amorphous Materials in the US offers chalcogenides called AMTIR that can be pressed into lenses, rather than having to diamond turn or grind them. In addition, Umicore offers infrared lenses made from a chalcogenide glass called GASIR. These types of moulded optics are used for automotive applications or thermal imaging applications.

‘Although the material is relatively expensive the price of making quite a complicated aspheric is low, because, rather than having to machine it, the material can be heated up and pressed into shape,’ explained Middleton at Crystran.

‘To increase the use of chalcogenides, practical demonstrations of working devices in many application spaces are needed,’ commented Lincoln. ‘People will believe it when they see it working.

‘There also needs to be reliable data on the properties of the material – what’s the loss, what are the opto-mechanical properties, etc,’ he added. ‘Comparative numbers against existing and well-used materials are very important as existing data can be misleading, particular for chalcogenide optical fibre losses.’

Qioptiq is in dialogue with Hewak at the ORC at the University of Southampton about the development of the materials. ‘We would certainly be looking to part-fund relevant activity areas under ChAMP; whether that is a PhD or post-doctoral research,’ commented Evans.

‘Towards the end of the ChAMP project, we’d [Qioptiq] hope to have developed a variant of the GLS material that would enable us to use it in future systems. We’d look to understand the exploitation route as well in partnership with the University of Southampton,’ Evans said, adding that, potentially, the company would implement new chalcogenides into a product within three to five years.

‘To really stand out we have to optimise the composition for each different application, rather than trying to adapt one particular chalcogenide for multiple uses,’ commented Lincoln. Because the applications are so diverse, there is no reason that one composition should work for all applications.

Ilika Technologies and Professor Brian Hayden’s chemistry group at the University of Southampton are conducting high-throughput screening of the material, that is, laying down a large array of different material compositions and measuring their physical properties. ‘That gives you the ability to know that the material you choose doesn’t just work, but it’s the very best material you can get. That is useful for industrial applications,’ Lincoln said.

‘One of the key aspects of optimising the material is so you can say: “this is the best material for a specific application”,’ he continued. ‘That gives you a really sustained competitive position, because if you know other compositions are not as good, your competition is always going to be behind you. Unless you’ve got the very best, you’re always vulnerable to someone coming up with a better composition. The optimisation is specifically to address that issue.

‘That’s a fairly unique thing to do in an emerging material system. The unknown about whether a material is the best composition for an application often suppresses uptake, because people are nervous that the next-best material will come along tomorrow. It not only means you know that the material has the right properties, but that these properties are at the most stable point in the manufacturing process, which keeps costs down.’

About the author

Greg Blackman is the editor for Electro Optics, Imaging & Machine Vision Europe, and Laser Systems Europe.

You can contact him at greg.blackman@europascience.com or on +44 (0) 1223 275 472.

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