FEATURE SINGLE PHOTON COUNTING
g
colleagues are working on silicon-based multipliers, with germanium absorbers. ‘You get all the benefits of silicon multiplication, but you have an absorber with a narrower bandgap semiconductor, and that could get up to 1.6µm at room temperature,’ he said. ‘There may be ways of incorporating tin into germanium that could take it even further. The challenge is to try and get this to work near room temperature.’ Such a device might be relatively easy to manufacture, and UK researchers ‘have a lead’ on such research, Buller added. SPEXS therefore includes industrial partners who ‘might be interested in either forming a supply chain and or being end-users’. Another strand of the SPEXS
programme is being led by the University of Sheffield, based on detectors made solely from InAs. This semiconductor’s bandgap allows absorption up to 3.3µm wavelengths, which might enable remote spectroscopic sensing of greenhouse gases like carbon dioxide and methane, explained Sheffield researcher Jo Shien Ng. This spectral region is already covered by conventional InAs photodiodes for thermal imaging. ‘People don’t realise that it actually could also work very well as for avalanche photodiodes,’ added Ng’s colleague Chee Hing Tan. ‘We found some promising properties over 10 years ago. We continued to work on this material, which is quite tricky, so it took us quite a few years.’
Making it real Tan explains that the InAs single photon detectors are effectively solid state versions of photomultiplier tubes. ‘It is very interesting for this project to see if it can produce the internal amplification without noise.’ The detectors can already detect signals comprising just 15
photons. To get sufficient gain to continue increasing the sensitivity involves achieving appropriate semiconductor doping profiles, Ng added. Their InAs project will take place in
two stages, revealed Tan. ‘The first phase would cover up to just over 3µm,’ he said. ‘Then there is a more ambitious phase of the project that aims to extend all the way to 5µm.’ That opens up spectroscopic possibilities, he observed, because many molecules absorb light in the 3-5µm range. Light in this spectrum can also penetrate smoke, and might be able to locate hotspots in forest fires, for example. Tan recognised that like InGaAs devices, such detectors will likely be expensive. ‘Right now our target has to be the high value market,’ he said. ‘Likely we are going to look at geospatial applications where the equipment is in the order of tens of thousands of pounds.’
“Light in this spectrum can also penetrate smoke, and might be able to locate hotspots in forest fires, for example”
The wavelength range covered by SPEXS ‘is particularly relevant for the technical requirement of environmental monitoring,’ noted Teledyne e2v’s Weatherup. ‘Of equal importance is the translation of such technology to mainstream manufacturing for volume and cost benefits,’ he added. That reflects Teledyne e2v’s advisory
role in SPEXS. The company became aware of SPEXS as a strategic partner in the QuantIC Hub in Glasgow, UK, as a result of its interest in quantum technologies for a range of sensing and imaging innovations. ‘We engaged
Real-time 3D reconstruction
The University of Sheffield is developing InAs single photon detectors
with Gerald and his team early in the process and became involved in technical exchanges to help develop our plans for future imaging sensors,’ Weatherup explained. ‘We are engaged in identifying collaboration and commercialisation opportunities that can be a practical route to achieving an economic impact from the programme. This is particularly focussed on the utilisation of our existing supply chain and in-house capabilities.’ Similarly, Leonardo is involved in
the project as an interested potential user that could purchase detectors for its integrated systems. Leonardo has assessed commercially available single photon detection possibilities, Lamb explained. ‘We’ve understood, in certain cases, the limitations,’ he said. ‘When the UK government funds academia, it should be funding a new generation of detectors. I’ve certainly been very prominent in arguing the case for a UK supply chain to encourage the development of these technologies. Other people in industry have a similar view.’ Lamb is clear that SPEXS must
320 metres RGB reference Lidar GPU
Teledyne e2v already makes silicon-based electron multiplying charge coupled devices (EMCCDs) and is interested in extending its detection capability to the infrared
16 Electro Optics June 2020
have both academic and commercial partners. ‘When you transfer technology from academia into what is effectively engineering for the commercial market, one has to have a very clear idea of what the market demand is, what the market requirement is, and what the end use is going to be,’ he said. ‘The point about SPEXS is that there is an industrial advisory group. The academics get something out of it because they understand how that technology is to be used in a real application. Very often academics don’t fully appreciate the environment where their invention might have to work. And that’s where the commercial engineer steps in to make it a reality.’ EO
@electrooptics |
www.electrooptics.com
Teledyne e2v
University of Sheffield
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