The team, led by Vladimir Bulović, Moungi Bawendi and Marc Baldo of the Energy Frontier Research Center for Excitonics at MIT, made its films on top of glass microscope slides. The films have a simple, two-layer structure. The bottom layer consists of colloidal nanocrystals (also known as quantum dots), which are nanometre-sized chunks of an inorganic semiconductor (lead sulphide) coated with a monolayer of fatty acids to passivate their surface. The top layer is a crystalline film made of an organic molecule called rubrene.

The nanocrystals absorb incoming infrared light, explains team member Mark Wilson. They then transfer this energy to the organic film. This energy, which is in the form of excitons (excited electron-hole pairs), is mobile and diffuses through the rubrene film.

High-energy singlet can emit visible light

“When two low-energy excitons collide, they can create a high-energy exciton, which we call a ‘singlet’ because of the spin physics in these materials,” he says. “The high-energy singlet can emit visible light, so, in short, we are able to change the colour of the light from infrared to the visible. Through this transformation, or ‘upconversion’, energy is conserved because we require two low-energy photons to generate each high-energy photon of visible light.” Such upconversion has many applications, including biological imaging, night vision, multidimensional displays and photovoltaics.

The advantage of excitonic upconversion is that it can operate efficiently at low light intensities – much lower even than that needed in the nonlinear optical processes that create the green light in common laser pointers, he tells “However, until now, upconversion of short-wave infrared light using excitons has proved difficult because it was a challenge to successfully harvest photons at these longer wavelengths (of more than 1 micron). This is because previous techniques typically relied on organometallic complexes to absorb the incoming light and these materials, which albeit strongly absorb light in the infrared, overwhelmingly dissipate the photon energy as heat, so that it is tough to actually do anything useful with it.”

Converting longer infrared wavelengths

“We used colloidal nanocrystals as the infrared-light sensitive materials to overcome this problem,” says Wilson. “We show that not only does this approach work, but that our devices are already quite efficient at upconverting light. The technology is not yet optimized and we are working on understanding how it works and so improving device performance.”

The team, reporting its work in Nature Photonics doi:10.1038/nphoton.2015.226, says that as well as trying to improve light conversion efficiency, it is also looking to lower the light intensity threshold for efficient operation and then improve the films so that they are able to convert longer infrared wavelengths of light (for example of around 1.5 microns). “If we succeed in doing this, our materials might be used to enhance the performance of industry-standard silicon camera technology,” explains Wilson. “As an example, short-wave infrared penetrates further into fog, so an infrared camera containing our film would be ideal for all-weather autonomous vehicles.”