More than half of the energy the Sun sends to Earth stays invisible to us. The human eye responds only to wavelengths between roughly 400 and 700 nanometres. Anything beyond that does reach the visual pigments in the retina, but carries too little energy to flip them: the molecular switch needs about 1.6 electron volts, and infrared photons deliver less. Night-vision devices compensate, but so far they offer only gradations of brightness, usually in green.
A team around Chengchang Fu, Xin Tang and Ge Mu at the Beijing Institute of Technology has now found a way around that limit. Writing in the journal Science Advances, the researchers describe a device that translates infrared radiation not into brightness but into colour – and does so in a way that lets wavelength and intensity be read off the hue.
The gain is more than cosmetic. The human eye distinguishes hues far more finely than steps in brightness. According to the study's calculations, this makes differences in infrared power visible that are around 200 times smaller than a purely monochrome display can convey.
Two emitting layers and an energy barrier
The conversion begins in a layer of quantum dots made of the semiconductor mercury telluride, each only about four nanometres across. In structures that small, energy levels are no longer continuous but broken into discrete steps. Short-wave infrared therefore triggers different and more numerous charge transitions than long-wave infrared – the number of positive charge carriers produced reveals which radiation arrived.
Above it sits an organic light-emitting diode with two stacked emissive layers: a red-emitting one below, a cyan-emitting one on top. They are separated by an energy barrier of about 0.82 electron volts. When few charge carriers arrive, the red layer captures all of them and the device glows faintly red. As their number grows, the barrier overflows, cyan mixes in, and the hue shifts. In the experiments, infrared light at 980, 1,550 and 2,000 nanometres each produced clearly different colours.
Because the layers are thin and largely transparent, the whole assembly could be built into a pair of glasses. The prototype weighs 23 grams and has an active area of about 3.6 square centimetres. Normal vision is preserved, with the infrared information laid over it as in an augmented-reality display.
The researchers see applications in night-vision technology, in sensing, and in identifying substances by their infrared signature. They have already tested a further step in mice: a miniaturised converter placed under the retina and coupled to the receptor for blue light made that receptor respond to infrared radiation, detectable in brain recordings. A visual prosthesis is still a long way off. As a laboratory prototype, though, the glasses show that the range of human sight can be extended with comparatively lean means.