Transforming Infrared into Visible Light

A new coating, illuminated by a low-cost infrared laser, can produce powerful beams of warm white light.

In recent years, LED technology has revolutionised the way we light up the world. Not only are these devices extremely energy efficient but a multitude of colours are possible.

But the light they give out spreads in all directions. For room illumination, where wide-beam lighting is needed, this is ideal. On the other hand, it frustrates applications like projectors or microscopes, which need a tightly-focused beam.

Now scientists at Germany's Philipps University in Marburg have come up with a novel tin-sulphide based material that forms a thin coating.

Illuminated with a cheap, infrared laser source, the coating soaks up the lower energy infrared photons and spits out visible white-coloured light reminiscent in colour to the output from a traditional tungsten filament bulb. The colour can also be tweaked by adjusting the energy input from the source laser.

The new coating, which is a white powder, relatively easy to make and stable at temperatures up to 300 degrees celsius, comprises clusters of tin and sulphur atoms, which arrange themselves into diamond-like shapes. Fixed to these core regions are short hydrocarbon chains.

This configuration yields a pool of electrons that can emit photons at a range of energies, enabling the material to produce the range of colours required so that the light emitted from the surface looks white.

This sort of approach is not new. Scientists have been making similar "phosphors" that can convert one colour of light to another like this for many years.

But what sets apart the new molecule, which is published this week in the journal Science, is that apart from being a warm white colour, the light issuing from the surface remains tightly-focused like the beam from the source laser with the photons emerging in the same direction in which they arrived.

According to lead author Nils Rosemann, their approach could open up new routes for advanced directed illumination technologies, particularly since the ingredients are extremely cheap...