A typical light sensor is like a well with a windlass: to bring up the bucket, you have to turn the crank. Similarly, a photodetector needs an external voltage to pull out an electric charge. But there are materials that convert light into current by themselves — like a spring that flows without a pump. One such material is zinc oxide, familiar from white paint and sunscreens.

Spectroscopy showed that europium ions were successfully embedded into the crystal lattice of zinc oxide — they glow red under irradiation. Adding cadmium changed the material's behavior: the barrier that hindered current flow between layers disappeared. The result is a junction that allows current to pass in only one direction — like a valve in the heart or a diode in an old vacuum tube.

The upshot: the device produces current when illuminated from ultraviolet to near-infrared, and it responds in 10 microseconds. That's faster than a lightning flash. Such sensors could work for years without recharging — for example, in smoke detectors or cameras on space probes.