Lasers change zinc oxide at lightspeeds

Future technology may involve sci-fi transformation techniques like the recently discovered ability to transform metals to semiconductors and vice versa, using the power of the laser.

A group of researchers from the Fritz Haber Institute of the Max Planck Society and the Humboldt-University of Berlin have discovered that a laser can convert a semiconductor to a metal and reverse it back in roughly the time it takes light to travel the width of a red blood cell.

As most of the technology used today relies on transistors, scientists have long tried to optimize them by modifying the material properties so they can be used more flexibly.

"Basically, it would be ideal to have just one material that can do it all, whenever you need it," said Professor Julia Stähler, Humboldt faculty and lead on the study.

The scientists on the team investigated the popular semiconductor zinc oxide and learned that by illuminating it with a laser, a technique known as photo-doping, the semiconductor surface can be turned into a metal – and back again.

This is achieved by photoexcitation – light modifies the electronic properties such that electrons suddenly move freely, and electrical current can flow, as it would in metal. Once the light is switched back off, the material also quickly returns to being a semiconductor.

Conventionally, the conductivity of semiconductors is altered by a chemical process called doping. Because semiconductors only do just that, conduct partially, the properties of the material are often changed to accommodate desired states, but the chemical doping change is permanent.

Metals, conversely, are much better at conducting electricity, therefore the capability to change properties at will is a large step in the right direction for adaptable tech.

"This mechanism is a completely new and surprising discovery," said Lukas Gierster, lead author and Ph.D. student in Stähler's group. "Three things in particular have surprised us: For one, photo- and chemical doping behave so much alike despite being fundamentally different mechanisms; two, gigantic changes can be reached with very low laser power; and three, switching the metal on and off happens quickly."

And it is quick. The conversion to a metal only takes 20 femtoseconds, or roughly, 20 millionths of a billionth of a second.

The speed of the re-formation of the semiconductor was especially surprising for the research group, as it was orders of magnitude faster than in previous studies. In other words, light is an ultrafast switch that has the force to alter the semiconducting properties of zinc oxide to a metallic behavior reversibly.

This discovery could be highly beneficial for high-frequency device applications and ultrafast optically controlled transistors, which could increase processing speed and simplify device design.

"Our gadgets could become faster – and thus smarter," said Stähler. "Low-power, ultrafast switching of conduction properties will provide us with high speed and design flexibility."

She and her colleagues are convinced that the same will prove true for other semiconducting materials, so that their discovery will likely reach much further than just zinc oxide.