Oldenburg-Milan research team uses flashes of light to accelerate electrons out of individual nanoparticles

Oldenburg. When light is converted into electricity or chemical energy - in solar cells, for example, or in plant cells during photosynthesis - it is because the incident light rays set the electrons in the materials in motion in a targeted manner. These electron movements take place on extremely short length scales of a few nanometres (a nanometre is a billionth of a metre) and on ultrafast time scales of a few femtoseconds (a femtosecond is a quadrillionth of a second). These processes are so complex that they cannot be followed in detail even with the best microscopes available to date. Researchers around the world are therefore working intensively on developing new techniques to visualise these processes.
Physicists at the University of Oldenburg have achieved a decisive breakthrough in this endeavour. In an article that has now been published in the renowned journal "Nature Photonics", the researchers from the "Ultrafast Nano-Optics" working group headed by Prof Dr Christoph Lienau report for the first time on experiments into the targeted acceleration of electrons from individual gold tips using ultrashort laser pulses.

"Together with partners from Prof Dr Giulio Cerullo's research group in Milan, we have built a special laser that allows us to generate extremely short laser pulses with a precisely adjustable temporal form of the electric field," explains Dr Petra Groß, who heads the project at the Institute of Physics in Oldenburg. "With these so-called phase-controlled pulses, we are able to knock electrons out of a gold tip just a few nanometres in size and accelerate them with precisely measured force in a direction determined by the light field." The scientists now feel like an experienced golfer who controls the trajectory of the ball with a sensitive tee shot. However, the accelerations are many times higher: while golfing achieves about a hundred times the acceleration of gravity, the electrons are accelerated to about one hundredth of the speed of light with an acceleration of ten to the power of twenty g, which is a trillion times higher.

"The gold tips serve as a particularly well-defined launch point for the electrons. Their structure is so simple that we can easily compare our experimental results with model calculations. We learn," says Lienau, "how electrons move on such short time and length scales." This knowledge is of central importance for understanding the even more complex electron movements in technologically relevant components such as solar cells. The Oldenburg scientists are therefore working intensively on utilising the newly developed experimental techniques to elucidate energy conversion processes in solar cells and biological nanostructures.