Researchers Discover Ultrafast Light-Driven Electron Slide Effect

Researchers demonstrate a previously unknown effect of electron acceleration with ultrashort laser pulses.

Researchers at Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) and the University of Rostock have discovered a groundbreaking effect in which electrons are strongly accelerated by just a single oscillation of a laser pulse. Previously, this effect was only visible with long laser pulses. Researchers have now observed the effect by using extremely sharp metallic needle tips. This discovery could contribute to the development of ultrafast optoelectronics. The results have now been published in Nature Physics*.

When an intense laser pulse hits a stationary electron, it performs a trembling motion at the frequency of the light field. However, this motion dies down after the pulse, and the electron comes to rest again at its original location. If, however, the light field changes its strength along the electron’s trajectory, the electron builds up an additional drift motion with each oscillation, which it retains even after the pulse. The spatial light intensity acts like a slope that the electron slides down.

This effect, known for decades, is called ponderomotive acceleration. However, due to the low spatial dependence of intensity even in focused light beams, this light-driven sliding effect can only be clearly observed for long-lasting laser pulses with many oscillations of the field. In a recent study, researchers have demonstrated pronounced ponderomotive acceleration during just a single light oscillation. The crucial trick was the use of sharp metallic needle tips, which exhibit an extremely strong spatial variation in light intensity when illuminated with laser light.

Fast electrons and razor-sharp needles
In experiments, the electrons released by the light could, for the first time, be assigned to individual cycles of the light field. For this purpose, tungsten needles with especially sharp tips only a few nanometers in size were produced in the laboratories of the research group led by Prof. Dr. Peter Hommelhoff at the Chair of Laser Physics at FAU using a special process and illuminated with optical laser pulses containing only around three field oscillations.

“Typically, we are particularly interested in the fast electrons released from the nanospikes, which we can precisely control with the waveform of the light pulse,” explains Dr. Jonas Heimerl, research associate at the Chair of Laser Physics. “For these, it is known that the ponderomotive motion is completely suppressed for sharp tips. Surprisingly, it was precisely in the signal of the slow electrons that we discovered a previously unknown and pronounced stripe pattern. Our experiments have even revealed an enhancement of the ponderomotive effects for the slow electrons.”

To compare with the experimental data, the research group led by Prof. Dr. Thomas Fennel at the University of Rostock conducted extensive numerical simulations that quantitatively describe the ponderomotive acceleration effect in a single light oscillation and demonstrate the far-reaching implications for the characterization and control of ultrafast electron dynamics.

“Ponderomotive acceleration is usually described as an effect averaged over many light oscillations. A fascinating aspect of our findings is that this can now be used to measure processes on the timescale of a fraction of a light oscillation,” explains Anne Herzig, a doctoral candidate in Thomas Fennel’s group.

“Although the fundamental physics of the near-field-induced stripe structures can in principle be explained with classical mechanics, they open up a new approach to characterizing the quantum effects of the emission process,” adds Anne Herzig. The insights gained were only possible thanks to the excellent interplay between experiment and theory and have the potential to expand the fundamental understanding of photoemission and enable new applications in ultrafast metrology and optoelectronics.