Researchers record ultrafast chorus dance of electrons on super-small particle

As reported in Science Advances, an international team of researchers observed how electrons, excited by ultrafast light pulses, danced in unison around a particle less than a nanometer in diameter. Researchers measured this dance with unprecedented precision, achieving the first measurement of its kind at the sub-nanometer scale.

The synchronized dance of electrons, known as plasmonic resonance, can confine light for brief periods of time. That light-trapping ability has been applied in a wide range of areas, from turning light into chemical energy to improving light-sensitive gadgets and even converting sunlight into electricity. While they've been studied extensively in systems from several centimeters across to those just 10 nanometers wide, this is the first time researchers were able to break the field's "nanometer barrier."

The study was conducted by researchers from the Department of Energy's SLAC National Accelerator Laboratory and Stanford University in collaboration with Ludwig-Maximilians-Universität München, University of Hamburg, DESY, Northwest Missouri State University, Politecnico di Milano, and the Max Planck Institute for the Structure and Dynamics of Matter.

Early studies have indicated that when plasmonic resonances unfold at incredibly small scales, new phenomena emerge, allowing light to be confined and controlled with unprecedented precision. This characteristic makes understanding exactly how resonances play out at small scales a very interesting topic for researchers.

To better understand plasmonic resonance, researchers first excite electrons around a particle, then wait for them to release their excess energy by emitting an electron. By timing that interval, scientists can determine whether true resonance -- with all electrons moving in unison -- has occurred, or if just one or two electrons were affected. However, these resonances happen at ultrafast timescales -- mere attoseconds, or billionths of a billionth of a second. Observation of these resonances in real time was beyond the reach of existing technologies.

Fortunately, advances in laser technology have enabled researchers to measure electron movements with attosecond precision.

Using attosecond, extreme ultraviolet light pulses, the team triggered and recorded the behavior of electrons within soccer-ball-shaped carbon molecules, informally known as "buckyballs," that measure just 0.7 nanometers in diameter. They precisely timed the process, from the instant light excited the electrons to the moment electrons were emitted, expelling excess energy and allowing the remaining electrons to relax into their usual orbits. Each cycle lasted between 50 to 300 attoseconds, and measurements indicated that the electrons were behaving with strong coherence, like disciplined dancers performing in unison.

"These findings demonstrate, for the first time, that attosecond measurements can provide valuable insights into plasmonic resonances at scales smaller than a nanometer," said Shubhadeep Biswas, the lead author on the paper and a SLAC project scientist.

This breakthrough allows researchers to evaluate a new range of super-small particles, revealing plasmonic characteristics that could enhance the efficiency of existing technologies and lead to novel applications.

"With this measurement, we are unlocking new insights into the interplay between electron coherence and light confinement at sub-nanometer scales," said Matthias Kling, professor of photon science and applied physics at Stanford University and the director of the Science, Research and Development Division at SLAC's Linac Coherent Light Source, a DOE Office of Science user facility. "This work demonstrates the power of attosecond techniques and opens the door to novel approaches in manipulating electrons in future ultrafast electronics, that could be operating at up to a million times higher frequencies than current technology."

"This cutting-edge research is opening new avenues for the development of ultra-compact, high-performance platforms, where light-matter interactions can be controlled by taking advantage of quantum effects emerging at the nanoscale," said Francesca Calegari, professor at the University of Hamburg, lead scientist at DESY.

This research at the Stanford PULSE Institute is part of the Ultrafast Chemical Sciences program supported by the DOE Office of Science.