Attosecond XUV laser triggers smallest, shortest dance of electrons ever recorded

The ability to manipulate light at the nanoscale has long been a dream for the field of photonics—and its implications range from more efficient solar cells to high-speed computing. But as researchers delved deeper into nanoplasmonics—the study of the way light interacts with free electrons in tiny metallic structures—they encountered a fundamental limitation: Existing theories and experimental techniques were largely confined to structures at least 10 nm in size. The behavior of plasmonic resonances at even smaller scales, especially below 1 nm, remained elusive.

This motivated a team of researchers to explore the way electrons move collectively at these unprecedented scales. It is well known that plasmons—coherent oscillations of electrons—enable extreme light confinement, but the tools to observe these effects in real time and at the subnanometer level were lacking.

Attosecond laser advances meet nanoplasmonics

Advances in attosecond laser technology now allow capturing events occurring within mere billionths of a billionth of a second, which was the breakthrough necessary to push the boundaries of this field. The result was the first measurement of how electrons, excited by ultrafast light pulses, danced in unison around a particle <1 nm in diameter— demonstrating that attosecond measurements can provide valuable insights for this area.

A recent paper published in Science Advances highlights collaborative work between researchers from SLAC National Accelerator Laboratory, Stanford University, Ludwig-Maximilians-Universität Munich, University of Hamburg, DESY, Northwest Missouri State University, Politecnico di Milano, and the Max Planck Institutes of Quantum Optics and the Structure and Dynamics of Matter. Our team focused on exploring the giant plasmon resonance (GPR) within the C₆₀ fullerene, a molecule composed of 60 carbon atoms arranged in a soccer-ball-like shape.

These fullerenes provide an extreme test case for nanoplasmonics, because they exhibit strong plasmonic resonances—despite their subnanometer size. Unlike larger plasmonic systems, where classical electromagnetic models can be used to describe the behavior of collective electron oscillations, fullerenes require a quantum mechanical treatment to fully capture the role of electron correlations in shaping the plasmonic response. Further, by demonstrating that subnanometer plasmons are governed by many-body electron correlations rather than single-particle excitations, this research challenges conventional wisdom in nanoplasmonics and opens new frontiers for ultrafast science.

Attosecond extreme ultraviolet (XUV) pulses trigger plasmonic excitations

Our experimental methodology used to study these effects involved attosecond extreme ultraviolet (XUV) pulses to trigger plasmonic excitations in C₆₀ and measure the time delay between the excitation event and the emission of electrons. This measurement, which ranged from 50 to 300 attoseconds, provided unprecedented insights into the way electron interactions contribute to the plasmonic response at subnanometer scales. Our experimental findings demonstrated that large-scale electron correlations significantly influence the observed plasmon linewidth and emission timing. Unlike the larger nanoparticles, whose plasmonic behavior is adequately described by classical models, our new findings require full-scale quantum descriptions involving electron correlations.

One of the central findings of this study is the role of many-body interactions in shaping the GPR in C₆₀. It revealed that when the collective plasmon is excited, due to large-scale electron-electron correlations, a transient attractive potential is formed in the system. The outgoing electron, which carries the energy for deexcitation, gets trapped in that transient potential and momentarily causes a photoemission delay. This insight challenges previous assumptions that plasmonic effects at nanoscales could be effectively described using classical electromagnetic theories and highlights the necessity of quantum mechanical approaches.

Our experiments used attosecond streaking spectroscopy, a sophisticated technique that allows us to track electron dynamics with attosecond precision. Using the technique, we isolated the effects of plasmonic resonance from other concurrent ionization processes. This enabled a clear interpretation of the correlation-induced photoemission delays observed in C₆₀, and theoretical simulations based on time-dependent density functional theory (TDDFT) provided further validation of the experimental results.

Broad range of future applications

Beyond its fundamental significance, this research has profound implications for future technological applications. The ability to manipulate and control plasmons at the subnanometer scale opens new possibilities for next-generation quantum computing, ultrafast data transmission, solar energy harvesting, advanced catalysis, and highly sensitive molecular detection. Our findings suggest that attosecond plasmonics could be harnessed to create novel light-based computing architectures capable of operating at petahertz (PHz) frequencies—several orders of magnitude faster than current semiconductor technologies. Moreover, the extreme light confinement enabled by these quantum plasmonic effects could enhance the sensitivity of molecular sensors and allow for real-time detection of chemical and biological interactions at the atomic scale.

While the streaking metrology approach used in this study represents a state-of-the-art technique, further improvements in laser pulse shaping and phase stabilization could push the time resolution down to single-digit attoseconds, providing even deeper insights into ultrafast electron dynamics. Moreover, the recent advancements of free-electron-lasers (FELs) such as the Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory, might be the game-changer. These sources provide high x-ray photon flux for a wide spectral range, and ultrashort pulse duration suitable for high temporal resolution studies. Integrating these experimental techniques with emerging quantum technologies could also open new avenues to explore light-matter interactions in hybrid quantum systems, where plasmonic excitations are coupled with solid-state qubits or superconducting circuits.

Another promising research direction is the study of hybrid plasmonic-quantum systems, where plasmonic excitations are coupled to quantum emitters such as quantum dots, excitons, or single-photon sources. These hybrid platforms could serve as building blocks for quantum networks, where information is encoded in entangled light-matter states and transmitted via nanoscale plasmonic waveguides. The ability to control plasmonic coherence on attosecond timescales could lead to significant advances in quantum optics and secure quantum communication technologies.

The implications extend across multiple disciplines—from fundamental physics and computational modeling to applied nanotechnology and biomedical engineering. As researchers continue to refine both experimental and theoretical techniques, the coming decade is likely to witness remarkable progress in our ability to manipulate and control plasmons at the quantum level, which will bring us closer to a future where light-based computing, ultrafast sensors, and quantum plasmonic devices become a practical reality. By pushing the limits of temporal and spatial resolution in electron dynamics, this study lays the foundation for a new era of scientific exploration, where the quantum nature of plasmonic excitations can be fully harnessed for transformative technological applications.