Electron microscopes are used to visualize the structure of solids, molecules, or nanoparticles with atomic resolution. However, most materials are not static. Rather, they interact, move, and reshape between initial and final configurations. For example, the interaction between light and matter. These interactions—defined by electrons pushed and pulled around by the oscillations of light—are extremely fast: light waves oscillate at attoseconds (one-billionth of one-billionth of one second.)
Until now, it has been very difficult to directly visualize these extremely fast processes in space and time. But a team of physicists has successfully recorded movies with attosecond time resolution in a transmission electron microscope, providing new insights into the functionality of nanomaterials and dielectric meta-atoms.
The results are published in Nature, in the paper, “Attosecond electron microscopy of sub-cycle optical dynamics.”
“If you look closely, almost all phenomena in optics, nanophotonics, or metamaterials occur on time scales below one oscillation period of a light wave,” explained Peter Baum, PhD, physics professor and head of the Light and Matter Group at the University of Konstanz. “To film the ultrafast interactions between light and matter, we, therefore, need a time resolution of attoseconds.” To achieve such an extreme recording speed, Baum’s group used the fast oscillations of a continuous-wave laser to convert the electron beam of an electron microscope into a sequence of ultrashort electron pulses.
In this process, a thin silicon membrane creates a periodic acceleration and deceleration of the electrons. “This modulation causes the electrons to catch up with each other. After some time, they convert into a train of ultrashort pulses,” explained David Nabben, a doctoral student in the Baum lab. Another laser wave creates the interaction with the sample object. The ultrashort electron pulses are then used to measure the object’s response to the laser light, frozen in time like in a stroboscope. In the end, the researchers obtain a movie of the processes with attosecond time resolution.
The scientists present several examples of time-resolved measurements in nanomaterials. The experiments show the emergence of chiral surface waves that can be controlled by the researchers to travel in a specific spatial direction, or characteristic time delays between different modes of radiation from nanoantennas. The scientists not only investigate such surface phenomena, but also film the electromagnetic processes inside a waveguide material.
The results illustrate a new development in nanophotonics and also the broad application range of the new attosecond electron microscopy. “The direct measurement of the electromagnetic functionality of materials as a function of space and time is not only of great value for fundamental science, but also opens up the way for new developments in photonic integrated circuits or metamaterials,” Nabben asserted.