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Electron microscopes give us an insight into the smallest details of materials and can visualize, for example, the structure of solids, molecules or nanoparticles with atomic resolution. However, most materials in nature are not static. They are constantly interacting, moving and reshaping between initial and final configurations.
One of the more general phenomena is the interaction between light and matter, which is ubiquitous in materials such as solar cells, displays or lasers. These interactions are defined by electrons being pushed and attracted by the oscillations of light, and the dynamics are extremely fast: the light waves oscillate atttoseconds, the billionth of a billionth of a second.
Until now, it has been very difficult to directly visualize these extremely fast processes in space and time, but that is exactly what a team of physicists from the University of Konstanz has managed to do. microscope, providing new insights into the functionality of nanomaterials and dielectric metaatoms. They recently published their results in Nature.
Generation of ultra-short electronic pulses
“If you look closely, almost all phenomena in optics, nanophotonics or metamaterials occur on time scales smaller than one oscillation period of a light wave,” explains Peter Baum, professor of physics and head of the Light and Matter Group at the l University of Konstanz.
‘To film the ultrafast interactions between light and matter, we therefore need a time resolution ofttoseconds.’ To achieve such an extreme recording rate, Baum’s research team uses the rapid oscillations of a continuous-wave laser to convert an electron microscope’s electron beam into a sequence of ultra-short electron pulses.
In this process, a thin silicon membrane creates a periodic acceleration and deceleration of the electrons. “This modulation causes the electrons to reach for each other. After some time, they convert into a train of ultrashort pulses,” explains David Nabben, a doctoral student and first author of the study.
Another laser wave creates the interaction with the reference object. The ultrashort electronic pulses are then used to measure the object’s response to laser light, frozen in time like in a stroboscope. Eventually, the researchers get a movie of the processes with a temporal resolution of anATOsecond.
Study of nanophotonic phenomena
In their study, the scientists present several examples of time-resolved measurements in nanomaterials. Experiments show, for example, the emergence of chiral surface waves that can be controlled by researchers to travel in a specific spatial direction, or characteristic time delays between different radiation modes of nanoantennas. Furthermore, scientists not only study such surface phenomena but also film electromagnetic processes within a waveguide material.
The results are very interesting for further developments in nanophotonics, but also demonstrate the wide range of applications of the new Attosecond Electron Microscopy. “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 paves the way for new developments in photonic integrated circuits or metamaterials,” says Nabben.
Peter Baum, Attosecond Electron Microscopy of Subcycle Optical Dynamics, Nature (2023). DOI: 10.1038/s41586-023-06074-9. www.nature.com/articles/s41586-023-06074-9
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