Electron microscopes give us an insight into the smallest details of matter and can, for example, make the atomic structure of materials, the structure of proteins or the shape of virus particles visible. Most of the materials in nature, however, are not static, but interact, move and constantly transform themselves. One of the most important phenomena is the interaction between light and matter, which is ubiquitous in plants, optical components, solar cells, displays and lasers, for example. These interactions - defined by electrons that are moved by the oscillations of light - take place on ultrafast time scales of attoseconds, i.e. 10-18Seconds or a billionth of a billionth of a second. So far it has not been possible to make the reaction dynamics of light and matter directly visible at such extreme speeds.
A team of physicists from the University of Konstanz and the Ludwig Maximilians University in Munich has now succeeded in combining a transmission electron microscope (TEM) with a continuous wave laser and thus designing the prototype of an attosecond electron microscope (A-TEM). The results appear in the latest issue of Science Advances .
Modulation of the electron beam
"Fundamental phenomena in optics, nanophotonics or metamaterials take place in attoseconds, that is, in a time span shorter than a light cycle", explains Prof. Dr. Peter Baum, first author of the study and head of the working group for light and matter at the Department of Physics at the University of Konstanz. “In order to be able to make ultrafast interactions between light and matter visible, a time resolution below the period of oscillation of the light is necessary.” Conventional transmission electron microscopes use a continuous electron beam to illuminate a sample and generate an image. In order to achieve such attosecond time resolution, Baum's group uses the rapid oscillations of a continuous wave laser to modulate the time of the electron beam inside the microscope.
Ultra-short electron pulses
A thin membrane with which the scientists break the symmetry of the optical oscillations of the laser wave is the key element in their experiment. In the laser-illuminated membrane, the electrons are accelerated and decelerated in quick succession. “This means that the electron beam is converted into a series of ultra-short electron pulses in the electron microscope,” says postdoctoral researcher Dr. Andrey Ryabov, first author of the study. With a further laser wave, which is split off from the first wave, an optical phenomenon is generated in an object to be examined. The object and its reaction to the laser light are then measured with the ultra-short electron pulses.
Easy adaptation, big impact
“The main advantage of our method is that we use the existing continuous electron beam inside the electron microscope instead of changing the electron source. As a result, we have a million times more electrons per second than in previous experiments, basically the full brightness of the source, which is the basic requirement for any practical application, ”continues Ryabov. Another advantage is that the necessary technical adjustments to the microscope are fairly simple and do not require any modifications to the electron gun.
As a result, it is now possible to achieve attosecond resolution in a whole series of space-time imaging processes, for example in time-resolved holography, waveform electron microscopy or laser-assisted electron spectroscopy. In the long term, the new attosecond electron microscopy could help to better understand the atomic origin of light-matter interactions in complex materials and biological substances and to optimize them for applications.
Reference: Ryabov A, Thurner JW, Nabben, Tsarev MV, Baum P. Attosecond metrology in a continuous-beam transmission electron microscope. Sci. Adv. 2020;6(46). doi:10.1126/sciadv.abb1393
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