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Filming electrons at work


New, patented method for time-resolved acquisition of dynamic processes based on switching of interference patterns. Physicists at TU Berlin's Institute of Optics and Atomic Physics have developed a new method that enables moving images of periodic processes to be acquired in a transmission electron microscope (TEM).

Examples of these processes include switching in state-of-the-art electronic components known as semiconductor nanostructures. Previously, it was not possible to observe the inner workings of such processes in detail.

Developed by Dr. Tolga Wagner under the supervision of Professor Dr. Michael Lehmann, the method is novel in that the research team invented a completely new shutter, or "gating" technique (since patented with which to "film electrons at work" inside a sample in a TEM.

The microscope is located at the TU Berlin campus in Berlin-Charlottenburg and has been optimized especially for electron holography (EH) research. The new method makes it easier to investigate how basic physical processes (e.g. charge carrier dynamics in semiconductor nanostructures) work.

"Researchers in the field of electron microscopy always try to keep the measurement conditions as stable as possible," says Dr. Wagner. High-resolution transmission electron microscopes are very sensitive to disruptive external factors such as vibrations, thermal instabilities, and electromagnetic field fluctuations. This is even truer for electron holography. In order to supply information on aspects such as potential distribution within a sample, electron holography requires two coherent electron waves to overlap (interference). This produces an interference pattern, the electron hologram, which can then be captured. For this to work, the electron waves must be in a stable arrangement relative to one another.

Dr. Wagner and his colleagues, however, purposely disrupt the measurement process. Instead of maintaining maximum stability, they only permit the interference for a short period. The information thus generated comes exclusively from the period in which the interference pattern occurred. There is (almost) no limit on how short the period can be. Time resolutions in the picosecond range (a millionth of the blink of an eye) are possible with a reasonable amount of effort. In addition, the set-up is very sensitive to disruptive external factors, which means it does not take much effort to suppress the interference.

"The basic idea behind our new method is that we use targeted disruptive factors to switch the interference on and off very quickly. That's the principle that our shutter method is based on, which is why we call it 'interference gating,'" explains Dr. Wagner. The position and width of the "gate" determine when information is captured and for how long. Using this technique, the physicists from TU Berlin managed to increase the time resolution of the transmission electron microscope from seconds to 25 nanoseconds.

These are the timescales for electronic processes in various areas, including semiconductors. "Using the time-resolved electron holography method we've developed, it's now possible to film changes in potential due to electrons' movement through semiconductors mere nanometers (a millionth of a millimeter) in size," adds Dr. Wagner.

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Source: TU Berlin