Experimental Physics/Electron- and Ion-Nanooptics

Time-Resolved Electron Holography

Transmission electron microscopy (TEM) is a key method for understanding materials at the nanoscale, as it provides access to structural or atomic origins of macroscopic observations. Electron holography (EH) further deepens this microscopic insight by directly measuring the associated electric and magnetic fields at the same spatial resolution (see, e.g., [1]). To date, EH has been limited to static samples. Extending it to dynamic processes is challenging because the temporal resolution of EH is limited by the long exposure times (typically on the order of seconds) required to obtain data that are well above noise.

For reversible processes, this hurdle is usually overcome by pump-probe techniques. Pump-probe measurements of periodic processes require time gating for the time-dependent signal. In TEM, the required shutter/gating is realized either by using fast direct detection cameras with readout times per frame in the submillisecond range or by gating the beam from the detector by transverse electric or magnetic fields. For accelerating voltages in the range of 200 to 300 kV, the transients that occur when switching the required high currents or voltages prohibit time resolutions below the millisecond range.

We have developed a simple but promising approach to temporal control by exploiting the high sensitivity of interference techniques such as electron holography to controlled instrumental instabilities [2]. In an off-axis EH setup, small instabilities can be easily generated by a mutual phase shift between the two partial waves [3]. In a proof-of-concept experiment, which essentially required only a PC sound card as a D/A converter and some batteries as a constant voltage source for the electron-optical biprism, we have realized time-resolved electron holography in an unmodified TEM with continuous illumination, which allows the measurement of periodically changing potential fluctuations in the sample with a time resolution in the microsecond range.

[1] G. Pozzi, M. Beleggia, T. Kasama, R.E. Dunin-Borkowski, Comptes Rendus Phys. 15 (2014) 126. 
[2] T. Niermann, M. Lehmann, T.Wagner,  Ultramicroscopy 182 (2017) 54–61.
[3] T. Wagner, T. Niermann, D. Berger, M. Lehmann, Proceedings EMC 2016.

In order to demonstrate the simple principle of "interference gating" and to give other scientists the opportunity to test the method themselves without much effort, an intuitive smartphone app was developed that provides the necessary control signals. Illustrated instructions on how to make an adapter cable (from a conventional headphone) to connect the smartphone to the TEM or oscilloscope ensure that the method can be tried out by anyone. For more information, including how to download the app, please contact Dr. Tolga Wagner.