A new degree of freedom for electron holography

A new degree of freedom for electron holography

Röder, F.; Lubk, A.; Houdellier, F.; Denneulin, T.; Snoeck, E.; Hÿtch, M. J.

Off-Axis Electron Holography permits the direct reconstruction of amplitude and phase of electron waves elastically scattered by an object (see, e.g., [1]). The technique employs the Möllenstedt biprism to mutually incline an object modulat-ed wave and a plane reference wave to form an interference pattern at the detec-tor plane. Limited coherence of the electron beam in presence of aberrations at-tenuates high spatial frequencies of the object exit wave spectrum, which is de-scribed by the sideband envelope function. We explore an extension of the con-ventional electron holography set-up given by deliberately tilting the reference wave independent from the object wave. This allows the transfer of spatial fre-quencies beyond the conventional sideband information limit as predicted by a generalized transfer theory for Off-Axis Electron Holography [2]. This is based on the idea that a reference wave tilted by q0 compensates the wave aberration for the spatial frequency q0 of the object wave spectrum. Thus, an off-axis hologram series with varying reference wave tilt allows in principle a linear synthesis of an effective coherent aperture with a radius reaching out beyond the conventional information limit. Furthermore, an object-independent measurement of aberra-tions as well as strain measurements by dark-field electron holography can be realized using this setup. The experimental realization of an arbitrarily tilted refer-ence wave is challenging and could be realized for the first time at the Hitachi HF3300C I2TEM at CEMES Toulouse for one direction [3]. We used an additional biprism placed in the illumination system. Three condenser lenses were adjusted to provide a demagnified image of the condenser biprism at the sample plane under parallel illumination. The pre-specimen deflectors were adapted to maintain the incident wave vector of the object wave and to realize a tilt of the reference wave as a function of the condenser biprism voltage. Finally, we have experimen-tally shown that dark-field holography can be conducted with an object-independent reference alleviating the need for a uniform area of known structure.

[1] H. Lichte, M. Lehmann, Rep. Prog. Phys. 71 (2008) 016102.
[2] F. Röder, A. Lubk, Ultramicoscopy 152 (2015) 63-74.
[3] F. Röder, F. Houdellier,T. Denneulin, E. Snoeck, M.J. Hÿtch, Ultramicoscopy 161 (2016) 23–40.

Keywords: electron holography; tilted reference wave; aperture systhesis; dark-field

  • Invited lecture (Conferences)
    PICO 2017 - Forth Conference on Frontiers of Aberration Corrected Electron Microscopy, 30.04.-04.05.2017, Kasteel Valsbroek, The Netherlands

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Publ.-Id: 25889