Coherent Electron Wavefront Engineering
The purpose of this project is to manipulate the wave front of coherent free electron wave functions to realize a novel electron diffraction imaging method. Spatial phase modulators that synthesize arbitrarily pulse shapes in space and time are available for coherent electromagnetic waves but coherent manipulation of electron beam is much less explored. For example, the direct demonstration of the quantum mechanical nature of free electrons via Young’s double‐slit interference experiment as described by R. P. Feynman  was reported only recently [2,3], even though the electron wave interference has been demonstrated more than 50 years ago .
A Boersch phase plate can shift the phase of electrons proportionally to the applied electrical potential, thereby allowing for in situ control of the electron phase shift. A device comprising multiple Boersch phase shifter elements will be able to modulate the wavefront of a coherent electron beam and control electron interference. In a recently published work, "Fabrication of low aspect ratio three-element Boersch phase shifters for voltage-controlled three electron beam interference" in Journal of Applied Physics , we developed a fabrication method utilizing the state-of-the-art electron beam lithography and reactive ion etching processes, a combination that is widely used for high-throughput and large-scale micro- and nanofabrication of electronic and photonic devices. Using the developed method, we fabricated a three-element phase shifter device with a metal–insulator–metal structure with 100-nm-thick ring electrodes and tested its electron transmission characteristics in a transmission electron microscope with a beam energy of 200 keV. We observed voltage-controlled evolution of electron interference, demonstrating the voltage-controlled electron phase shift using the fabricated device with a phase shift of π rad per 1 V. We analyze the experimental results in comparison with a three-dimensional electrostatic simulation. Furthermore, we discuss the possible improvements in terms of beam deflection and crosstalk between phase shifter elements in a five-layer device structure.
 R. P. Feynman, R. B. Leighton, M. Sands, The Feynman Lectures on Physics (Addition‐Wesley, Menlo Park, CA, 1965), Vol. III. III, Ch. 1, section 1‐5.
 S. Frabboni, G. C. Gazzadi, G. Pozzi, Young’s double‐slit interference experiment with electrons, Am. J. Phys. 75, 1053 (2007).
 R. Bach, D. Pope, S.‐H. Liou, H. Batelaan, Controlled double‐slit electron diffraction, New J. Physics 15, 033018 (2013).
 C. Joensson, Elektroneninterferenzen an mehreren kuenstlich hergestellten Feinspalten, Z. Physik 161, 454 (1961).
 P. Thakkar, V. A. Guzenko, P.-H. Lum R. E. Dunin-Borkowski, J. P. Abrahams, S. Tsujino, Fabrication of low aspect ratio three-element Boersch phase shifters for voltage-controlled three electron beam interference, J. Appl. Phys. 128, 134502 (2020).
Pooja Thakkar (PSI, BIO & SNI, Uni Basel)
Vitaliy Guzenko, Dr. (PSI, PSD)
Jan Pieter Abrahams, Prof. Dr. (PSI, BIO & SNI, BZ, Uni Basel)
S. Tsujino, Dr. (PSI, BIO & SNI, Uni Basel)
Peng-Han Lu (FZ Jülich & RWTH Aachen)
Rafal E. Dunin-Borkowski, Prof. Dr. (FZ Jülich)