Opinion


Electron Holography for Everyone – Here It Comes Edgar Voelkl HoloWerk LLC, 4 Seneca Forest Ct, Germantown, MD 20876


HoloWerk@mac.com For seventy years electron holography has been attempted


with varying degrees of success. I believe the time is right to produce an electron holography microscope that would make electron holography a mainstream analysis tool. Background. Since Zernike’s invention in 1933, imaging


the phase of the complex object wave has been an ongoing effort [1]. Of the many different approaches, (in-line) holog- raphy, invented by Gabor in 1948, is the most famous [2]. Te invention of the electron biprism by G. Möllenstedt in 1954 and the long-time classified work by E.N. Leith released in 1962 opened the door to off-axis electron holography (EH), the standard EH technique today [3,4]. Further methods like Scherzer focus, defocus series, and differential phase contrast highlight the importance of imaging the phase, which could be summarized by simply stating that the sample first and foremost causes a change in the phase of the electron wave [5–7]. Of the phase imaging techniques, Zernike-type phase contrast and specifically differential phase contrast have seen a recent transition to mainstream popularity because of their “ease of use” through careful hardware and soſtware/work- flow development. Recent electron holography developments. Off-axis EH,


however, remains a complex application largely because of the physics involved: (a) amplitude splitters for large angles do not exist for electrons (requiring the area of interest to be near a sample-free area); (b) electrons are fermions, causing a significant limitation for lateral coherence; and (c) the holo- gram must be processed first before the phase image becomes visible. Nonetheless, at Oak Ridge National Laboratory, early work


on extending phase sensitivity via averaging stroboscopically acquired holograms started in 1995 [8]. However, the stability of the illumination system denied that goal back then. In 2005, the arrival of Cs correctors brought about the needed stability of the illumination system. In 2007 an exposure time of several minutes with minimal loss of fringe contrast was announced [9]. Te increased illumination stability also allowed for the averaging of stroboscopically acquired holograms reported in 2010 [10], delivering high sensitivity, for example, to small mag- netic fields. Related workflow development was announced in 2018 following recent computer-based improvements [11]. Single-electron detectors. Around 2005 a key improve-


ment on the detector side arrived quietly: single-electron detection. Conventional cameras negatively impact phase resolution in EH because fine interference fringes must be recorded at low sampling rates. Single-electron detectors on the other hand have very good specifications for the modu- lation transfer function (MTF) and the detection quantum


34 doi:10.1017/S1551929518001074


Figure 1: Adding electron lenses and deflectors (not shown) between the objective lens and the biprism (blue circle) of a standard transmission electron microscope would allow electron holography to become a mainstream applica- tion, as explained in the text.


www.microscopy-today.com • 2019 January


efficiency (DQE), which make them ideal for EH. As shown in 2015, a hologram recorded on a single-electron detec- tion camera vastly improves the information contained in the phase image (conventionally referred to as phase resolu- tion). With an average electron count of four per pixel, the direct image showed neither the sample nor the interference fringes—only noise. Aſter processing, comparatively well- defined phase images were obtained that allowed the sample thickness to be measured to within ∼2 nm [12]. Tin samples, and definitely electric and magnetic fields,


shiſt the phase of the electron beam, and so it comes as no sur- prise that the phase image can carry a strong signal. In the clas- sical image of electrons as particles, half of them go through the sample and the other half provides the reference beam. In the quantum mechanical description, however, each electron “interacts” with the sample, contrary to the classical particle perspective (ignoring inelastically scattered electrons). Tus EH holography not only records the strongest signals from the sample (it records amplitude and phase), it also benefits from electrons that classically would not be considered as interact- ing with the sample at all. Tis makes EH the most sensitive phase imaging technique. Not yet a mainstream technique. Even with sophisti-


cated workf lows, EH is not a mainstream application for one simple reason: changing magnification interrupts continu- ous imaging of the phase. There are two aspects of this: (a) the magnification change typically includes removing the sample from the field of view, re-aligning the illumination


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