Prospects for quantitative and time-resolved double and continuous exposure off-axis electron holography

Abstract

The technique of double exposure electron holography, which is based on the superposition of two off-axis electron holograms, was originally introduced before the availability of digital image processing to allow differences between electron-optical phases encoded in two electron holograms to be visualised directly without the need for holographic reconstruction. Here, we review the original method and show how it can now be extended to permit quantitative studies of phase shifts that oscillate in time. We begin with a description of the theory of off-axis electron hologram formation for a time-dependent electron wave that results from the excitation of a specimen using an external stimulus with a square, sinusoidal, triangular or other temporal dependence. We refer to the more general method as continuous exposure electron holography, present preliminary experimental measurements and discuss how the technique can be used to image electrostatic potentials and magnetic fields during high frequency switching experiments.

Quantitative measurement of charge accumulation along a quasi-one-dimensional W5O14 nanowire during electron field emission

“…The fluctuations in measured current were not observed to be associated with corresponding fluctuations in electrostatic potential or electric field. Indeed, no double exposure (i.e., Moiré) effects 30,31 were observed in electron holograms recorded using 6 s exposure times, whether or not the nanowire was field emitting. In addition to the instabilities, the emission current started to drop at an applied bias of 180 V and stopped completely at 188 V (see the ESI † for further details).…”

Holographic imaging of electromagnetic fields via electron-light quantum interference

“…The introduction of temporal resolution in electron holography is more challenging, and so far, the adopted schemes use temporal gating working in the microsecond time scale ( 20 , 21 ). Reaching the ultrafast domain could become a reality owing to recent developments in the ultrafast transmission electron microscope (UEM), in which femtosecond lasers are used to create ultrafast electron pulses ( 22 , 23 ).…”