Electron Microscopy with Structured Electrons

Abstract

Nanofabricated diffractive optics are new tools for coherently dividing and manipulating electron wavefunctions, similar to how spatial light modulators are used to engineer light waves in optics. For example, diffraction holograms can be used in a scanning transmission electron microscope to remove aberrations [1], implement new types of interferometric phase contrast [2,3], and produce electron probes with phase vortices and quantized angular momentum [4,5].We are currently investigating the use of nanoscale gratings for off-axis STEM interferometry ( Figure 1). We have used a single grating in a scanning transmission electron microscope (STEM) to form a pathseparated electron interferometer [6]. In this configuration, single electrons are coherently divided before the specimen, and then focused into two or more focused probes at the specimen. We are developing ways to use this for electron interferometry and STEM-holography [7]. Advantages of using a grating as an amplitude-type beamsplitters over Moellenstedt biprisms in conventional electron holography is that (a) it preserves the symmetry of the probe, essentially creating multiple copies of it, (b) it loosens the coherent requirements of the initial electron beam, potentially enabling electron holography in an expanded range of instruments, (c) eliminates the voltage stability requirements of a biprism, and (d) allows for structured probes by incorporating additional holographic features into the grating. Nanostructured electron optics also can be used to implement new types of on-axis interferometric phase contrast in STEM (Figure 2). For example, in Matched Illumination-Detector Interferometry STEM (MIDI-STEM), a transverse phase modulation is introduced to a STEM probe using a nanostructured phase plate in the probe-defining condenser aperture [2,3]. The diffraction plane of the probe is imaged at each probe position. Virtual detector zones are defined on the fast imaging detector. Nonuniform phase introduced by the specimen across the probe results in intensity shifts between the pre-defined zones. For an appropriately defined phase plate, this can provide linear phase contrast at high resolutions in aberration-corrected STEM.

Probing the limits of the rigid-intensity-shift model in differential-phase-contrast scanning transmission electron microscopy

“…A simple probe shape with reduced tail intensity is a Gaussian probe, cf. [68]. A less elegant method -but one perhaps simpler to employ since such phase plates are not yet widely available and inserting them into electron microscopes is non-trivial -would be to use judicious combinations of the several condenser apertures typically available in the microscope.…”