Benjamin J. McMorran

Electron Vortex Beams with High Quanta of Orbital Angular Momentum

Diffraction holograms are used to create electron vortex beams that should enable higher-resolution imaging. Electron beams with helical wavefronts carrying orbital angular momentum are expected to provide new capabilities for electron microscopy and other applications. We used nanofabricated diffraction holograms in an electron microscope to produce multiple electron vortex beams with well-defined topological charge. Beams carrying quantized amounts of orbital angular momentum (up to 100ℏ) per electron were observed. We describe how the electrons can exhibit such orbital motion in free space in the absence of any confining potential or external field, and discuss how these beams can be applied to improved electron microscopy of magnetic and biological specimens.

An electron Talbot interferometer

We report the first demonstration of a Talbot interferometer for electrons. The interferometer was used to image the Talbot carpet behind a nano-fabricated material grating. The Talbot interferometer design uses two identical gratings, and is particularly sensitive to distortions of the incident wavefronts. To illustrate this we used our interferometer to measure the curvature of concave wavefronts in a weakly focused electron beam. We describe how this wavefront curvature demagnified the Talbot revivals, and we discuss further applications for electron Talbot interferometers.

Efficient linear phase contrast in scanning transmission electron microscopy with matched illumination and detector interferometry

The ability to image light elements in soft matter at atomic resolution enables unprecedented insight into the structure and properties of molecular heterostructures and beam-sensitive nanomaterials. In this study, we introduce a scanning transmission electron microscopy technique combining a pre-specimen phase plate designed to produce a probe with structured phase with a high-speed direct electron detector to generate nearly linear contrast images with high efficiency. We demonstrate this method by using both experiment and simulation to simultaneously image the atomic-scale structure of weakly scattering amorphous carbon and strongly scattering gold nanoparticles. Our method demonstrates strong contrast for both materials, making it a promising candidate for structural determination of heterogeneous soft/hard matter samples even at low electron doses comparable to traditional phase-contrast transmission electron microscopy. Simulated images demonstrate the extension of this technique to the challenging problem of structural determination of biological material at the surface of inorganic crystals.

Roadmap on structured light

Structured light refers to the generation and application of custom light fields. As the tools and technology to create and detect structured light have evolved, steadily the applications have begun to emerge. This roadmap touches on the key fields within structured light from the perspective of experts in those areas, providing insight into the current state and the challenges their respective fields face. Collectively the roadmap outlines the venerable nature of structured light research and the exciting prospects for the future that are yet to be realized.

Efficient diffractive phase optics for electrons

Electron diffraction gratings can be used to imprint well-defined phase structure onto an electron beam. For example, diffraction gratings have been used to prepare electron beams with unique phase dislocations, such as electron vortex beams, which hold promise for the development of new imaging and spectroscopy techniques for the study of materials. However, beam intensity loss associated with absorption, scattering, and diffraction by a binary transmission grating drastically reduces the current in the beam, and thus the possible detected signal strength it may generate. Here we describe electron-transparent phase gratings that efficiently diffract transmitted electrons. These phase gratings produce electron beams with the high current necessary to generate detectable signal upon interaction with a material. The phase grating design detailed here allows for fabrication of much more complex grating structures with extremely fine features. The diffracted beams produced by these gratings are widely separated and carry the designed phase structure with high fidelity. In this work, we outline a fabrication method for high-efficiency electron diffraction gratings and present measurements of the performance of a set of simple prototypical gratings in a transmission electron microscope. We present a model for electron diffraction gratings that can be used to optimize the performance of diffractive electron optics. We also present several new holograms that utilize manipulation of phase to produce new types of highly efficient electron beams.

Tailoring magnetic energies to form dipole skyrmions and skyrmion lattices

Author(s): Montoya, SA; Couture, S; Chess, JJ; Lee, JCT; Kent, N; Henze, D; Sinha, SK; Im, MY; Kevan, SD; Fischer, P; McMorran, BJ; Lomakin, V; Roy, S; Fullerton, EE | Abstract:

An integrated electrochromic nanoplasmonic optical switch.

We demonstrate active switching of light through a nanoslit based plasmonic devices using electrochromic Prussian blue nanocrystals, and achieve large (∼95%) transmission modulation by switching the nanocrystals between oxidized and reduced states.

Synthesizing skyrmion bound pairs in Fe-Gd thin films

We show that properly engineered amorphous Fe-Gd alloy thin films with perpendicular magnetic anisotropy exhibit bound pairs of like-polarity, opposite helicity skyrmions at room temperature. Magnetic mirror symmetry planes present in the stripe phase, instead of chiral exchange, determine the internal skyrmion structure and the net achirality of the skyrmion phase. Our study shows that stripe domain engineering in amorphous alloy thin films may enable the creation of skyrmion phases with technologically desirable properties.

Propagation of Vortex Electron Wave Functions in a Magnetic Field

The physics of coherent beams of photons carrying axial orbital angular momentum (OAM) is well understood, and such beams, sometimes known as vortex beams, have found applications in optics and microscopy. Recently electron beams carrying very large values of axial OAM have been generated. In the absence of coupling to an external electromagnetic field, the propagation of such vortex electron beams is virtually identical mathematically to that of vortex photon beams propagating in a medium with a homogeneous index of refraction. But when coupled to an external electromagnetic field, the propagation of vortex electron beams is distinctly different from photons. Here we use the exact path integral solution to Schrodingers equation to examine the time evolution of an electron wave function carrying axial OAM. Interestingly we find that the nonzero OAM wave function can be obtained from the zero OAM wave function, in the case considered here, simply by multipling it by an appropriate time and position dependent prefactor. Hence adding OAM and propagating it can in this case be replaced by first propagating then adding OAM. Also, the results shown provide an explicit illustration of the fact that the gyromagnetic ratio for OAM is unity. We also propose a novel version of the Bohm-Aharonov effect using vortex electron beams.

Reversal of patterned Co/Pd multilayers with graded magnetic anisotropy

Magnetization reversal and the effect of patterning have been investigated in full-film and dot arrays of Co/Pd multilayers, using the first-order reversal curve and scanning electron microscopy with polarization analysis techniques. The effect of patterning is most pronounced in low sputtering pressure films, where the size of contiguous domains is larger than the dot size. Upon patterning, each dot must have its own domain nucleation site and domain propagation is limited within the dot. In graded anisotropy samples, the magnetically soft layer facilitates the magnetization reversal, once the reverse domains have nucleated.