Tyler R. Harvey

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.

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.

Origins and demonstrations of electrons with orbital angular momentum

The surprising message of Allen et al. (Allen et al. 1992 Phys. Rev. A 45, 8185 (doi:10.1103/PhysRevA.45.8185)) was that photons could possess orbital angular momentum in free space, which subsequently launched advancements in optical manipulation, microscopy, quantum optics, communications, many more fields. It has recently been shown that this result also applies to quantum mechanical wave functions describing massive particles (matter waves). This article discusses how electron wave functions can be imprinted with quantized phase vortices in analogous ways to twisted light, demonstrating that charged particles with non-zero rest mass can possess orbital angular momentum in free space. With Allen et al. as a bridge, connections are made between this recent work in electron vortex wave functions and much earlier works, extending a 175 year old tradition in matter wave vortices. This article is part of the themed issue ‘Optical orbital angular momentum’.

Stern-Gerlach-like approach to electron orbital angular momentum measurement

Many methods now exist to prepare free electrons into orbital angular momentum states, and the predicted applications of these electron states as probes of materials and scattering processes are numerous. The development of electron orbital angular momentum measurement techniques has lagged behind. We show that coupling between electron orbital angular momentum and a spatially varying magnetic field produces an angular momentum-dependent focusing effect. We propose a design for an orbital angular momentum measurement device built on this principle. As the method of measurement is non-interferometric, the device works equally well for mixed, superposed and pure final orbital angular momentum states. The energy and orbital angular momentum distributions of inelastically scattered electrons may be simultaneously measurable with this technique.

Observation of nanoscale magnetic fields using twisted electron beams

Electron waves give an unprecedented enhancement to the field of microscopy by providing higher resolving power compared to their optical counterpart. Further information about a specimen, such as electric and magnetic features, can be revealed in electron microscopy because electrons possess both a magnetic moment and charge. In-plane magnetic structures in materials can be studied experimentally using the effect of the Lorentz force. On the other hand, full mapping of the magnetic field has hitherto remained challenging. Here we measure a nanoscale out-of-plane magnetic field by interfering a highly twisted electron vortex beam with a reference wave. We implement a recently developed holographic technique to manipulate the electron wavefunction, which gives free electrons an additional unbounded quantized magnetic moment along their propagation direction. Our finding demonstrates that full reconstruction of all three components of nanoscale magnetic fields is possible without tilting the specimen.Beyond high resolving power, electron microscopy can be used to study both the electronic and magnetic properties of a sample. Here, Grillo et al. combine electron vortex beams with holographic detection to measure out-of-plane nanoscale magnetic fields.

Streamlined approach to mapping the magnetic induction of skyrmionic materials

Recently, Lorentz transmission electron microscopy (LTEM) has helped researchers advance the emerging field of magnetic skyrmions. These magnetic quasi-particles, composed of topologically non-trivial magnetization textures, have a large potential for application as information carriers in low-power memory and logic devices. LTEM is one of a very few techniques for direct, real-space imaging of magnetic features at the nanoscale. For Fresnel-contrast LTEM, the transport of intensity equation (TIE) is the tool of choice for quantitative reconstruction of the local magnetic induction through the sample thickness. Typically, this analysis requires collection of at least three images. Here, we show that for uniform, thin, magnetic films, which includes many skyrmionic samples, the magnetic induction can be quantitatively determined from a single defocused image using a simplified TIE approach.

Efficient sorting of free electron orbital angular momentum

We propose a method for sorting electrons by orbital angular momentum (OAM). Several methods now exist to prepare electron wavefunctions in OAM states, but no technique has been developed for efficient, parallel measurement of pure and mixed electron OAM states. The proposed technique draws inspiration from the recent demonstration of the sorting of OAM through modal transformation. We show that the same transformation can be performed with electrostatic electron optical elements. Specifically, we show that a charged needle and an array of electrodes perform the transformation and phase correction necessary to sort orbital angular momentum states. This device may enable the analysis of the spatial mode distribution of inelastically scattered electrons.

Development of STEM-Holography

Low-atomic number materials play a crucial role in life sciences, medicine, and the carbon energy cycle. However, our ability to image these materials at the atomic length scale is limited because they do not scatter electrons at high-angles in the same way a crystalline or high atomic number material does. Additionally, these materials are easily damaged under electron beam illumination. To get around these issues, bold efforts have been made in the fields of electron holography [2] and ptychography [3, 4], leading to myriad techniques that can potentially achieve sub-nanometer resolution. Additionally, offaxis electron holography has been developed and applied in many research groups [5 8], pushing the boundaries of electron microscopy with unprecedented feats such as the atomic resolution electrostatic potential mapping of graphene sheets [9].

Characterization of Electron Orbital Angular Momentum Transfer to Nanoparticle Plasmon Modes

We observed the decay of an electron vortex beam from a state with orbital angular momentum l = 1ħ to l = 0 by interaction with gold nanoparticle surface plasmon modes. Several optical studies have induced plasmon vortices using optical vortices and circularly polarized light and suggested their use in nanophotonic and plasmonic devices [1,2]. Direct observation of angular momentum transfer from electron vortices allows for unique identification of the orbital angular momentum associated with localized plasmon excitations down to the nanometer scale.

Atomic-resolution Imaging Using Cs-corrected Vortex Beams

Phase gratings have been shown to produce electron beams with orbital angular momentum as demonstrated by numerous groups, and show promise for electron magnetic circular dichroism (EMCD) at the atomic scale [1, 2]. A linear diffraction grating will produce diffracted beams with a separation determined by the pitch or spacing between grating lines. A grating with a central line that forks into j + 1 central lines will produce a set of diffracted beams each containing discrete units of orbital angular momentum m = j × n in the n th diffraction order [2]. The discontinuity in the center of the grating imparts a “vortex”-type phase on the diffracted beams. We have built such a forked grating with one discontinuity, a radius of 30 m, and a grating pitch of 80 nm using focused ion beam patterning on a 50 nm thick SiN window. Figure 1 shows a low-magnification SEM image of the grating, and the inset shows the discontinuity (fork) in the center at higher magnification. The SiN thin film was patterned as a phase grating rather than an amplitude grating, and thus the 30 nm trench depth does not extend through the full thickness of the SiN thin film. Amplitude gratings have a theoretical maximum diffraction efficiency of 10.1% into the first order, but this grating achieves ~20% efficiency due to the phase grating design. High diffraction efficiency is a critical consideration for the application of diffractive optics in STEM imaging and spectroscopy.