50 research outputs found
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 on electrons with electrostatic optical elements. Specifically, we show that a charged needle and an array of electrodes perform the transformation and phase correction necessary to sort OAM states. This device may enable the analysis of the spatial mode distribution of inelastically scattered electrons
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Constraints and opportunities for integrated multifunctional forest management in the Cairngorms region of Scotland
Scottish forestry policy promotes integrated multifunctional forestry and continued forest expansion, and the private forestry sector is key to achieving these goals. However, a variety of factors can constrain multifunctional forestry, particularly in upland areas. Management objectives for private forests are also not always widely known and the potential impacts of government policy, and particularly conservation designations, on the delivery of multifunctional forestry have not been explored. Furthermore, the effects of landownership on forest management are unclear. The key aim of this research was to evaluate how forests and woodlands in the Cairngorms are being managed, and to determine the main constraints and opportunities to the further application of a multifunctional approach to forest management in the region at various scales.
The research incorporated GIS analysis, a postal questionnaire survey and semi-structured interviews. A typology was developed which identified three key management themes: sustainable multifunctional management; restricted functionality forestry; and dual function management. Forest owner preferences, economic concerns, government policy and the wider land use context all influence forest management. No consistent relationship was evident between specific types of landownership and forest management functionality. A link was apparent between species and structural diversity and forest multifunctionality. Overall, 'social' objectives were relatively weak, particularly for private forests, while 'environmental' objectives were consistently strong and 'productive/ economic' objectives varied in importance.
The themes of constraint and opportunity were: spatial fragmentation and integration; forestry markets; bureaucracy and policy integration; public pressure and public support; and unpredictability and forest resource resilience. Many constraints result from temporal, spatial and organizational scale mis-matches between social and natural systems. The matching up of scales at the management, organizational and market levels and the treatment of forests as complex social-ecological systems is recommended. Further recommendations include: further land management integration; increased policy regionalization; timber market localization; and enhanced long-term planning
Probing Light Atoms at Sub-nanometer Resolution: Realization of Scanning Transmission Electron Microscope Holography
Atomic resolution imaging in transmission electron microscopy (TEM) and
scanning TEM (STEM) of light elements in electron-transparent materials has
long been a challenge. Biomolecular materials, for example, are rapidly altered
when illuminated with electrons. These issues have driven the development of
TEM and STEM techniques that enable the structural analysis of electron
beam-sensitive and weakly scattering nano-materials. Here, we demonstrate such
a technique, STEM holography, capable of absolute phase and amplitude object
wave measurement with respect to a vacuum reference wave. We use an
amplitude-dividing nanofabricated grating to prepare multiple spatially
separated electron diffraction probe beams focused at the sample plane, such
that one beam transmits through the specimen while the others pass through
vacuum. We raster-scan the diffracted probes over the region of interest. We
configure the post specimen imaging system of the microscope to diffraction
mode, overlapping the probes to form an interference pattern at the detector.
Using a fast-readout, direct electron detector, we record and analyze the
interference fringes at each position in a 2D raster scan to reconstruct the
complex transfer function of the specimen, t(x). We apply this technique to
image a standard target specimen consisting of gold nanoparticles on a thin
amorphous carbon substrate, and demonstrate 2.4 angstrom resolution phase
images. We find that STEM holography offers higher phase-contrast of the
amorphous material while maintaining Au atomic lattice resolution when compared
with high angle annular dark field STEM.Comment: 9 pages, 5 figures in main text, 1 supplemental figure in the
appendi
An electron Talbot interferometer
The Talbot effect, in which a wave imprinted with transverse periodicity
reconstructs itself at regular intervals, is a diffraction phenomenon that
occurs in many physical systems. Here we present the first observation of the
Talbot effect for electron de Broglie waves behind a nanofabricated
transmission grating. This was thought to be difficult because of Coulomb
interactions between electrons and nanostructure gratings, yet we were able to
map out the entire near-field interference pattern, the "Talbot carpet", behind
a grating. We did this using a Talbot interferometer, in which Talbot
interference fringes from one grating are moire'-filtered by a 2nd grating.
This arrangement has served for optical, X-ray, and atom interferometry, but
never before for electrons. Talbot interferometers are particularly sensitive
to distortions of the incident wavefronts, and to illustrate this we used our
Talbot interferometer to measure the wavefront curvature of a weakly focused
electron beam. Here we report how this wavefront curvature demagnified the
Talbot revivals, and we discuss applications for electron Talbot
interferometers.Comment: 5 pages, 5 figures, updated version with abstrac
Interpretable and efficient contrast in scanning transmission electron microscopy with a diffraction grating beamsplitter
Efficient imaging of biomolecules, 2D materials and electromagnetic fields
depends on retrieval of the phase of transmitted electrons. We demonstrate a
method to measure phase in a scanning transmission electron microscope using a
nanofabricated diffraction grating to produce multiple probe beams. The
measured phase is more interpretable than phase-contrast scanning transmission
electron microscopy techniques without an off-axis reference wave, and the
resolution could surpass that of off-axis electron holography. We apply the
technique to image nanoparticles, carbon sub- strates and electric fields. The
contrast observed in experiments agrees well with contrast predicted in
simulations