MALTS: A Tool to Simulate Lorentz Transmission Electron Microscopy From Micromagnetic Simulations

Here we describe the development of the MALTS software which is a generalized tool that simulates Lorentz Transmission Electron Microscopy (LTEM) contrast of magnetic nanostructures. Complex magnetic nanostructures typically have multiple stable domain structures. MALTS works in conjunction with the open access micromagnetic software Object Oriented Micromagnetic Framework or MuMax. Magnetically stable trial magnetization states of the object of interest are input into MALTS and simulated LTEM images are output. MALTS computes the magnetic and electric phases accrued by the transmitted electrons via the Aharonov-Bohm expressions. Transfer and envelope functions are used to simulate the progression of the electron wave through the microscope lenses. The final contrast image due to these effects is determined by Fourier Optics. Similar approaches have been used previously for simulations of specific cases of LTEM contrast. The novelty here is the integration with micromagnetic codes via a simple user interface enabling the computation of the contrast from any structure. The output from MALTS is in good agreement with both experimental data and published LTEM simulations. A widely-available generalized code for the analysis of Lorentz contrast is a much needed step towards the use of LTEM as a standardized laboratory technique.

Linear to Circular Polarization Reflector with Transmission Band

A frequency selective surface (FSS) which can be utilized as a diplexer for circular polarization (CP) applications is proposed. The structure consists of two dipole-based FSS placed parallel to each other. The dipoles in one array are rotated by 90° with respect to those in the other. For an angle of incidence of 45° at one frequency band the structure allows a CP signal to be transmitted while at a further band it converts a linearly polarized (LP) signal to CP upon reflection. Full-wave simulation results validated the concept.

Linear-to-Circular Polarization Reflector With Transmission Band

A new class of polarizing surface is proposed that in a given frequency band can reflect incident linearly polarized waves with circular polarization (CP) while at other frequencies is transparent allowing incident waves to transmit unaffected. The proposed structure consists of two parallel anisotropic frequency selective surfaces (FSSs) that independently interact with TE or TM waves, respectively. The FSSs are designed to, respectively, transmit TE and TM waves within the same transmission frequency range, so that the combined structure is transparent to all polarizations in this band. Likewise, the two arrays are designed to, respectively, reflect TE and TM incident waves in a common reflection band, so that all polarizations are fully reflected in this range; if the separation of the two arrays is such that the TE and TM components of an incident wave polarized at slant 45° experience a 90° phase shift, reflection will occur in CP. The concept and performance limitations are theoretically investigated using transmission line theory as well as full wave results. The predicted performance is validated by means of experimental results on a fabricated prototype. The proposed structure is pertinent for employment as a quasi-optical diplexer in CP dual-band systems such as reflector antennas.

Exploiting Human Resource Requirements to Infer Human Movement Patterns for Use in Modelling Disease Transmission Systems: An Example from Eastern Province, Zambia

In this research, an agent-based model (ABM) was developed to generate human movement routes between homes and water resources in a rural setting, given commonly available geospatial datasets on population distribution, land cover and landscape resources. ABMs are an object-oriented computational approach to modelling a system, focusing on the interactions of autonomous agents, and aiming to assess the impact of these agents and their interactions on the system as a whole. An A* pathfinding algorithm was implemented to produce walking routes, given data on the terrain in the area. A* is an extension of Dijkstra's algorithm with an enhanced time performance through the use of heuristics. In this example, it was possible to impute daily activity movement patterns to the water resource for all villages in a 75 km long study transect across the Luangwa Valley, Zambia, and the simulated human movements were statistically similar to empirical observations on travel times to the water resource (Chi-squared, 95% confidence interval). This indicates that it is possible to produce realistic data regarding human movements without costly measurement as is commonly achieved, for example, through GPS, or retrospective or real-time diaries. The approach is transferable between different geographical locations, and the product can be useful in providing an insight into human movement patterns, and therefore has use in many human exposure-related applications, specifically epidemiological research in rural areas, where spatial heterogeneity in the disease landscape, and space-time proximity of individuals, can play a crucial role in disease spread.