Coherent control of radiation patterns of nonlinear multiphoton processes in nanoparticles

We propose a scheme for the coherent control of light waves and currents in metallic nanospheres which applies independently of the nonlinear multiphoton processes at the origin of waves and currents. We derive conditions on the external control field which enable us to change the radiation pattern and suppress radiative losses or to reduce absorption, enabling the particle to behave as a perfect scatterer or as a perfect absorber. The control introduces narrow features in the response of the particles that result in high sensitivity to small variations in the local environment, including subwavelength spatial shifts

Dataset on coherent control of fields and induced currents in nonlinear multiphoton processes in a nanosphere

We model a scheme for the coherent control of light waves and currents in metallic nanospheres which applies independently of the nonlinear multiphoton processes at the origin of waves and currents. Using exact mathematical formulae, we calculate numerically with a custom fortran code the effect of an external control field which enable us to change the radiation pattern and suppress radiative losses or to reduce absorption, enabling the particle to behave as a perfect scatterer or as a perfect absorber. Data are provided in tabular, comma delimited value format and illustrate narrow features in the response of the particles that result in high sensitivity to small variations in the local environment, including subwavelength spatial shifts

Universal behavior in finite 2D kinetic ferromagnets

We show that quenching and coarsening of the kinetic Ising model with a non-conserved order parameter displays the same average evolution for finite systems of arbitrary size. This multiscale similarity applies not just within the characteristic length scale of the system, but also at all length and time scales, including in the regime where simple dynamical scaling fails for finite systems. As a consequence, beyond rapidly decaying finite size effects, the evolution of correlations in small systems exactly parallels arbitrarily larger cases. We suggest that dynamical scaling data collapse can be partially restored with an area law-like correction, but unlike the multiscale behavior, this does not hold for distances approaching a quarter of the system size

Two beam toy model for dislocation contrast in ECCI

Dislocation contrast in the SEM, as observed though electron channelling contrast imaging (ECCI), is commonly treated analogously to the contrast in the TEM. This perception is based on early studies done for dislocations parallel with the surface where the surface relaxation is negligible. However, for threading dislocations (TD) that interact with the surface (normal or inclined), as is the case for nitrides materials, g b type invisibility criteria are no longer fully applicable to ECCI, especially in forward geometry [1]. Dislocations change locally the lattice curvature and Bragg diffraction conditions in the crystal, affecting the form and diffracting behaviour of the electron wavefunction in that region. More explicitly, Howie and Whelan [2] had shown that dislocation contrast is the result of interband transitions between Bloch waves states which, in turn, are caused by the change in the displacement field, u(r), around the dislocation or local “strain”. Dynamical models have been used successfully to both predict and characterise dislocations in ECCI [3]. Nevertheless, the behaviour of dislocation contrast in ECCI in particular and diffraction contrast in the SEM in general remains somewhat opaque. In the work we investigate the behaviour of contrast causing strain as a means of insight into this problem

Characterization of the blue emission of Tm/Er co-implanted GaN

Comparative studies have been carried out on the cathodoluminescence (CL) and photoluminescence (PL) properties of GaN implanted with Tin and GaN co-implanted with Tin and a low concentration of Er. Room temperature CL spectra were acquired in an electron probe microanalyser to investigate the rare earth emission. The room temperature CL intensity exhibits a strong dependence on the annealing temperature of the implanted samples. The results of CL temperature dependence are reported for blue emission (similar to 477 nm) which is due to intra 4f-shell electron transitions ((1)G(4)-> H-3(6)) associated with Tm3+ ions. The 477 nm blue CL emission is enhanced strongly as the annealing temperature increases up to 1200 degrees C. Blue PL emission has also been observed from the sample annealed at 1200 degrees C. To our knowledge, this is the first observation of blue PL emission from Tin implanted GaN samples. Intra-4f transitions from the D-1(2) level (similar to 465 nm emission lines) of Tm3+ ions in GaN have been observed in GaN:Tm films at temperatures between 20-200 K. We will discuss the temperature dependent Tm3+ emission in both GaN:Tm,Er and GaN:Tm samples

Dynamical simulations of transmission Kikuchi diffraction (TKD) patterns

Truly nanostructured materials pose a significant spatial resolution challenge to the conventional Electron Backscatter Diffraction (EBSD) characterization technique. Nevertheless, the interaction volume can be reduced by the use of electron transparent samples and the acquisition of electron backscatterlike patterns (EBSP) in transmission mode instead. These transmission Kikuchi diffraction (TKD) patterns are typically acquired by mounting a thin foil, similar to transmission electron microscopy (TEM), and tilting it at a slight angle (20◦ -30◦ ) from horizontal towards a standard EBSD camera

Defect evolution and interplay in n-type InN

The nature and interplay of intrinsic point and extended defects in n-type Si-doped InN epilayers with free carrier concentrations up to 6.6x10E20cm-3 are studied using positron annihilation spectroscopy and transmission electron microscopy and compared to results from undoped irradiated films. In as-grown Si-doped samples, V_In-V_N complexes are the dominant III-sublattice related vacancy defects. Enhanced formation of larger V_In-mV_N clusters is observed at the interface, which speaks for high concentrations of additional V_N in the near-interface region and coincides with an increase in the density of screw and edge type dislocations in that area.Comment: 4 pages, 3 figure

Energy-weighted dynamical scattering simulations of electron diffraction modalites in the scanning electron microscope

Transmission Kikuchi diffraction (TKD) has been gaining momentum as a high resolution alternative to electron back-scattered diffraction (EBSD), adding to the existing electron diffraction modalities in the scanning electron microscope (SEM). The image simulation of any of these measurement techniques requires an energy dependent diffraction model for which, in turn, knowledge of electron energies and diffraction distances distributions is required. We identify the sample-detector geometry and the effect of inelastic events on the diffracting electron beam as the important factors to be considered when predicting these distributions. However, tractable models taking into account inelastic scattering explicitly are lacking. In this study, we expand the Monte Carlo (MC) energy-weighting dynamical simulations models used for EBSD [1] and ECP [2] to the TKD case. We show that the foil thickness in TKD can be used as a means of energy filtering and compare band sharpness in the different modalities. The current model is shown to correctly predict TKD patterns and, through the dictionary indexing approach, to produce higher quality indexed TKD maps than conventional Hough transform approach, especially close to grain boundaries

Accelerating the density-functional tight-binding method using graphical processing units

Acceleration of the density-functional tight-binding (DFTB) method on single and multiple graphical processing units (GPUs) was accomplished using the MAGMA linear algebra library. Two major computational bottlenecks of DFTB ground-state calculations were addressed in our implementation: the Hamiltonian matrix diagonalization and the density matrix construction. The code was implemented and benchmarked on two different computer systems: (1) the SUMMIT IBM Power9 supercomputer at the Oak Ridge National Laboratory Leadership Computing Facility (OLCF) with 1 to 6 NVIDIA Volta V100 GPUs per computer node, and (2) an in-house Intel Xeon computer with 1 to 2 NVIDIA Tesla P100 GPUs. The performance and parallel scalability were measured for three molecular models of 1-, 2- and 3-dimensional chemical systems, represented by carbon nanotubes, covalent organic frameworks, and water clusters