Observation of Transient and Asymptotic Driven Structural States of Tungsten Exposed to Radiation.

Combining spatially resolved x-ray Laue diffraction with atomic-scale simulations, we observe how ion-irradiated tungsten undergoes a series of nonlinear structural transformations with increasing radiation exposure. Nanoscale defect-induced deformations accumulating above 0.02 displacements per atom (dpa) lead to highly fluctuating strains at ∼0.1 dpa, collapsing into a driven quasisteady structural state above ∼1 dpa. The driven asymptotic state is characterized by finely dispersed vacancy defects coexisting with an extended dislocation network and exhibits positive volumetric swelling, due to the creation of new crystallographic planes through self-interstitial coalescence, but negative lattice strain.

Unusual Otto excitation dynamics and enhanced coupling of light to TE plasmons in graphene.

Transverse-electric (TE) plasmons are a unique and unusual aspect of graphene's plasmonic response that are predicted to manifest when the sign of imaginary part of conductivity changes to negative near the spectral onset of interband transitions. Although thus far, a feasible platform for the direct experimental detection of TE plasmons at finite temperature is yet to be suggested. Here we analyze the dynamics of Otto-Kretschmann excitation of TE plasmons in graphene. We show that TE plasmons supported by graphene in an Otto configuration unusually exhibit a cutoff thickness between the coupling prism and the graphene layer that forbids their efficient coupling to an incident wave in the case of a single-layer graphene at typical finite temperatures. In contrast, significantly increased coupling in the case of an N-layer graphene insulator stack, owing to an N-fold increase of the effective graphene conductivity as the insulator thickness approaches zero, is predicted to provide a TE plasmon resonance that is easily detectable at room temperature.

Embedding metal electrodes in thick active layers for ITO-free plasmonic organic solar cells with improved performance.

We propose and numerically investigate the optical performance of a novel plasmonic organic solar cell with metallic nanowire electrodes embedded within the active layer. A significant improvement (~15%) in optical absorption over both a conventional ITO organic solar cell and a conventional plasmonic organic solar cell with top-loaded metallic grating is predicted in the proposed structure. Optimal positioning of the embedded metal electrodes (EME) is shown to preserve the condition for their strong plasmonic coupling with the metallic back-plane, meanwhile halving the hole path length to the anode which allows for a thicker active layer that increases the optical path length of propagating modes. With a smaller sheet resistance than a typical 100 nm thick ITO film transparent electrode, and an increased optical absorption and hole collection efficiency, our EME scheme could be an excellent alternative to ITO organic solar cells.

Incorporation of nanovoids into metallic gratings for broadband plasmonic organic solar cells.

We present investigation and optimization of a newly proposed plasmonic organic solar cell geometry based on the incorporation of nanovoids into conventional rectangular backplane gratings. Hybridization of strongly localized plasmonic modes of the nanovoids with Fabry-Perot cavity modes originating from surface plasmon reflection at the grating elements is shown to significantly boost performance in the long wavelength regime. This constitutes improved broadband operation while maintaining absorption enhancements at short wavelengths derived from conventional rectangular grating. Our calculations predict a figure of merit enhancement of up to 41% compared to when the nanovoid indented grating is absent. This is a significant improvement over the previously considered rectangular grating structures, which is further shown to be maintained over the entire angular range.

Microstructure of a heavily irradiated metal exposed to a spectrum of atomic recoils.

At temperatures below the onset of vacancy migration, metals exposed to energetic ions develop dynamically fluctuating steady-state microstructures. Statistical properties of these microstructures in the asymptotic high exposure limit are not universal and vary depending on the energy and mass of the incident ions. We develop a model for the microstructure of an ion-irradiated metal under athermal conditions, where internal stress fluctuations dominate the kinetics of structural evolution. The balance between defect production and recombination depends sensitively not only on the total exposure to irradiation, defined by the fluence, but also on the energy of the incident particles. The model predicts the defect content in the high dose limit as an integral of the spectrum of primary knock-on atom energies, with the finding that low energy ions produce a significantly higher amount of damage than high energy ions at comparable levels of exposure to radiation.

An empirical potential for simulating hydrogen isotope retention in highly irradiated tungsten.

We describe the parameterization of a tungsten-hydrogen empirical potential designed for use with large-scale molecular dynamics simulations of highly irradiated tungsten containing hydrogen isotope atoms, and report test results. Particular attention has been paid to getting good elastic properties, including the relaxation volumes of small defect clusters, and to the interaction energy between hydrogen isotopes and typical irradiation-induced defects in tungsten. We conclude that the energy ordering of defects changes with the ratio of H atoms to point defects, indicating that this potential is suitable for exploring mechanisms of trap mutation, including vacancy loop to plate-like void transformations.

Plasmon nanofocusing in a dielectric hemisphere covered in tapered metal film.

We propose and analyze a new type of mechanically robust optical nanofocusing probe with minimized external environmental interference. The probe consists of a dielectric optical fiber terminated by a dielectric hemisphere - both covered in thin gold film whose thickness is reduced (tapered) along the surface of the hemisphere toward its tip. Thus the proposed probe combines the advantages of the diffraction-limited focusing due to annular propagation of the plasmon with its nanofocusing by a tapered metal wedge (i.e. a metal film with reducing local thickness). The numerical finite-element analysis demonstrates strongly subwavelength resolution of the described structure with the achievable size of the focal spot of ~20 nm with up to ~150 times enhancement of the local electric field intensity. Detailed physical interpretations of the obtained results are presented and possible application as a new type of SNOM probe for subwavelength imaging, spectroscopy and sensing are also discussed.

Wavelength-dependent transmission through sharp 90 degrees bends in sub-wavelength metallic slot waveguides.

In this paper, we present a comprehensive numerical study of the wavelength-dependence of transmission through sharp 90 degrees bends in metallic slot waveguides with sub-wavelength localization and varying geometrical parameters. In particular, it is demonstrated that increasing the plasmon wavelength results in a significant increase (up to nearly 100%) of transmission through the bend, combined with a reduction in the mode asymmetry in the second arm of the bend. The mode asymmetry and its relaxation are explained by interference of the transmitted mode with non-propagating and leaky modes generated at the bend. Comparison with the two-dimensional case of a metal-dielectric-metal waveguide is also conducted, showing significant differences for the slot waveguides based on the presence of different non-propagating and leaky modes.

Enhanced resolution beyond the Abbe diffraction limit with wavelength-scale solid immersion lenses.

The solid immersion lens (SIL) is a well-developed near-field optical device for imaging and data storage. Recent experiments have demonstrated high-quality imaging beyond the diffraction limit by nanoscale lenses in an SIL-type implementation [Nature 460, 498 (2009)]; we call these nSIL. A question arises as to what resolution is obtainable with an nSIL. From full three-dimensional, finite-difference time-domain calculations, we demonstrate that the FWHM of the focal spot of an objective-lens-nSIL system can be reduced by greater than 25% compared to a regular macroscopic SIL.

Morphological analysis of 3d atom probe data using Minkowski functionals.

We present a morphological analysis of atom probe data of nanoscale microstructural features, using methods developed by the astrophysics community to describe the shape of superclusters of galaxies. We describe second-phase regions using Minkowski functionals, representing the regions' volume, surface area, mean curvature and Euler characteristic. The alloy data in this work show microstructures that can be described as sponge-like, filament-like, plate-like, and sphere-like at different concentration levels, and we find quantitative measurements of these features. To reduce user decision-making in constructing isosurfaces and to enhance the accuracy of the analysis a maximum likelihood based denoising filter was developed. We show that this filter performs significantly better than a simple Gaussian smoothing filter. We also interpolate the data using natural cubic splines, to refine voxel sizes and to refine the surface. We demonstrate that it is possible to find a mathematically well-defined, quantitative description of microstructure from atomistic datasets, to sub-voxel resolution, without user-tuneable parameters.

Top-down, decoupled control of constitutive parameters in electromagnetic metamaterials with dielectric resonators of internal anisotropy.

A meta-atom platform providing decoupled tuning for the constitutive wave parameters remains as a challenging problem, since the proposition of Pendry. Here we propose an electromagnetic meta-atom design of internal anisotropy (εr ≠ εθ), as a pathway for decoupling of the effective- permittivity εeff and permeability µeff. Deriving effective parameters for anisotropic meta-atom from the first principles, and then subsequent inverse-solving the obtained decoupled solution for a target set of εeff and µeff, we also achieve an analytic, top-down determination for the internal structure of a meta-atom. To realize the anisotropy from isotropic materials, a particle of spatial permittivity modulation in r or θ direction is proposed. As an application example, a matched zero index dielectric meta-atom is demonstrated, to enable the super-funneling of a 50λ-wide flux through a sub-λ slit; unharnessing the flux collection limit dictated by the λ-zone.

Direct Optical Probing of Transverse Electric Mode in Graphene.

Unique electrodynamic response of graphene implies a manifestation of an unusual propagating and localised transverse-electric (TE) mode near the spectral onset of interband transitions. However, excitation and further detection of the TE mode supported by graphene is considered to be a challenge for it is extremely sensitive to excitation environment and phase matching condition adherence. Here for the first time, we experimentally prove an existence of the TE mode by its direct optical probing, demonstrating significant coupling to an incident wave in electrically doped multilayer graphene sheet at room temperature. We believe that proposed technique of careful phase matching and obtained access to graphene's TE excitation would stimulate further studies of this unique phenomenon, and enable its potential employing in various fields of photonics as well as for characterization of graphene.

Plasmonic excitations of 1D metal-dielectric interfaces in 2D systems: 1D surface plasmon polaritons.

Surface plasmon-polariton (SPP) excitations of metal-dielectric interfaces are a fundamental light-matter interaction which has attracted interest as a route to spatial confinement of light far beyond that offered by conventional dielectric optical devices. Conventionally, SPPs have been studied in noble-metal structures, where the SPPs are intrinsically bound to a 2D metal-dielectric interface. Meanwhile, recent advances in the growth of hybrid 2D crystals, which comprise laterally connected domains of distinct atomically thin materials, provide the first realistic platform on which a 2D metal-dielectric system with a truly 1D metal-dielectric interface can be achieved. Here we show for the first time that 1D metal-dielectric interfaces support a fundamental 1D plasmonic mode (1DSPP) which exhibits cutoff behavior that provides dramatically improved light confinement in 2D systems. The 1DSPP constitutes a new basic category of plasmon as the missing 1D member of the plasmon family: 3D bulk plasmon, 2DSPP, 1DSPP, and 0D localized SP.

Computational steering in Monte Carlo simulations of thin film polystyrene.

High molecular weight polymer systems show very long relaxation times, of the order of milliseconds or more. This time-scale proves practically inaccessible for atomic-scale dynamical simulation such as molecular dynamics. Even with a Monte Carlo (MC) simulation, the generation of statistically independent configurations is non-trivial. Many moves have been proposed to enhance the efficiency of MC simulation of polymers. Each is described by a proposal density Q(x'; x): the probability of selecting the trial state x' given that the system is in the current state x. This proposal density must be parametrized for a particular chain length, chemistry and temperature. Choosing the correct set of parameters can greatly increase the rate at which the system explores its configuration space. Computational steering (CS) provides a new methodology for a systematic search to optimize the proposal densities for individual moves, and to combine groups of moves to greatly improve the equilibration of a model polymer system. We show that monitoring the correlation time of the system is an ideal single parameter for characterizing the efficiency of a proposal density function, and that this is best evaluated by a distributed network of replicas of the system, with the operator making decisions based on the averages generated over these replicas. We have developed an MC code for simulating an anisotropic atomistic bead model which implements the CS paradigm. We report simulations of thin film polystyrene.