Challenges to extracting spatial information about double P dopants in Si from STM images.

The design and implementation of dopant-based silicon nanoscale devices rely heavily on knowing precisely the locations of phosphorous dopants in their host crystal. One potential solution combines scanning tunneling microscopy (STM) imaging with atomistic tight-binding simulations to reverse-engineer dopant coordinates. This work shows that such an approach may not be straightforwardly extended to double-dopant systems.

Fermi-Level Pinning in ErAs Nanoparticles Embedded in III-V Semiconductors.

Embedding rare-earth monopnictide nanoparticles into III-V semiconductors enables unique optical, electrical, and thermal properties for THz photoconductive switches, tunnel junctions, and thermoelectric devices. Despite the high structural quality and control over growth, particle size (<3 nm), and density, the underlying electronic structure of these nanocomposite materials has only been hypothesized. Structural and electronic properties of ErAs nanoparticles with different shapes and sizes (cubic to spherical, 1.

Emerging Nontrivial Topology in Ultrathin Films of Rare-Earth Pnictides.

Thin films of rare-earth monopnictide (RE-V) semimetals are expected to turn into semiconductors due to quantum confinement effects (QCE), lifting the overlap between electron pockets at Brillouin zone edges (X) and hole pockets at the zone center (Γ). Instead, using LaSb as an example, we find the emergence of the quantum spin Hall (QSH) insulator phase in (001)-oriented films as the thickness is reduced to 7, 5, or 3 monolayers (MLs). This is attributed to a strong QCE on the in-plane electron pockets and the lack of quantum confinement on the out-of-plane pocket projected onto the zone center, resulting in a band inversion.

Evaluating Glyph Design for Showing Large-Magnitude-Range Quantum Spins.

We present experimental results to explore a form of bivariate glyphs for representing large-magnitude-range vectors. The glyphs meet two conditions: (1) two visual dimensions are separable; and (2) one of the two visual dimensions uses a categorical representation (e.g.

From single-particle-like to interaction-mediated plasmonic resonances in graphene nanoantennas.

Plasmonic nanostructures attract tremendous attention as they confine electromagnetic fields well below the diffraction limit while simultaneously sustaining extreme local field enhancements. To fully exploit these properties, the identification and classification of resonances in such nanostructures is crucial. Recently, a novel figure of merit for resonance classification has been proposed and its applicability was demonstrated mostly to toy model systems.

Spin relaxation of a donor electron coupled to interface states.

An electron spin qubit in a silicon donor atom is a promising candidate for quantum information processing because of its long coherence time. To be sensed with a single-electron transistor, the donor atom is usually located near an interface, where the donor states can be coupled with interface states. Here we study the phonon-assisted spin-relaxation mechanisms when a donor is coupled to confined (quantum-dot-like) interface states.

Spin decoherence in a two-qubit CPHASE gate: the critical role of tunneling noise.

Rapid progress in semiconductor spin qubits has enabled experimental demonstrations of a two-qubit logic gate. Understanding spin decoherence in a two-qubit logic gate is necessary for optimal qubit operation. We study spin decoherence due to charge noise for two electrons in a double quantum dot used for a two-qubit controlled-phase gate.

Controlling the layer localization of gapless states in bilayer graphene with a gate voltage.

Experiments in gated bilayer graphene with stacking domain walls present topological gapless states protected by no-valley mixing. Here we research these states under gate voltages using atomistic models, which allow us to elucidate their origin. We find that the gate potential controls the layer localization of the two states, which switches non-trivially between layers depending on the applied gate voltage magnitude.

Validation of SplitVectors Encoding for Quantitative Visualization of Large-Magnitude-Range Vector Fields.

We designed and evaluated SplitVectors, a new vector field display approach to help scientists perform new discrimination tasks on large-magnitude-range scientific data shown in three-dimensional (3D) visualization environments. SplitVectors uses scientific notation to display vector magnitude, thus improving legibility. We present an empirical study comparing the SplitVectors approach with three other approaches - direct linear representation, logarithmic, and text display commonly used in scientific visualizations.

Hole spins in an InAs/GaAs quantum dot molecule subject to lateral electric fields.

There has been tremendous progress in manipulating electron and hole-spin states in quantum dots or quantum dot molecules (QDMs) with growth-direction (vertical) electric fields and optical excitations. However, the response of carriers in QDMs to an in-plane (lateral) electric field remains largely unexplored. We computationally explore spin-mixing interactions in the molecular states of single holes confined in vertically stacked InAs/GaAs QDMs using atomistic tight-binding simulations.

Approaching the quantum limit for plasmonics: linear atomic chains.

Optical excitations in atomic-scale materials can be strongly mixed, with contributions from both single-particle transitions and collective response. This complicates the quantum description of these excitations, because there is no clear way to define their quantization. To develop a quantum theory for these optical excitations, they must first be characterized so that single-particle-like and collective excitations can be identified.

Exciton-plasmon interactions in quantum dot-gold nanoparticle structures.

We present a self-assembly method to construct CdSe/ZnS quantum dot-gold nanoparticle complexes. This method allows us to form complexes with relatively good control of the composition and structure that can be used for detailed study of the exciton-plasmon interactions. We determine the contribution of the polarization-dependent near-field enhancement, which may enhance the absorption by nearly two orders of magnitude and that of the exciton coupling to plasmon modes, which modifies the exciton decay rate.

Superresolution four-wave mixing microscopy.

We report on the development of a superresolution four-wave mixing microscope with spatial resolution approaching 130 nm which represents better than twice the diffraction limit at 800 nm while retaining the ability to acquire materials- and chemical- specific contrast. The resolution enhancement is achieved by narrowing the microscope's excitation volume in the focal plane through the combined use of a Toraldo-style pupil phase filter with the multiplicative nature of four-wave mixing.

Plasmonic properties of metallic nanoparticles: the effects of size quantization.

We examine the size quantization of plasmons in metallic nanoparticles using time-dependent density functional theory. For small particles in the quantum limit, we identify "quantum core plasmons" and "classical surface plasmons", both of which are collective oscillations comprised of multiple single-particle transitions. As particle size increases, the response of the classical surface plasmons becomes much larger than that of the quantum core plasmons.

Coherent stokes scattering from gold nanorods: critical dimensions and multicolor near-resonant plasmon excitation.

In this study, we detail the coherent Stokes scattering from gold nanorods in ensemble and single particle measurements. An increase of more than an order of magnitude was observed in the surface plasmon resonance enhancement of coherent Stokes scattering by gold nanorods for small changes in nanorod dimensions. The impact of this dimensional change is, in general, smaller when probed by single color linear and non-linear techniques.

Comparison of the sensitivity and image contrast in spontaneous Raman and coherent Stokes Raman scattering microscopy of geometry-controlled samples.

We experimentally compare the performance and image contrast of spontaneous Raman and coherent Stokes Raman scattering microscopy. We demonstrate the differences between these techniques on a series of geometry-controlled samples that range in complexity from a point (array of tips) to one-dimensional (line grating) and, lastly, two-dimensional (checkereboard) microstructure. Through the use of this sample series, a comparison of the focal volume, achievable signal-to-noise, and resulting image contrast is made.

Effect of mechanical strain on the optical properties of quantum dots: controlling exciton shape, orientation, and phase with a mechanical strain.

We show how a nanomechanical strain can be used to dynamically reengineer the optics of quantum dots, giving a tool to manipulate mechanoexciton shape, orientation, fine structure splitting, and optical transitions, transfer carriers between dots, and interact qubits for quantum processing. Most importantly, a nanomechanical strain reengineers both the magnitude and phase of the exciton exchange coupling to tune exchange splittings, change the phase of spin mixing, and rotate the polarization of mechanoexcitons, providing phase and energy control of excitons.

Discrete dipole approximation for the study of radiation dynamics in a magnetodielectric environment.

We develop a general computational approach, based on the discrete dipole approximation, for the study of radiation dynamics near or inside an object with arbitrary linear dielectric permittivity, and magnetic permeability tensors. Our method can account for dispersion and losses and provides insight on the role of local-field corrections in discrete magnetodielectric structures. We illustrate our method in the case of a source inside a magneto-dielectric, isotropic sphere for which the spontaneous emission rate of a source can be computed analytically.

Optical response of strongly coupled quantum dot-metal nanoparticle systems: double peaked Fano structure and bistability.

In this communication, we study the optical response of a semiconductor quantum dot (SQD) coupled with a metal nanoparticle (MNP). In particular, we explore the relationship between the size of the constituents and the response of the system. We identify, here, three distinct regimes of behavior in the strong field limit that each exhibit novel properties.

Effects of inhomogeneous fields in superresolving structured-illumination microscopy.

The increased resolution attained by structured illumination is based on the degree to which high spatial frequencies can be down converted into the passband of the imaging system. To effectively do this, a high contrast high-frequency illumination pattern is required. We show how the use of high numerical aperture (1.

Mapping the plasmon resonances of metallic nanoantennas.

We study the light scattering and surface plasmon resonances of Au nanorods that are commonly used as optical nanoantennas in analogy to dipole radio antennas for chemical and biodetection field-enhanced spectroscopies and scanned-probe microscopies. With the use of the boundary element method, we calculate the nanorod near-field and far-field response to show how the nanorod shape and dimensions determine its optical response. A full mapping of the size (length and radius) dependence for Au nanorods is obtained.

Accelerating Scientific Discovery Through Computation and Visualization III. Tight-Binding Wave Functions for Quantum Dots.

This is the third in a series of articles that describe, through examples, how the Scientific Applications and Visualization Group (SAVG) at NIST has utilized high performance parallel computing, visualization, and machine learning to accelerate scientific discovery. In this article we focus on the use of high performance computing and visualization for simulations of nanotechnology.

Coupled dipole method with an exact long-wavelength limit and improved accuracy at finite frequencies.

We present a new formulation of the coupled dipole method that accounts for local-field effects and is exact in the long-wavelength limit. This formulation also leads to improved accuracy of the description of light-scattering processes at finite frequencies.

Local-field correction for an interstitial impurity in a crystal.

The local-field correction experienced by an interstitial impurity in a crystal with cubic symmetry is derived by use of a rigorous, self-consistent, semimicroscopic description of spontaneous emission in a microcavity. We compute the local-field factor for various positions of the impurity inside the crystal. Furthermore, we demonstrate that the local-field factor can be computed from a simple electrostatic model as a rapidly converging lattice sum.

Local imaging of photonic structures: image contrast from impedance mismatch.

Photonic structures made from square arrays of air holes in Si(3)N(x) membranes are locally imaged by near-field optical microscopy in illumination mode. Holes with diameters smaller than and larger than the wavelength of light are investigated. Counterintuitively, the holes appear dark and the film is bright in transmission images for both hole sizes.