510 research outputs found
Numerical Investigation of Thermal Stress Convention in Nonisothermal Gases Under Microgravity Conditions
Reported here are our results of our numerical/theoretical investigation into the effects of thermal stress in nonisothermal gases under microgravity conditions. The first part of the report consists of a brief summary of the accomplishments and conclusions of our work. The second part consists of two manuscripts, one being a paper presented at the 1998 MSAD Fluid Physics workshop, and the other to appear in Physics of Fluids
Multipole expansion of Bessel and Gaussian beams for Mie scattering calculations
Multipole expansions of Bessel and Gaussian beams, suitable for use in Mie scattering calculations, are derived. These results allow Mie scattering calculations to be carried out considerably faster than existing methods, something that is of particular interest for time evolution simulations where large numbers of scattering calculations must be performed. An analytic result is derived for the Bessel beam that improves on a previously published expression requiring the evaluation of an integral. An analogous expression containing a single integral, similar to existing results quoted, but not derived, in literature, is derived for a Gaussian beam,valid from the paraxial limit all the way to arbitrarily high numerical apertures
Investigation of Thermal Creep and Thermal Stress Effects in Microgravity Physical Vapor Transport
Reported here are the results of our numerical investigation into the mechanisms which affect the transport and growth processes in physical vapor transport (PVT) crystal growth ampoules. The first part of the report consists of a brief summary of the major accomplishments and conclusions of our work. The second part consists of two manuscripts, submitted to the Journal of Crystal Growth, which provided a detailed description of the findings in our investigation
A T matrix method based upon scalar basis functions
A surface integral formulation is developed for the T matrix of a homogenous and isotropic particle of arbitrary shape, which employs scalar basis functions represented by the translation matrix elements of the vector spherical wave functions. The formulation begins with the volume integral equation for scattering by the particle, which is transformed so that the vector and dyadic components in the equation are replaced with associated dipole and multipole level scalar harmonic wave functions. The approach leads to a volume integral formulation for the T matrix, which can be extended, by the use of Green's identities, to the surface integral formulation. The result is shown to be equivalent to the traditional surface integral formulas based on the VSWF basis
Optical Studies of Zero-Field Magnetization of CdMnTe Quantum Dots: Influence of Average Size and Composition of Quantum Dots
We show that through the resonant optical excitation of spin-polarized
excitons into CdMnTe magnetic quantum dots, we can induce a macroscopic
magnetization of the Mn impurities. We observe very broad (4 meV linewidth)
emission lines of single dots, which are consistent with the formation of
strongly confined exciton magnetic polarons. Therefore we attribute the
optically induced magnetization of the magnetic dots results to the formation
of spin-polarized exciton magnetic polarons. We find that the photo-induced
magnetization of magnetic polarons is weaker for larger dots which emit at
lower energies within the QD distribution. We also show that the photo-induced
magnetization is stronger for quantum dots with lower Mn concentration, which
we ascribe to weaker Mn-Mn interaction between the nearest neighbors within the
dots. Due to particular stability of the exciton magnetic polarons in QDs,
where the localization of the electrons and holes is comparable to the magnetic
exchange interaction, this optically induced spin alignment persists to
temperatures as high as 160 K.Comment: 26 pages, 7 figs - submitted for publicatio
Optical binding mechanisms: a conceptual model for Gaussian beam traps
Optical binding interactions between laser-trapped spherical microparticles
are familiar in a wide range of trapping configurations. Recently it has been
demonstrated that these experiments can be accurately modeled using Mie
scattering or coupled dipole models. This can help confirm the physical
phenomena underlying the inter-particle interactions, but does not necessarily
develop a conceptual understanding of the effects that can lead to future
predictions. Here we interpret results from a Mie scattering model to obtain a
physical description which predict the behavior and trends for chains of
trapped particles in Gaussian beam traps. In particular, it describes the
non-uniform particle spacing and how it changes with the number of particles.
We go further than simply \emph{demonstrating} agreement, by showing that the
mechanisms ``hidden'' within a mathematically and computationally demanding Mie
scattering description can be explained in easily-understood terms.Comment: Preprint of manuscript submitted to Optics Expres
Directed assembly of optically bound matter
We present a study of optically bound matter formation in a counter-propagating evanescent field, exploiting total internal reflection on a prism surface. Small ensembles of silica microspheres are assembled in a controlled manner using optical tweezers. The structures and dynamics of the resulting optically bound chains are interpreted using a simulation implementing generalized Lorentz-Mie theory. In particular, we observe enhancement of the scattering force along the propagation direction of the optically bound colloidal chains leading to a microscopic analogue of a driven pendulum which, at least superficially, resembles Newton’s cradle
Probing the inter-layer exciton physics in a MoS/MoSe/MoS van der Waals heterostructure
Stacking atomic monolayers of semiconducting transition metal dichalcogenides
(TMDs) has emerged as an effective way to engineer their properties. In
principle, the staggered band alignment of TMD heterostructures should result
in the formation of inter-layer excitons with long lifetimes and robust valley
polarization. However, these features have been observed simultaneously only in
MoSe/WSe heterostructures. Here we report on the observation of long
lived inter-layer exciton emission in a MoS/MoSe/MoS trilayer van
der Waals heterostructure. The inter-layer nature of the observed transition is
confirmed by photoluminescence spectroscopy, as well as by analyzing the
temporal, excitation power and temperature dependence of the inter-layer
emission peak. The observed complex photoluminescence dynamics suggests the
presence of quasi-degenerate momentum-direct and momentum-indirect bandgaps. We
show that circularly polarized optical pumping results in long lived valley
polarization of inter-layer exciton. Intriguingly, the inter-layer exciton
photoluminescence has helicity opposite to the excitation. Our results show
that through a careful choice of the TMDs forming the van der Waals
heterostructure it is possible to control the circular polarization of the
inter-layer exciton emission.Comment: 19 pages, 3 figures. Just accepted for publication in Nano Letters
(http://pubs.acs.org/doi/10.1021/acs.nanolett.7b03184
T-Matrix Method and its Applications to Electromagnetic Scattering by Particles: A Current Perspective
This note serves as a short introduction to the reprint of our article "T-matrix computations of light scattering by nonspherical particles: a review" (JQSRT 1996; 55:535:75). We first discuss the motivation for writing that article and explain its historical context. This is followed by a short overview of more recent developments
Small spacecraft power and thermal subsystems
This white paper provides a general guide to the conceptual design of satellite power and thermal control subsystems with special emphasis on the unique design aspects associated with small satellites. The operating principles of these technologies are explained and performance characteristics of current and projected components are provided. A tutorial is presented on the design process for both power and thermal subsystems, with emphasis on unique issues relevant to small satellites. The ability of existing technology to meet future performance requirements is discussed. Conclusions and observations are presented that stress cost-effective, high-performance design solutions
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