arXiv subjects

David H. Foster

Publications and source records attributed to David H. Foster.

8 recordsLinked to original sources

arXiv2014

Band alignment and directional stability in abrupt and polar-compensated Si/ZnS interface calculations

We perform a first principles investigation of Si/ZnS interface properties for the [111], [100], and [110] directions, including single-substitution polar-compensated interfaces. The asymmetry of general interface directions poses known challenges for standard methods of calculation: a multiplicity of interface distinctions, artificial electric fields, and indeterminacy of orientation stability. By placing each distinct interface in a variety of supercell environments, we demonstrate that the spread of both band offsets and interface enthalpies is acceptably small for reasonable cell lengths, removing the need for corrections involving inappropriate assumptions or computationally expensive structures. Both the orientation and the ionic character of abrupt (111) zinc blende interfaces are shown to affect band alignment and interface enthalpy. We find that the band offsets for the compensated and abrupt (111) and (100) interfaces lie on a strongly bimodal distribution of total width greater than 1.2 eV, while the (110) band offset lies near the distribution midpoint. The midpoint agrees with previous experiments on (100) interfaces, but only one peak of the distribution agrees with (111) interface experiments, indicating that the grown macroscopic (111) interfaces had significant selectivity among the possible microscopic interfaces. The polar-compensated interfaces are shown to be more stable than the corresponding abrupt interfaces over most growth conditions.

cond-mat.mtrl-sci↗

arXiv2013

Multiscale modeling of solar cells with interface phenomena

We describe a mathematical model for heterojunctions in semiconductors which can be used, e.g., for modeling higher efficiency solar cells. The continuum model involves well-known drift-diffusion equations posed away from the interface. These are coupled with interface conditions with a nonhomogeneous jump for the potential, and Robin-like interface conditions for carrier transport. The interface conditions arise from approximating the interface region by a lower-dimensional manifold. The data for the interface conditions are calculated by a Density Functional Theory (DFT) model over a few atomic layers comprising the interface region. We propose a domain decomposition method (DDM) approach to decouple the continuum model on subdomains which is implemented in every step of the Gummel iteration. We show results for CIGS/CdS, Si/ZnS, and Si/GaAs heterojunctions.

physics.comp-ph↗

arXiv2009

Degenerate Perturbation Theory Describing the Mixing of Orbital Angular Momentum Modes in Fabry-Pérot Cavity Resonators

We present an analytic perturbation theory which extends the paraxial approximation for a common cylindrically symmetric stable optical resonator and incorporates the differential, polarization-dependent reflectivity of a Bragg mirror. The degeneracy of Laguerre-Gauss modes with distinct orbital angular momentum (OAM) and polarization, but identical transverse order N, will become observably lifted at sufficiently small size and high finesse. The resulting paraxial eigenmodes possess two distinct OAM components, the fractional composition subtly depending on mirror structure.

physics.optics↗

arXiv2007

Goos-Haenchen induced vector eigenmodes in a dome cavity

We demonstrate numerically calculated electromagnetic eigenmodes of a 3D dome cavity resonator that owe their shape and character entirely to the Goos-Haenchen effect. The V-shaped modes, which have purely TE or TM polarization, are well described by a 2D billiard map with the Goos-Haenchen shift included. A phase space plot of this augmented billiard map reveals a saddle-node bifurcation; the stable periodic orbit that is created in the bifurcation corresponds to the numerically calculated eigenmode, dictating the angle of its "V". A transition from a fundamental Gaussian to a TM V mode has been observed as the cavity is lengthened to become nearly hemispherical.

physics.optics↗

arXiv2004

Bragg-induced orbital angular-momentum mixing in paraxial high-finesse cavities

Numerical calculation of vector electromagnetic modes of plano-concave microcavities reveals that the polarization-dependent reflectivity of a flat Bragg mirror can lead to unexpected cavity field distributions for nominally paraxial modes. Even in a rotationally symmetric resonator, certain pairs of orbital angular momenta are necessarily mixed in an excitation-independent way to form doublets. A characteristic mixing angle is identified, which even in the paraxial limit can be designed to have large values. This correction to Gaussian theory is zeroth-order in deviations from paraxiality. We discuss the resulting nonuniform polarization fields. Observation will require small cavities with sufficiently high Q. Possible applications are proposed.

physics.optics↗

arXiv2004

Spatial and polarization structure in micro-dome resonators: effects of a Bragg mirror

Micro-domes based on a combination of metallic and dielectric multilayer mirrors are studied using a fully vectorial numerical basis-expansion method that accurately accounts for the effects of an arbitrary Bragg stack and can efficiently cover a large range of dome shapes and sizes. Results are examined from three different viewpoints: (i) the ray-optics limit, (ii) the (semi-) confocal limit for which exact wave solutions are known, and (iii) the paraxial approximation using vectorial Gaussian beams.

physics.optics↗

arXiv2004

Methods for 3-D vector microcavity problems involving a planar dielectric mirror

We develop and demonstrate two numerical methods for solving the class of open cavity problems which involve a curved, cylindrically symmetric conducting mirror facing a planar dielectric stack. Such dome-shaped cavities are useful due to their tight focusing of light onto the flat surface. The first method uses the Bessel wave basis. From this method evolves a two-basis method, which ultimately uses a multipole basis. Each method is developed for both the scalar field and the electromagnetic vector field and explicit ``end user'' formulas are given. All of these methods characterize the arbitrary dielectric stack mirror entirely by its 2\times2 transfer matrices for s- and p-polarization. We explain both theoretical and practical limitations to our method. Non-trivial demonstrations are given, including one of a stack-induced effect (the mixing of near-degenerate Laguerre-Gaussian modes) that may persist arbitrarily far into the paraxial limit. Cavities as large as 50 λare treated, far exceeding any vectorial solutions previously reported.

physics.optics↗

arXiv2003

Directional Tunneling Escape from Nearly Spherical Optical Resonators

We report the surprising observation of directional tunneling escape from nearly spherical fused-silica optical resonators, in which most of the phase space is filled with nonchaotic regular trajectories. Experimental and theoretical studies of the dependence of the far-field emission pattern on both the degree of deformation and the excitation condition show that nonperturbative phase-space structures in the internal ray dynamics profoundly affect tunneling leakage of the whispering-gallery modes.

physics.optics↗