Private Returns to Education in Ghana: Implications for Investments in Schooling and Migration

This study examines private returns to schooling in Ghana over a seven-year period, and the implications for school investments and migration. Using data from the 1992 and 1999 Ghana living standards surveys and ordinary least squares technique, we find that the private returns to schooling at higher levels of education have increased for both female and male workers. For female workers, the return to an additional year of secondary schooling increased from 7.3% in 1992 to 12.3% in 1999. In the case of tertiary education, the change is from 11.4% in 1992 to 18.4% in 1999. For male workers the return to an additional year of secondary education decreased from about 7% to 6%, while the return to tertiary education increased from about 13% to 19%. Generally, the rising rates of return at higher school levels have coincided with a similar trend in school attendance rates for female and male children. The spatial analysis implies a rural-urban gap in the returns to an additional year of tertiary education. Linking these results to migration, the data show a relatively low incidence of rural-to-urban migration, notwithstanding relatively higher earnings in urban areas. To sustain the gains realized in educational attainment, lingering issues of gender equity need to be addressed by policy makers so that females are not left behind in the intergenerational race for improvements in quality of life

Plasmonics: Chip-based component devices and metamaterials

Dispersion control and active materials integration have yielded plasmonic components including i) three-dimensional single layer plasmonic metamaterials ii) all-optical, electro-optic and field effect modulation of plasmon propagation iii) plasmon-enhanced absorption in solar cells

Design of a film surface roughness-minimizing molecular beam epitaxy

Molecular beam epitaxy of germanium was used along with kinetic Monte Carlo simulations to study time-varying processing parameters and their effect on surface morphology. Epitaxial Ge films were deposited on highly oriented Ge(001) substrates, with reflection high-energy electron diffraction as a real-time sensor. The Monte Carlo simulations were used to model the growth process, and physical parameters were determined during growth under time-varying flux. A reduced version of the simulations was generated, enabling the application on an optimization algorithm. Temperature profiles were then computed that minimize surface roughness subject to various experimental constraints. The final roughness after two layers of growth was reduced to 0.32, compared to 0.36 at the maximum growth temperature. The study presented here is an initial demonstration of a general approach that could also be used to optimize properties in other materials and deposition processes

Plasmonics: Localization and guiding of electromagnetic energy in metal/dielectric structures

We review the basic physics of surface-plasmon excitations occurring at metal/dielectric interfaces with special emphasis on the possibility of using such excitations for the localization of electromagnetic energy in one, two, and three dimensions, in a context of applications in sensing and waveguiding for functional photonic devices. Localized plasmon resonances occurring in metallic nanoparticles are discussed both for single particles and particle ensembles, focusing on the generation of confined light fields enabling enhancement of Raman-scattering and nonlinear processes. We then survey the basic properties of interface plasmons propagating along flat boundaries of thin metallic films, with applications for waveguiding along patterned films, stripes, and nanowires. Interactions between plasmonic structures and optically active media are also discussed

Cooperative behavior of quantum dipole emitters coupled to a zero-index nanoscale waveguide

We study cooperative behavior of quantum dipole emitters coupled to a rectangular waveguide with dielectric core and silver cladding. We investigate cooperative emission and inter-emitter entanglement generation phenomena for emitters whose resonant frequencies are near the frequency cutoff of the waveguide, where the waveguide effectively behaves as zero-index metamaterial. We show that coupling emitters to a zero-index waveguide allows one to relax the constraint on precision positioning of emitters for observing inter-emitter entanglement generation and extend the spatial scale at which the superradiance can be observed

Elimination sequence optimization for SPAR

SPAR is a large-scale computer program for finite element structural analysis. The program allows user specification of the order in which the joints of a structure are to be eliminated since this order can have significant influence over solution performance, in terms of both storage requirements and computer time. An efficient elimination sequence can improve performance by over 50% for some problems. Obtaining such sequences, however, requires the expertise of an experienced user and can take hours of tedious effort to affect. Thus, an automatic elimination sequence optimizer would enhance productivity by reducing the analysts' problem definition time and by lowering computer costs. Two possible methods for automating the elimination sequence specifications were examined. Several algorithms based on the graph theory representations of sparse matrices were studied with mixed results. Significant improvement in the program performance was achieved, but sequencing by an experienced user still yields substantially better results. The initial results provide encouraging evidence that the potential benefits of such an automatic sequencer would be well worth the effort

Measurement of the direct energy gap of coherently strained Sn_xGe_(1–x)/Ge(001) heterostructures

The direct energy gap has been measured for coherently strained Sn_xGe_(1–x) alloys on Ge(001) substrates with 0.035 < x < 0.115 and film thickness 50–200 nm. The energy gap determined from infrared transmittance data for coherently strained Sn_xGe_(1–x) alloys indicates a large alloy contribution and a small strain contribution to the decrease in direct energy gap with increasing Sn composition. These results are consistent with a deformation potential model for changes in the valence and conduction band density of states with coherency strain for this alloy system

Purcell Enhancement of Parametric Luminescence: Bright and Broadband Nonlinear Light Emission in Metamaterials

Single-photon and correlated two-photon sources are important elements for optical information systems. Nonlinear downconversion light sources are robust and stable emitters of single photons and entangled photon pairs. However, the rate of downconverted light emission, dictated by the properties of low-symmetry nonlinear crystals, is typically very small, leading to significant constrains in device design and integration. In this paper, we show that the principles for spontaneous emission control (i.e. Purcell effect) of isolated emitters in nanoscale structures, such as metamaterials, can be generalized to describe the enhancement of nonlinear light generation processes such as parametric down conversion. We develop a novel theoretical framework for quantum nonlinear emission in a general anisotropic, dispersive and lossy media. We further find that spontaneous parametric downconversion in media with hyperbolic dispersion is broadband and phase-mismatch-free. We predict a 1000-fold enhancement of the downconverted emission rate with up to 105 photon pairs per second in experimentally realistic nanostructures. Our theoretical formalism and approach to Purcell enhancement of nonlinear optical processes, provides a framework for description of quantum nonlinear optical phenomena in complex nanophotonic structures.Comment: 29 pages, 10 figure

Water-Splitting Photoelectrolysis Reaction Rate via Microscopic Imaging of Evolved Oxygen Bubbles

Bubble formation and growth on a water-splitting semiconductor photoelectrode under illumination with above-bandgap radiation provide a direct measurement of the gas-evolving reaction rate. Optical microscopy was used to record the bubble growth on single-crystal strontium titanate immersed in basic aqueous electrolyte and illuminated with UV light at 351/364 nm from a focused argon laser. By analyzing the bubble size as a function of time, the water-splitting reaction rate was determined for varying light intensities and was compared to photocurrent measurements. Bubble nucleation was explored on an illuminated flat surface, as well as the subsequent light scattering and electrode shielding due to the bubble. This technique allows a quantitative examination of the actual gas evolution rate during photoelectrochemical water splitting, independent of current measurements