GaN-based nanowires hold great promise for solid state lighting applications because of their waveguiding properties and the ability to grow nonpolar GaN nanowire-based heterostructures, which could lead to increased light extraction and improved internal quantum efficiency, respectively. In addition, GaN nanowires can be grown directly on Si substrates, providing an inexpensive and scalable platform for device fabrication. We use finite difference time domain photonic simulations to explore light extraction efficiency enhancement in GaN nanowire-based light-emitting diodes (LEDs) on Si. Emission polarization and the placement of the emission source along the length of the nanowire were taken into consideration. We find that the optimal placement of the emission source is determined by the light reflection at the nanowire-air and nanowire-substrate interfaces and the coupling of emitted radiation into the waveguided modes, resulting in extraction efficiencies of up to 50%. Our approach to optimizing light extraction via simulation techniques can be applied to more realistic large-scale devices to guide experimental work towards nanowire-based LEDs with potentially greater efficiencies than their thin-film counterparts.
Compact neutron sources for neutron capture therapy hold the promise of permitting wide availability for this therapeutic modality for cancer treatment. Most accelerator based neutron source concepts for this purpose are centered on (p,n) reactions using bombarding energies several hundred keV to 1-2 MeV above the reaction threshold producing high neutron yield. The neutron energies in the range of hundreds of keV to 1-2 MeV require considerable moderation and/or filtration, which reduces the output epithermal neutron flux, and fast neutron contamination is always present. Operating with proton energies closer to the threshold decreases neutron yield but allows for smaller, more efficient filters and moderators, which results in less reduction of the epithermal flux by the moderator/filter assembly. Work by this collaboration is examining the balance between total neutron yield and filter/modulator efficiency in order to achieve intense epithermal beams with low fast neutron contamination. As the first stage of this project, neutron yield and spectrum measurements on 9Be(p,n) and 7Li(p,n) reactions have been made and the results will be presented along with neutronic calculations for these systems. A radio frequency quadrupole accelerator is being used in this work.
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