Carrier lifetime and mobility enhancement in nearly defect-free core-shell nanowires measured using time-resolved terahertz spectroscopy.

We have used transient terahertz photoconductivity measurements to assess the efficacy of two-temperature growth and core-shell encapsulation techniques on the electronic properties of GaAs nanowires. We demonstrate that two-temperature growth of the GaAs core leads to an almost doubling in charge-carrier mobility and a tripling of carrier lifetime. In addition, overcoating the GaAs core with a larger-bandgap material is shown to reduce the density of surface traps by 82%, thereby enhancing the charge conductivity.

Defect-free GaAs/AlGaAs Core-shell nanowires on Si substrates

We report straight and vertically aligned defect-free GaAs nanowires grown on Si(111) substrates by metal-organic chemical vapor deposition. By deposition of thin GaAs buffer layers on Si substrates, these nanowires could be grown on the buffer layers with much less stringent conditions as otherwise imposed by epitaxy of III-V compounds on Si. Also, crystal-defect-free GaAs nanowires were grown by using either a two-temperature growth mode consisting of a short initial nucleation step under higher temperature followed by subsequent growth under lower temperature or a rapid growth rate mode with high source flow rate. These two growth modes not only eliminated planar crystallographic defects but also significantly reduced tapering. Core-shell GaAs-AlGaAs nanowires grown by the two-temperature growth mode showed improved optical properties with strong photoluminescence and long carrier life times. © 2011 American Chemical Society.

Structural and optical characterization of vertical GaAs/GaP core-shell nanowires grown on Si substrates

GaAs nanowires were grown on Si (111) substrates. By coating a thin GaAs buffer layer on Si surface and using a two-temperature growth, the morphology and crystal structure of GaAs nanowires were dramatically improved. The strained GaAs/GaP core-shell nanowires, based on the improved GaAs nanowires with a shell thickness of 25 nm, showed a significant shift in emission energy of 260 meV from the unstrained GaAs nanowires. © 2010 IEEE.

Imaging and optical properties of single core-shell GaAs-AlGaAs nanowires

We study the optical properties of a single core-shell GaAs-AlGaAs nanowire (grown by VLS method) using the technique of micro-photoluminescence and spatially-resolved photoluminescence imaging. We observe large linear polarization anisotropy in emission and excitation of nanowires.

Resonant excitation and imaging of nonequilibrium exciton spins in single core-shell GaAs-AlGaAs nanowires.

Nonequilibrium spin distributions in single GaAs/AlGaAs core-shell nanowires are excited using resonant polarized excitation at 10 K. At all excitation energies, we observe strong photoluminescence polarization due to suppressed radiative recombination of excitons with dipoles aligned perpendicular to the nanowire. Excitation resonances are observed at 1- or 2-LO phonon energies above the exciton ground states. Using rate equation modeling, we show that, at the lowest energies, strongly nonequilibrium spin distributions are present and we estimate their spin relaxation rate.

Nearly intrinsic exciton lifetimes in single twin-free GaAsAlGaAs core-shell nanowire heterostructures

CW and time-resolved photoluminescence measurements are used to investigate exciton recombination dynamics in GaAsAlGaAs heterostructure nanowires grown with a recently developed technique which minimizes twinning. A thin capping layer is deposited to eliminate the possibility of oxidation of the AlGaAs shell as a source of oxygen defects in the GaAs core. We observe exciton lifetimes of ∼1 ns, comparable to high quality two-dimensional double heterostructures. These GaAs nanowires allow one to observe state filling and many-body effects resulting from the increased carrier densities accessible with pulsed laser excitation. © 2008 American Institute of Physics.

Growth, structural and optical properties of GaAs/AlGaAs Core/shell nanowires with and without quantum well shells

We have investigated the growth, structural properties and photoluminescence of novel GaAs/AlGaAs radial heterostructure nanowires, fabricated by metalorganic chemical vapour deposition. The effect of growth temperature on nanowire morphology is discussed. Strong photoluminescence is observed from GaAs nanowires with AlGaAs shells. Core/multishell nanowires, of GaAs cores clad in several alternating layers of thick AlGaAs barrier shells and thin GaAs quantum well shells, exhibit a blue-shifted photoluminescence peak believed to arise from quantum confinement effects. A novel two-temperature growth procedure for obtaining GaAs cores is introduced, and other nanowire heterostructures are addressed. © 2006 IEEE.

Temperature dependence of photoluminescence from single core-shell GaAs-AlGaAs nanowires

Temperature-dependent polarized microphotoluminescence measurements of single GaAsAlGaAs core-shell nanowires are used to probe their electronic states. The low-temperature emission from these wires is strongly enhanced compared with that observed in bare GaAs nanowires and is strongly polarized, reflecting the dielectric mismatch between the nanowire and the surrounding air. The temperature-dependent band gap of the nanowires is seen to be somewhat different from that observed in bulk GaAs, and the PL rapidly quenches above 120 K, with an activation energy of 17 meV reflecting the presence of nonradiative defects. © 2006 American Institute of Physics.

Mode division multiplexing exploring hollow-core photonic bandgap fibers

We review our recent exploratory investigations on mode division multiplexing using hollow-core photonic bandgap fibers (HC-PBGFs). Compared with traditional multimode fibers, HC-PBGFs have several attractive features such as ultra-low nonlinearities, low-loss transmission window around 2 μm etc. After having discussed the potential and challenges of using HC-PBGFs as transmission fibers for mode multiplexing applications, we will report a number of recent proof-of-concept results obtained in our group using direct detection receivers. The first one is the transmission of two 10.7 Gbit/s non-return to zero (NRZ) data signals over a 30 m 7-cell HC-PBGF using the offset mode launching method. In another experiment, a short piece of 19-cell HC-PBGF was used to transmit two 20 Gbit/s NRZ channels using a spatial light modulator for precise mode excitation. Bit-error-ratio (BER) performances below the forward-error-correction (FEC) threshold limit (3.3×10-3) are confirmed for both data channels when they propagate simultaneously. © 2013 IEEE.

Award Winner in the Young Investigator Category, 2014 Society for Biomaterials Annual Meeting and Exposition, Denver, Colorado, April 16-19, 2014: Periodically perforated core-shell collagen biomaterials balance cell infiltration, bioactivity, and mechanical properties.

Orthopedic tissue engineering requires biomaterials with robust mechanics as well as adequate porosity and permeability to support cell motility, proliferation, and new extracellular matrix (ECM) synthesis. While collagen-glycosaminoglycan (CG) scaffolds have been developed for a range of tissue engineering applications, they exhibit poor mechanical properties. Building on previous work in our lab that described composite CG biomaterials containing a porous scaffold core and nonporous CG membrane shell inspired by mechanically efficient core-shell composites in nature, this study explores an approach to improve cellular infiltration and metabolic health within these core-shell composites. We use indentation analyses to demonstrate that CG membranes, while less permeable than porous CG scaffolds, show similar permeability to dense materials such as small intestine submucosa (SIS). We also describe a simple method to fabricate CG membranes with organized arrays of microscale perforations. We demonstrate that perforated membranes support improved tenocyte migration into CG scaffolds, and that migration is enhanced by platelet-derived growth factor BB-mediated chemotaxis. CG core-shell composites fabricated with perforated membranes display scaffold-membrane integration with significantly improved tensile properties compared to scaffolds without membrane shells. Finally, we show that perforated membrane-scaffold composites support sustained tenocyte metabolic activity as well as improved cell infiltration and reduced expression of hypoxia-inducible factor 1α compared to composites with nonperforated membranes. These results will guide the design of improved biomaterials for tendon repair that are mechanically competent while also supporting infiltration of exogenous cells and other extrinsic mediators of wound healing.

Mode multiplexing at 2×20Gbps over 19-cell hollow-core photonic band gap fibre

This paper demonstrates the first mode-multiplexed system over 19-cell hollow-core photonic band gap fibre, at 2×20Gbps using the LP0,1 and LP2,1-like modes. © 2012 OSA.

Hybrid core carbon fiber composite sandwich panels: Fabrication and mechanical response

Carbon fiber reinforced polymer (CFRP) composite sandwich panels with hybrid foam filled CFRP pyramidal lattice cores have been assembled from a carbon fiber braided net, 3D woven face sheets and various polymeric foams, and infused with an epoxy resin using a vacuum assisted resin transfer process. Sandwich panels with a fixed CFRP truss mass have been fabricated using a variety of closed cell polymer and syntactic foams, resulting in core densities ranging from 44-482kgm-3. The through thickness and in-plane shear modulus and strength of the cores increased with increasing foam density. The use of low compressive strength foams within the core was found to result in a significant reduction in the compressive strength contributed by the CFRP trusses. X-ray tomography led to the discovery that the trusses develop an elliptical cross-section shape during pressure assisted resin transfer. The ellipticity of the truss cross-sections increased, and the lattice contribution to the core strength decreased as the foam density was reduced. Micromechanical modeling was used to investigate the relationships between the mechanical properties and volume fractions of the core materials and truss topology of the hybrid core. The specific strength and moduli of the hybrid cores lay between those of the CFRP lattices and foams used to fabricate them. However, their volumetric and gravimetric energy absorptions significantly exceeded those of the materials from which they were fabricated. They compare favorably with other lightweight energy absorbing materials and structures. © 2013.

Mode multiplexing at 2×20Gbps over 19-cell hollow-core photonic band gap fibre

This paper demonstrates the first mode-multiplexed system over 19-cell hollow-core photonic band gap fibre, at 2×20Gbps using the LP0,1 and LP2,1-like modes. © 2012 OSA.

The low velocity impact response of sandwich beams with a corrugated core or a Y-frame core

The low speed impact responses of simply-supported and clamped sandwich beams with corrugated and Y-frame cores have been measured in a drop-weight apparatus at 5 m s-1. The AISI 304 stainless steel sandwich beams comprised two identical face sheets and represented 1:20 scale versions of ship hull designs. No significant rate effects were observed at impact speeds representative of ship collisions: the drop-weight responses were comparable to the ones measured quasi-statically. Moreover, the corrugated and Y-frame core beams had similar performances. Three-dimensional finite element (FE) models simulated the experiments and were in good agreement with the measurements. The simulations demonstrated correctly that the sandwich beams collapsed by core indentation under both quasi-static loading and in the drop-weight experiments. These FE models were then used to investigate the sensitivity of impact response to (i) velocity, over a wider range of velocities than achievable with the drop-weight apparatus, and (ii) the presence of the back face sheet. The dynamic responses of sandwich beams with both front and back face sheets were found to be within 20% of the quasi-static responses for speeds less than approximately 5 m s-1. This suggests that quasi-static considerations are adequate to model the collision of a sandwich ship hull. By contrast, beams without a back face collapsed by Brazier buckling under quasi-static loading conditions, and by core indentation at a loading velocity of 5 m s-1. Thus, dynamic considerations are needed in ship hull designs that do not employ a back face. © 2014 Elsevier Ltd. All rights reserved.