Huiwen Xu

Effect of catalyst composition on carbon nanotube growth

Transmission electron microscopy was used to probe the compositions of individual Co–Mo bimetal catalyst particles and the morphologies of carbon nanotubes (CNTs) catalyzed by these particles under flowing carbon monoxide at 700 °C. It was found that the composition of the catalyst particle at a CNT tip and the distribution of Co within the particle largely determine the morphology of the CNT. A particle with low Co content ( 85 at. %) tends to produce onion-like structures. These observations provide insight into the CNT growth mechanisms.

2.0 μm wavelength InAs quantum dashes grown on a GaAs substrate using a metamorphic buffer layer

We describe optical and structure characteristics of InAs quantum dashes grown on a GaAs substrate using an AlGaAsSb metamorphic buffer. The metamorphic buffer increases the lattice constant of the growth matrix from 5.653 to 5.869 A. The increased lattice constant of the growth matrix yields a lattice mismatch with the InAs active region of only 3.2% and accommodates a large In content to access emission wavelengths >2.0 μm. From our comparison with quantum dot structures, we conclude that the elongated quantum dash formation is due to asymmetric surface bonds in the zinc blende crystal structure that control surface migration in low strain conditions.

Mg(B,O)2 precipitation in MgB2

MgB2 samples prepared by solid-state reaction were investigated using high-resolution transmission electron microscopy (HREM), x-ray energy-dispersive spectroscopy (EDX), electron energy-loss spectroscopy (EELS), and energy-filtered imaging. Large amounts of coherent precipitates with a size range from about 5 nm up to about 100 nm were found in the MgB2 crystallite matrices. The precipitates are of different shapes including sphere, ellipsoid, and faceted polyhedron depending on the size of the precipitates. EDX and EELS analyses confirm that smaller precipitates contain magnesium, boron and oxygen while larger faceted precipitates contain mainly magnesium and oxygen, implying that the oxygen content increases with precipitate size. HREM and electron diffraction investigations found that the precipitates have the same crystal lattice structure as that of MgB2 but with various composition modulations depending on the composition of the precipitates. The precipitates transform to the MgO phase after long e...

Effect of strain-compensation in stacked 1.3μm InAs∕GaAs quantum dot active regions grown by metalorganic chemical vapor deposition

We have introduced tensile layers embedded in a GaAs matrix to compensate compressive strain in stacked 1.3μm InAs quantum dot (QD) active regions. The effects of the strain compensation are systematically investigated in five-stack and ten-stack QD structures where we have inserted InxGa1−xP (x=0.30 or 0.36) layers. High-resolution x-ray diffraction spectra quantify the overall strain in each sample and indicate >35% strain reduction can be accomplished. Both atomic force and transmission electron microscope images confirm that strain compensation improves material crystallinity and QD uniformity. With aggressive strain compensation, room temperature QD photoluminescence intensity is significantly increased demonstrating a reduced defect density.

Multi-Colour Nanowire Photonic Crystal Laser Pixels

Emerging applications such as solid-state lighting and display technologies require micro-scale vertically emitting lasers with controllable distinct lasing wavelengths and broad wavelength tunability arranged in desired geometrical patterns to form “super-pixels”. Conventional edge-emitting lasers and current surface-emitting lasers that require abrupt changes in semiconductor bandgaps or cavity length are not a viable solution. Here, we successfully address these challenges by introducing a new paradigm that extends the laser tuning range additively by employing multiple monolithically grown gain sections each with a different emission centre wavelength. We demonstrate this using broad gain-bandwidth III-nitride multiple quantum well (MQW) heterostructures and a novel top-down nanowire photonic crystal nanofabrication. We obtain single-mode lasing in the blue-violet spectral region with a remarkable 60 nm of tuning (or 16% of the nominal centre wavelength) that is determined purely by the photonic crystal geometry. This approach can be extended to cover the entire visible spectrum.

Single-mode lasing of GaN nanowire-pairs

Stable single-mode lasing operation from a pair of coupled GaN nanowires is demonstrated through optical pumping. GaN nanowires with different lengths were placed side-by-side in contact to form a coupled cavity through nanoprobe manipulation. Unlike individual nanowire lasers, which operate in a combined multiple transverse and multiple longitude mode oscillation, a coupled nanowire-pair provides a mode selection mechanism through the Vernier effect, which can strongly enhance the free spectrum range between adjacent resonant modes and generate a stable single-mode operation with a high side-mode suppression ratio.

Strain-relieved, dislocation-free InxGa1−xAs∕GaAs(001) heterostructure by nanoscale-patterned growth

A strain-relieved, dislocation-free InxGa1−xAs layer is selectively grown on nanoscale SiO2-patterned GaAs(001) by molecular beam epitaxy. By localizing the epitaxial area to a periodic array of nanoscale circular holes opened in a SiO2 mask and allowing the InxGa1−xAs epilayers selectively grown on adjacent holes to coalesce over the SiO2 mask by lateral overgrowth, the strain of the resulting InxGa1−xAs layer (x=0.06) is relieved with a dramatically decreased generation of misfit dislocations. These experimental results qualitatively support the basic idea of the Luryi-Suhir proposal [Appl. Phys. Lett. 49, 140 (1986)].

Gold Substrate-Induced Single-Mode Lasing of GaN Nanowires.

We demonstrate a method for mode-selection by coupling a GaN nanowire laser to an underlying gold substrate. Multimode lasing of GaN nanowires is converted to single-mode behavior following placement onto a gold film. A mode-dependent loss is generated by the absorbing substrate to suppress multiple transverse-mode operation with a concomitant increase in lasing threshold of only ∼13%. This method provides greater flexibility in realizing practical single-mode nanowire lasers and offers insight into the design of metal-contacted nanoscale optoelectronics.

Tabletop single-shot extreme ultraviolet Fourier transform holography of an extended object

We demonstrate single and multi-shot Fourier transform holography with the use of a tabletop extreme ultraviolet laser. The reference wave was produced by a Fresnel zone plate with a central opening that allowed the incident beam to illuminate the sample directly. The high reference wave intensity allows for larger objects to be imaged compared to mask-based lensless Fourier transform holography techniques. We obtain a spatial resolution of 169 nm from a single laser pulse and a resolution of 128 nm from an accumulation of 20 laser pulses for an object ~11x11μm(2) in size. This experiment utilized a tabletop extreme ultraviolet laser that produces a highly coherent ~1.2 ns laser pulse at 46.9 nm wavelength.

Distributed feedback gallium nitride nanowire lasers

Achieving single-mode laser operation in nanowire lasers remains a challenge due to a lack of mode selection approaches. We have implemented single-mode lasing using distributed feedback by externally coupling gallium nitride nanowires to a dielectric grating to achieve mode-control. The effective periodicity of the grating experienced by the nanowire was altered using nanomanipulation to change the angular alignment between the nanowire and the grating. The effective periodicity controls the spectral location of the distributed feedback stop-band. Single-mode emission was achieved at an alignment, where the designed periodicity of the grating was experienced by the nanowire.