High pulsed power VCSEL arrays with polymer microlenses formed by photoacid diffusion.

We demonstrate millimeters-long VCSEL linear arrays with SU-8 epoxy-based microlenses that are directly patterned and cross-linked on the output apertures by a simple, photoacid-diffusion-aided photolithography technique. The linear arrays are capable of delivering >7 W of peak pulsed output power. By exploiting the photoacid diffusion effect, it is possible to produce a range of microlens structures with height and radius of curvature ranging from approximately ten to tens of microns. Simulation and experimental results show that the far-field beam divergence can be reduced by a factor of up to 7 in VCSELs integrated with optimal microlens dimensions.

Highly uniform zinc blende GaAs nanowires on Si(111) using a controlled chemical oxide template.

GaAs-based nanowires (NWs) can be grown without extrinsic catalyst using the Ga-assisted vapor-liquid-solid method in an epitaxy reactor, on Si(111) substrates covered with native oxide. Despite its wide use, the conventional method fails to provide a good control over uniformity, reproducibility, and yield of vertical NWs. The nucleation of GaAs NWs is very sensitive to the properties of the native oxide such as chemical composition, roughness and porosity. Consequently, samples grown under the same conditions on Si(111) substrates from different manufacturing batches often produce dramatically different growth results. In order to remove the dependence on wafer batch, a controlled chemical oxidation process is developed to replace the native oxide on Si(111) substrate with a reproducible chemical oxide. A high yield (exceeding 90%) of vertical GaAs NWs is achieved with excellent uniformity on chemical oxide-covered substrate. As an added advantage, the crystalline quality is significantly improved over that of GaAs NWs grown on native oxide-covered substrate, and pure zinc blende crystal structure can be achieved with minimal defects. In addition, the chemical oxide can be used as a template for producing different combinations of NW densities and sizes in parallel on the same wafer using the same growth conditions.

A Barium Titanate-on-Oxide Insulator Optoelectronics Platform.

Electro-optic modulators are among the most important building blocks in optical communication networks. Lithium niobate, for example, has traditionally been widely used to fabricate high-speed optical modulators due to its large Pockels effect. Another material, barium titanate, nominally has a 50 times stronger r-parameter and would ordinarily be a more attractive material choice for such modulators or other applications. In practice, barium titanate thin films for optical waveguide devices are usually grown on magnesium oxide due to its low refractive index, allowing vertical mode confinement. However, the crystal quality is normally degraded. Here, a group of scandate-based substrates with small lattice mismatch and low refractive index compared to that of barium titanate is identified, thus concurrently satisfying high crystal quality and vertical optical mode confinement. This work provides a platform for nonlinear on-chip optoelectronics and can be promising for waveguide-based optical devices such as Mach-Zehnder modulators, wavelength division multiplexing, and quantum optics-on-chip.

Methylenetetrahydrofolate reductase polymorphisms and homocysteine-lowering effect of vitamin therapy in Singaporean stroke patients.

BACKGROUND AND PURPOSE: Increased plasma total homocysteine (tHcy) levels are a risk factor for stroke and can be reduced with vitamin therapy. However, data on the tHcy-lowering effects of vitamins are limited largely to white populations. Thus, we aimed to determine in Singaporean patients with recent stroke: (1) the efficacy of vitamin therapy (folic acid, vitamin B12, and B6) on lowering tHcy, and (2) whether efficacy is modified by Methylenetetrahydrofolate reductase (MTHFR) gene polymorphism(s). METHODS: A total of 443 eligible patients were recruited after presenting with ischemic stroke within the past 7 months. Patients were randomized to receive either placebo or vitamins. Fasting blood samples collected at baseline and at 1 year were assayed for levels of plasma tHcy. Patients were genotyped for MTHFR C677T and A1298C polymorphisms. RESULTS: Mean baseline tHcy was similar in the 2 groups (placebo 13.7 micromol/L; vitamins 14.0 micromol/L; P=0.70). At 1 year, mean tHcy was 14.5 micromol/L in the placebo group compared with 10.7 micromol/L in the vitamin group (difference 3.8 micromol/L; 95% CI, 2.8 to 4.8 micromol/L; P<0.0001). MTHFR C677T genotype was an independent determinant of tHcy levels at baseline (P=0.005), but A1298C was not (P=0.08). Neither polymorphism significantly influenced the effect of vitamin therapy on tHcy at 1 year. The magnitude of the reduction in tHcy levels at 1 year with vitamin therapy was similar, irrespective of MTHFR genotypes. CONCLUSIONS: Vitamin therapy reduces mean tHcy levels by 3.8 micromol/L in the Singaporean stroke population studied. MTHFR C677T but not A1298C is independently associated with tHcy levels at baseline, and neither impacts the tHcy-lowering effect of vitamins used in this study.