Catastrophic optical damage in 808†nm broad area laser diodes: a study of the dark line defect propagation.

We present a study of the propagation of dark line defects (DLDs) in catastrophically damaged 808 nm laser diodes, based on cathodoluminescence (CL) measurements and laser mode propagation simulations. Room temperature CL images show blurred DLDs running parallel to the laser cavity. Remarkably, low temperature images reveal their true morphology: the blurred lines are resolved as parallel narrow discontinuous DLDs. This morphology does not match the usually reported molten front scenario of DLD propagation. Low temperature images show that DLDs consist of a sequence of catastrophic optical damage (COD) events separated a few micrometers from each other. Consequently, a different propagation scheme is proposed. The points where the CODs occur suffer a temperature increase and these hot spots play a capital role in the propagation of the DLDs. Their influence on the beam distribution is modelled using finite element methods. The calculations evidence changes on the intensity distribution of the laser that qualitatively reproduce the DLD shapes. Additionally, the COD events result in the generation of defects in the region that surrounds them. The successive CODs in the discontinuous DLDs are rationalized in terms of the enhanced laser absorption in these sensitized regions where the laser beam is concentrated by thermal lensing.

Effect of thermal lensing and the micrometric degraded regions on the catastrophic optical damage process of high-power laser diodes.

Catastrophic optical damage (COD) is one of the processes limiting the lifetime of high-power laser diodes. The understanding of this degradation phenomenon is critical to improve the laser power and lifetime for practical applications. In this Letter, we analyze the defect propagation inside the cavity of quantum well (QW) high-power laser diodes presenting COD. For this, we studied the effect of highly localized thermal gradients and degraded regions on the laser field distribution. Finite element method (FEM) simulations are compared to experimental cathodoluminescence (CL) measurements. The presence of micrometric hot spots inside the QW induces the thermal lensing of the laser field. The laser self-focusing inside the cavity eventually generates a new hot spot, and, in a repetitive way, a sequence of hot spots would be created. This would account for the propagation of the dark line defects (DLDs) that are characteristic of this degradation mode.

Local electric field enhancement at the heterojunction of Si/SiGe axially heterostructured nanowires under laser illumination.

We present a phenomenon concerning electromagnetic enhancement at the heterojunction region of axially heterostructured Si/SiGe nanowires when the nanowire is illuminated by a focused laser beam. The local electric field is sensed by micro Raman spectroscopy, which allows the enhancement of the Raman signal arising from the heterojunction region to be revealed; the Raman signal per unit volume increases at least ten times with respect to the homogeneous Si and SiGe nanowire segments. In order to explore the physical meaning of this phenomenon, a three-dimensional solution of the Maxwell equations of the interaction between the focused laser beam and the nanowire was carried out by finite element methods. A local enhancement of the electric field at the heterojunction was deduced. However, the magnitude of the electromagnetic field enhancement only approaches the experimental one when the free carriers are considered, showing enhanced absorption at the carrier depleted heterojunction region. The existence of this effect promises a way of improving photon harvesting using axially heterostructured semiconductor nanowires.

Ancestry and TPMT-VNTR Polymorphism: Relationship with Hematological Toxicity in Uruguayan Patients with Acute Lymphoblastic Leukemia.

6-Mercaptopurine (6-MP) is a thiopurine drug widely used in childhood acute lymphoblastic leukemia (ALL) therapy. Genes such as TPMT and NUDT15 have an outstanding role in 6-MP metabolism. Mutations in both genes explain a significant portion of hematological toxicities suffered by ALL Uruguayan pediatric patients. A variable number tandem repeat in the TPMT promoter (TPMT-VNTR) has been associated with TPMT expression. This VNTR has a conservative architecture (AnBmC). To explore new causes of hematological toxicities related to ALL therapy, we genotyped the TPMT-VNTR of 130 Uruguayan pediatric patients. Additionally, individual genetic ancestry was estimated by 45 ancestry-informative markers (AIMs). Hematological toxicity was measured as the number of leukopenia events and 6-MP dose along the maintenance phase. As previously reported, we found TPMT*2 and TPMT*3C alleles were associated to TPMT-VNTR A2BC and AB2C, respectively. However, contrasting with other reports, TPMT*3A allele was found in a heterogeneous genetic background in linkage equilibrium. Patients carrying more than 5 A repeats present a significant higher number of leukopenia events among patients without TPMT and/or NUDT15 variants. Native American ancestry and the number of A repeats were significantly correlated with the number of leukopenia events. However, the correlation between Native American ancestry and the number of leukopenia events was lost when the number of A repeats was considered as covariate. This suggests that TPMT-VNTR alleles are more relevant than Native American ancestry in the hematological toxicity. Our results emphasize that TPMT-VNTR may be used as a pharmacogenetic biomarker to predict 6-MP-related hematological toxicity in ALL childhood therapy.

Association of TCF7L2 mutation and atypical diabetes in a Uruguayan population.

AIM: To investigate if mutations in TCF7L2 are associated with "atypical diabetes" in the Uruguayan population. METHODS: Healthy, nondiabetic controls (n = 133) and patients with type 2 diabetes (n = 177) were selected from among the presenting population at level-3 referral healthcare centers in Uruguay. Patients with type 2 diabetes were subgrouped according to "atypical diabetes" (n = 92) and "classical diabetes" (n = 85). Genotyping for the rs12255372 and rs7903146 single nucleotide polymorphisms (SNPs) in the TCFTL2 gene was carried out with TaqMan® probes. Random samples were sequenced by Macrogen Ltd. (South Korea). Statistical analysis of the SNP data was carried out with the SNPStats online tool (http://bioinfo.iconcologia.net/SNPstats). The best inheritance model was chosen according to the lowest values of Akaike's information criterion and Bayesian information criterion. Differences between groups were determined by unpaired t-tests after checking the normal distribution or were converted to normalize the data. The association of SNPs was tested for matched case-control samples by using χ2 analysis and calculation of odds ratios (ORs) with 95% confidence intervals (CIs). All statistical tests were performed using SPSS v10.0 and EpiInfo7 statistical packages. Significant statistical differences were assumed in all cases showing adjusted P < 0.05. RESULTS: We genotyped two TCF7L2 SNPs (rs7903146 and rs12255372) in a population-based sample of 310 Uruguayan subjects, including 133 healthy control subjects and 177 clinical diagnosed with type 2 diabetes. For both SNPs analyzed, the best model was the dominant type: rs12255372 = G/G vs G/T+T/T, OR = 0.63, 95%CI: 0.40-0.98, P < 0.05 and rs7903146 = C/C vs C/T+T/T, OR = 0.79, 95%CI: 0.41-1.55, P = 0.3. The rs12255372 SNP showed high association with the type 2 diabetes cases (OR = 1.60, 95%CI: 1.20-2.51, P < 0.05). However, when the type 2 diabetics group was analyzed according to the atypical and classical subgroupings, the association with diabetes existed only for rs12255372 and the classical subgroup (vs controls: OR = 2.1, 95%CI: 1.21-3.75, P < 0.05); no significant differences were found for either SNP or atypical diabetes. CONCLUSION: This is the first time SNPs_TCF7L2 were genotyped in a diabetic population stratified by genotype instead of phenotype. Classical and atypical patients showed statistical differences.

A novel mutation in the OAR domain of the ARX gene.

Mutations in ARX gene should be considered in patients with mental disability or/and epilepsy. It is an X-linked gene that has pleiotropic effects. Here, we report the case of a boy diagnosed with Ohtahara syndrome. We performed the molecular analysis of the gene and identified a new missense mutation.

The phenotype masks the genotype: A possible new expression of diabetes.

The concept of a new form of diabetes, with signs of both types 1 and 2, has not been often considered, until recently. It is of immense interest to explore the role of the admixture that characterizes the Uruguayan population (higher and different from other Latin America countries) for the presence of such expression of that particular disease. We describe here a child who possibly presents with this expression. He had typical signs of both diabetic conditions: type 1 (young age, positive immunologic and genetic markers, ketoacidosis) and type 2 (obesity [body mass index = 36 kg/m(2)] and acanthosis nigricans). In spite of complying with the established guidelines, therapeutic and nutritional control, quality of life and good metabolic control, the patient's obesity had been continually increasing. Looking for a genetic explanation, we studied three single nucleotide polymorphisms involved in three different metabolic pathways (peroxisome proliferator-activated receptor gamma 2, insulin receptor substrate-1 and uncoupling protein-2) associated with insulin resistance. Our patient showed three mutations, GG, GA, GG, associated with insulin resistance that explains obesity associated with limited response to the commonly used drugs. According to the clinical presentation and the genetic and immunological background, we considered that this patient presents with a new form of diabetes. We have termed this particular disease "hybrid diabetes" because of the involvement of genes associated with both the classical type of diabetes. However, at least in an admixed population such as in Uruguay, clinical classification would not strictly dictate the choice of treatment.