Multi-bounce self-mixing in terahertz metasurface external-cavity lasers.

The effects of optical feedback on a terahertz (THz) quantum-cascade metasurface vertical-external-cavity surface-emitting laser (QC-VECSEL) are investigated via self-mixing. A single-mode 2.80 THz QC-VECSEL operating in continuous-wave is subjected to various optical feedback conditions (i.e., feedback strength, round-trip time, and angular misalignment) while variations in its terminal voltage associated with self-mixing are monitored. Due to its large radiating aperture and near-Gaussian beam shape, we find that the QC-VECSEL is strongly susceptible to optical feedback, which is robust against misalignment of external optics. This, in addition to the use of a high-reflectance flat output coupler, results in high feedback levels associated with multiple round-trips within the external cavity-a phenomenon not typically observed for ridge-waveguide QC-lasers. Thus, a new theoretical model is established to describe self-mixing in the QC-VECSEL. The stability of the device under variable optical feedback conditions is also studied. Any mechanical instabilities of the external cavity (such as vibrations of the output coupler), are enhanced due to feedback and result in low-frequency oscillations of the terminal voltage. The work reveals how the self-mixing response differs for the QC-VECSEL architecture, informs other systems in which optical feedback is unavoidable, and paves the way for QC-VECSEL self-mixing applications.

Molecular epidemiology of hepatitis B virus infection in Tanzania.

Despite the significant public health problems associated with hepatitis B virus (HBV) in sub-Saharan Africa, many countries in this region do not have systematic HBV surveillance or genetic information on HBV circulating locally. Here, we report on the genetic characterization of 772 HBV strains from Tanzania. Phylogenetic analysis of the S-gene sequences showed prevalence of HBV genotype A (HBV/A, n=671, 86.9 %), followed by genotypes D (HBV/D, n=95, 12.3 %) and E (HBV/E, n=6, 0.8 %). All HBV/A sequences were further classified into subtype A1, while the HBV/D sequences were assigned to a new cluster. Among the Tanzanian sequences, 84 % of HBV/A1 and 94 % of HBV/D were unique. The Tanzanian and global HBV/A1 sequences were compared and were completely intermixed in the phylogenetic tree, with the Tanzanian sequences frequently generating long terminal branches, indicating a long history of HBV/A1 infections in the country. The time to the most recent common ancestor was estimated to be 188 years ago [95 % highest posterior density (HPD): 132 to 265 years] for HBV/A1 and 127 years ago (95 % HPD: 79 to 192 years) for HBV/D. The Bayesian skyline plot showed that the number of transmissions 'exploded' exponentially between 1960-1970 for HBV/A1 and 1970-1990 for HBV/D, with the effective population of HBV/A1 having expanded twice as much as that of HBV/D. The data suggest that Tanzania is at least a part of the geographic origin of the HBV/A1 subtype. A recent increase in the transmission rate and significant HBV genetic diversity should be taken into consideration when devising public health interventions to control HBV infections in Tanzania.

Evaluation of performance characteristics of hepatitis B e antigen serologic assays.

BACKGROUND: Hepatitis B e antigen (HBeAg) is considered an indicator of high hepatitis B virus (HBV) replication. Performance characteristics of commercially available HBeAg assays have not been determined, thus it is unknown whether lack of HBeAg detection is because of test sensitivity or HBV basal core promoter and precore mutations. OBJECTIVES: We studied the correlation between HBeAg reactivity with HBV DNA levels in three commercially available HBeAg assays using 335 HBsAg and HBV DNA positive serum/plasma samples. STUDY DESIGN: Diagnostic sensitivity was determined by serial dilutions of a WHO HBeAg standard. The limit of HBeAg detection estimated through regression was 1 IU/mL (Centaur), 97 IU/mL (DiaSorin) and 129 IU/mL (Vitros). Of these 335 samples, enough sample volume remained in 253 samples for head-to-head comparison of the assays. RESULTS: 81 (32%), 41 (16%) and 36 (14%) of the samples were HBeAg positive by the Centaur, DiaSorin and Vitros assays, respectively. Compared to the FDA-approved Centaur assay the specificity of the other two assays was 98%, while sensitivity was 47% for the DiaSorin assay and 41% for the Vitros assay. Significant association was found between HBeAg positive samples and HBV DNA levels >20,000 IU/mL; 31% of HBeAg negative samples (Centaur) had HBV DNA levels >20,000 IU/mL, 26% of HBeAg positive samples had HBV DNA levels <20,000 IU/mL and 5 HBeAg positive samples had HBV DNA levels <2000 IU/mL. CONCLUSION: Discordance was seen between these HBeAg assays, indicating reliance on HBeAg alone as a marker of high HBV replication can be misleading. Detection and quantification of HBV DNA remains the accurate and reliable marker of HBV replication.