High performance ultra-compact SOI waveguide crossing.

Waveguide crossing is an important integrated photonic component that will be routinely used for high-density and large-scale photonic integrated circuits, such as optical switches and routers. Several techniques have been reported in achieving high performance waveguide crossings on a silicon-on-insulator photonic platform, i.e., low-loss and low-crosstalk waveguide crossings based on multimode interference, bi-layer tapering, optical transformation, metamaterials, and subwavelength gratings. Until recently, not much attention has been given to the reduction of the footprint of waveguide crossings. Here we experimentally demonstrate an ultra-compact waveguide crossing on silicon photonic platform with a footprint only ~1 × 1 µm2. Our simulations show that it has a low insertion loss (< 0.175 dB) and low crosstalk (< -37dB) across the whole C-band, while the fabricated one has an insertion loss < 0.28 dB and crosstalk around -30 dB for the C-band.

Metabolic and Transcriptional Analysis of Recombinant Saccharomyces Cerevisiae for Xylose Fermentation: A Feasible and Efficient Approach.

Lignocellulose is an abundant xylose-containing biomass found in agricultural wastes, and has arisen as a suitable alternative to fossil fuels for the production of bioethanol. Although Saccharomyces cerevisiae has been thoroughly used for the production of bioethanol, its potential to utilize lignocellulose remains poorly understood. In this work, xylose-metabolic genes of Pichia stipitis and Candida tropicalis, under the control of different promoters, were introduced into S. cerevisiae. RNA-seq analysis was use to examine the response of S. cerevisiae metabolism to the introduction of xylose-metabolic genes. The use of the PGK1 promoter to drive xylitol dehydrogenase (XDH) expression, instead of the TEF1 promoter, improved xylose utilization in "XR-pXDH" strain by overexpressing xylose reductase (XR) and XDH form C. tropicalis, enhancing the production of xylitol (13.66 ± 0.54 g/L after 6 days fermentation). Overexpression of xylulokinase and XR/XDH from P. stipitis remarkably decreased xylitol accumulation (1.13 ± 0.06 g/L and 0.89 ± 0.04 g/L xylitol, respectively) and increased ethanol production (196.14 % and 148.50 % increases during the xylose utilization stage, respectively), in comparison with the results of XR-pXDH. This result may be produced due to the enhanced xylose transport, Embden-Meyerhof and pentose phosphate pathways, as well as alleviated oxidative stress. The low xylose consumption rate in these recombinant as well as alleviated strains comparing with P. stipitis and C. tropicalis may be explained by the insufficient supplementation of NADPH and NAD +. The results obtained in this work provide new insights on the potential utilization of xylose using bioengineered S. cerevisiae strains.