Temperature-independent silicon subwavelength grating waveguides.

We demonstrate, by experiment and numerical calculations, temperature-independent subwavelength grating waveguides with a periodic composite core composed of alternating regions of silicon and SU-8 polymer. The polymer has a negative thermo-optic (TO) material coefficient that cancels the large positive TO effect of the silicon. Measurements and Bloch mode calculations were carried out over a range of silicon-polymer duty ratios. The lowest measured TO coefficient at a wavelength of 1550 nm is 1.8×10(-6) K(-1); 2 orders of magnitude smaller than a conventional silicon photonic wire waveguide. Calculations predict the possibility of complete cancellation of the silicon waveguide temperature dependence.

Single-etch grating coupler for micrometric silicon rib waveguides.

Grating couplers are widely used as an efficient and versatile fiber-chip coupling structure in nanometric silicon wire waveguides. The implementation of efficient grating couplers in micrometric silicon-on-insulator (SOI) rib waveguides is, however, challenging, since the coupler waveguide region is multimode. Here we experimentally demonstrate grating couplers in 1.5 µm-thick SOI rib waveguides with a coupling efficiency of -2.2 dB and a 3 dB bandwidth of 40 nm. An inverse taper is used to adiabatically transform the interconnection waveguide mode to the optimum grating coupler excitation field with negligible higher order Bloch mode excitation. Couplers are fabricated in the same etch step as the waveguides using i-line stepper lithography. The benefits of wafer-scale testing and device characterization without facet preparation are thus attained at no additional cost.

A high-resolution silicon-on-insulator arrayed waveguide grating microspectrometer with sub-micrometer aperture waveguides.

We demonstrate a 50-channel high-resolution arrayed waveguide grating microspectrometer with a 0.2 nm channel spacing on a silicon-on-insulator (SOI) platform. The chip size is 8 mm x 8 mm. High channel density and spectral resolution are achieved using high aspect ratio 0.6 mum x 1.5 mum waveguide apertures to inject the light into the input combiner and to intercept different spectral channels at the output combiner focal region. The measured crosstalk is <-10 dB, the 3 dB channel bandwidth is 0.15 nm, and the insertion loss is -17 dB near the central wavelength of lambda = 1.545 mum.

The N-terminal domain that distinguishes yeast from bacterial RNase III contains a dimerization signal required for efficient double-stranded RNA cleavage.

Yeast Rnt1 is a member of the double-stranded RNA (dsRNA)-specific RNase III family identified by conserved dsRNA binding (dsRBD) and nuclease domains. Comparative sequence analyses have revealed an additional N-terminal domain unique to the eukaryotic homologues of RNase III. The deletion of this domain from Rnt1 slowed growth and led to mild accumulation of unprocessed 25S pre-rRNA. In vitro, deletion of the N-terminal domain reduced the rate of RNA cleavage under physiological salt concentration. Size exclusion chromatography and cross-linking assays indicated that the N-terminal domain and the dsRBD self-interact to stabilize the Rnt1 homodimer. In addition, an interaction between the N-terminal domain and the dsRBD was identified by a two-hybrid assay. The results suggest that the eukaryotic N-terminal domain of Rnt1 ensures efficient dsRNA cleavage by mediating the assembly of optimum Rnt1-RNA ribonucleoprotein complex.

Monolithically integrated asymmetric graded and step-index couplers for microphotonic waveguides.

A monolithically integrated asymmetric graded index (GRIN) or step-index (GRIN) mode converters for microphotonic waveguides are proposed and described. The design parameters and tolerances are calculated for amorphous silicon (a-Si) couplers integrated with silicon-on-insulator waveguides. The GRIN and step-index couplers operate over a wide wavelength range with low polarization dependence, and the lithographic resolution needed is only +/-1 microm. Finally, experimental results are presented for a single layer 3 microm thick step-index a-Si coupler integrated on a 0.8 microm thick SOI waveguide. The measured variation of coupling efficiency with coupler length is in agreement with theory, with an optimal coupling length of 15 microm for this device.

Purification and characterization of Saccharomyces cerevisiae Rnt1p nuclease.

In this article, we have described methods used to purify Rnt1p and study its biochemical properties. Rnt1p can be easily purified from bacteria as N-terminal His6-tagged protein and its activity may be monitored in vitro. Rnt1p cleaves the RNA by binding to a cleavage site followed by hydrolysis and product release. The kinetic parameters of Rnt1p are similar to those of other nucleases, including bacterial RNase III. The ability of Rnt1p to bind substrate without cleaving it in the absence of divalent metal ions provides a convenient means to study RNA recognition and binding independent of catalysis. The gel mobility shift and in-the-gel cleavage assays described here reveal the formation of two Rnt1p-RNA complexes with different cleavage activities, suggesting that the protein may bind the substrate in two different forms or through a two-step binding reaction.

Interference effect in scattering loss of high-index-contrast planar waveguides caused by boundary reflections.

We present theoretical and experimental results on an interference effect caused by boundary reflections on the optical scattering loss in high-index-contrast planar waveguides. Analytical expressions for the polarization-dependent scattering loss are derived using a surface Green's function. For high-index-contrast waveguides of submicrometer dimensions a significant deviation from accepted theory arises, including scattering loss suppression owing to a thin-film interference effect. Our theoretical predictions are confirmed by loss measurements on silicon-on-insulator channel waveguides.

Spiral-path high-sensitivity silicon photonic wire molecular sensor with temperature-independent response.

We demonstrate a new silicon photonic wire waveguide evanescent field (PWEF) sensor that exploits the strong evanescent field of the transverse magnetic mode of this high-index-contrast, submicrometer-dimension waveguide. High sensitivity is achieved by using a 2 mm long double-spiral waveguide structure that fits within a compact circular area of 150 microm diameter, facilitating compatibility with commercial spotting apparatus and the fabrication of densely spaced sensor arrays. By incorporating the PWEF sensor element into a balanced waveguide Mach-Zehnder interferometer circuit, a minimum detectable mass of approximately 10 fg of streptavidin protein is demonstrated with near temperature-independent response.

[Aftercare and rehabilitation of HIV patients in Western Africa]. / Nachsorge und Rehabilitation bei HIV-Patienten in Westafrika.

Thanks to the modern therapy protocols, terminal prognosis of Aids disease is no longer certainty. It has now been updated to a chronic disease with good survival rates demanding follow-up and rehabilitative measures. Rehabilitative care in Western Africa is still rare. The care for Aids patients mainly concentrates on psychosocial activities offered by non-governmental Aids associations. The care is mainly outpatient; the objective is to improve the patients' compliance. Compared to western countries, African HIV-infected patients suffer even more from discrimination and are outcast from their family. Hospital-based rehabilitation is being established. Objectives are primarily to improve compliance and, secondly, to restore physical and mental efficiency as well. Favourable patient compliance is considered to be a good prognostic factor resulting in preventing recurrences and expensive therapy.

The RNase III family: a conserved structure and expanding functions in eukaryotic dsRNA metabolism.

The last few years have witnessed the appreciation of dsRNA as a regulator of gene expression, a potential antiviral agent, and a tumor suppressor. However, in spite of these clear effects on the cell function, the mechanism that controls dsRNA maturation and stability remains unknown. Recently, the discovery of eukaryotic orthologues of the bacterial dsRNA specific ribonuclease III (RNase III) suggested a central role for these enzymes in the regulation of dsRNA and eukaryotic RNA metabolism in general. This article reviews the structure-function features of the eukaryotic RNase III family and their roles in dsRNA metabolism with an emphasis on the yeast RNase III. Yeast RNase III is involved in the maturation of the majority of snRNAs, snoRNAs, and rRNA. In addition, perturbation of the expression level of yeast RNase III alters meiosis and causes sterility. These basic functions of the yeast RNase III appear to be widely conserved which makes it a good model to understand the importance of eukaryotic dsRNA metabolism.

Solution structure of conserved AGNN tetraloops: insights into Rnt1p RNA processing.

Rnt1p, the yeast orthologue of RNase III, cleaves rRNAs, snRNAs and snoRNAs at a stem capped with conserved AGNN tetraloop. Here we show that 9 bp long stems ending with AGAA or AGUC tetraloops bind to Rnt1p and direct specific but sequence-independent RNA cleavage when provided with stems longer than 13 bp. The solution structures of these two tetraloops reveal a common fold for the terminal loop stabilized by non-canonical A-A or A-C pairs and extensive base stacking. The conserved nucleotides are stacked at the 5' side of the loop, exposing their Watson-Crick and Hoogsteen faces for recognition by Rnt1p. These results indicate that yeast RNase III recognizes the fold of a conserved single-stranded tetraloop to direct specific dsRNA cleavage.

Eliminating the birefringence in silicon-on-insulator ridge waveguides by use of cladding stress.

We propose and demonstrate the use of the cladding stress-induced photoelastic effect to eliminate modal birefringence in silicon-on-insulator (SOI) ridge waveguides. Birefringence-free operation was achieved for waveguides with otherwise large birefringence by use of properly chosen thickness and stress of the upper cladding layer. With the stress levels typically found in cladding materials such as SiO2, the birefringence modification range can be as large as 10(-3). In arrayed waveguide grating demultiplexers that were fabricated in a SOI platform, we demonstrated the reduction of the birefringence from 1.2 x 10(-3) (without the upper cladding) to 4.5 x 10(-5) when a 0.8-microm oxide upper cladding with a stress of -320 MPa (compressive) was used. Because the index changes induced by the stress are orders of magnitude smaller than the waveguide core-cladding index contrast, the associated mode mismatch loss is negligible.