Light Turning Mirrors in SiON Optical Waveguides for Hybrid Integration with CMOS Photo-detectors

A new method is proposed for hybrid integration of SiON optical waveguides and standard CMOS photo-detectors based on anisotropic etching of 45° facets in a Si substrate. After removal of anisotropically etched Si structures in cladding SiO2, the fabricated total-internal-reflection mirrors can direct the output of the waveguides to photo-detectors placed on top of the chip. The metal-free fabrication process, designed to create these mirrors, is convenient for batch production. Fourier optics based simulations predict that the reflection efficiency of the mirrors is 68.5 %. The far field pattern obtained from the fabricated device is similar to the simulated one

Direct measurement of the on-chip insertion loss of high finesse microring resonators in Si3N4-SiO2 technology.

Microring resonators show the possibility for designing Very Large Scale Integrated (VLSI) photonic circuits by cascading them. In order to realize the devices, the on-chip insertion loss becomes an important parameter. The direct measurement of the on-chip insertion loss of a high finesse microring resonator will be presented. Its value (0.1 ± 0.1) dB is low, in agreement with calculations

45° light turning mirrors for hybrid integration of silica optical waveguides and photo-detectors

For hybrid integration of an optical chip with an electronic chip with photo diodes and electronic processing, light must be coupled from the optical chip to the electronic chip. This paper presents a method to fabricate metal-free 45° quasi-total internal reflecting mirrors in optical chips that enable 90° out-of-plane light coupling between flip-chip bonded chips. This method is fully compatible with fabrication of conventional optical chips. The mirrors are created using anisotropic etching of 45° facets in a Si substrate followed by fabrication of optical structures. After removal of the mirror-defining Si structures by isotropic etching, the obtained air-optical structure interface directs the output of the waveguides to out-of-plane photo detectors that are mounted flip-chip on the optical chip. Simulations show a reflection efficiency of 72.3 %, while experimentally 47% was measured on a not fully optimized first batch

PKS 1004+13: A High-Inclination, Highly-Absorbed Radio-Loud QSO -- The First Radio-Loud BAL QSO at Low Redshift?

The existence of BAL outflows in only radio-quiet QSOs was thought to be an important clue to mass ejection and the radio-loud - radio-quiet dichotomy. Recently a few radio-loud BAL QSOs have been discovered at high redshift. We present evidence that PKS 1004+13 is a radio-loud BAL QSO. It would be the first known at low-redshift (z = 0.24), and one of the most radio luminous. For PKS 1004+13, there appear to be broad absorption troughs of O VI, N V, Si IV, and C IV, indicating high-ionization outflows up to about 10,000 km/s. There are also two strong, broad (~500 km/s), high-ionization, associated absorption systems that show partial covering of the continuum source. The strong UV absorption we have detected suggests that the extreme soft-X-ray weakness of PKS 1004+13 is primarily the result of absorption. The large radio-lobe dominance indicates BAL and associated gas at high inclinations to the central engine axis, perhaps in a line-of-sight that passes through an accretion disk wind.Comment: To appear in Ap.J. Letters, 1999 (June or July); 4 pages, 5 figure

Metal-insulator transition in YHx: scaling of the sub-THz conductivity

The established scaling laws of the conductivity with temperature and doping are strong indications for the quantum nature of the metal-insulator transition in YH$_x$. Here we report the first results on the frequency scaling of the conductivity. Samples were brought from the insulating to the metallic phase by carrier doping via illumination. In the metallic phase, the sub-terahertz conductivity coincides with the dc data. These results do not agree with the simplest picture of a quantum-phase transition.Comment: 4 pages, accepted to PR

Sensitivity analysis based dimension reduction of multiscale models

In this paper, the sensitivity analysis of a single scale model is employed in order to reduce the input dimensionality of the related multiscale model, in this way, improving the efficiency of its uncertainty estimation. The approach is illustrated with two examples: a reaction model and the standard Ornstein–Uhlenbeck process. Additionally, a counterexample shows that an uncertain input should not be excluded from uncertainty quantification without estimating the response sensitivity to this parameter. In particular, an analysis of the function defining the relation between single scale components is required to understand whether single scale sensitivity analysis can be used to reduce the dimensionality of the overall multiscale model input space

Commercialization of vegetable production in Alamata Woreda, Northern Ethiopia: processes and impact

The Raya valley in Tigray where Alamata Woreda is located, has fertile soil, suitable climate and rich water resources to grow various crops including vegetables. Surface water from seasonal rivers/streams and small dams and ground water extracted from deep and shallow wells with various water lifting devices are the two main sources of water for irrigation in the Woreda. A participatory rural appraisal (PRA) study conducted by the Woreda stakeholders and facilitated by IPMS identified (irrigated) vegetables as a potential marketable commodity in 2005. Using the commodity value chain approach, production, input supply and marketing problems and opportunities were identified. Major problems were lack of interest partly as a result of market failure in the past, lack of agronomic and irrigation knowledge and skills resulting in lack of use of advanced agronomic inputs (e.g. seeds) and underutilization of modern irrigation facilitates (most of the deep wells established were not used and/or underutilized). Different extension approaches were used including study tours to change the mind-set and to acquire knowledge for experts and farmers. Following various production interventions, market linkages were created which resulted in better prices (from 0.70Birr/kg before 2005 to 3-5Birr/kg in the following years). Farmer to farmer communications, trainings, workshops and media coverage facilitated the further dissemination of knowledge and skills between PAs in Alamata and neighboring Woredas. As a result of these interventions, the area of irrigated onion, pepper and tomato tripled in size from 351 ha in 2004/05 to 1113 ha in 2008/09. The lion share of this increase was due to a ten fold increase in onion area from 84 ha in 2004/05 to 824 ha in 2008/09. Most of this increase took place in the spate irrigated areas where plots previously used for cereal crops (sorghum and teff) were converted to vegetables. Both women and men farmers benefited from the intervention. Many farmers managed to construct houses in town and were able to own different assets. The further expansion of the (irrigated) vegetable production in Alamata is feasible. However, more attention needs to be paid to improving productivity, especially in the spate irrigated areas since no clear evidence was found that area increase was accompanied by productivity increase, indicating lack of adequate institutional and farmers’ knowledge and skills. Also adverse weather conditions during the 2008 harvesting season, resulted in considerable crop spoilage and lower prices – indicating the risk associated with this commodity under rain-fed conditions. Finally, potential salinity problems should also be taken into account

Thermohaline mixing in low-mass giants: RGB and beyond

Thermohaline mixing has recently been proposed to occur in low mass red giants, with large consequence for the chemical yields of low mass stars. We investigate the role of thermohaline mixing during the evolution of stars between 1 Msun and 3 Msun. We use a stellar evolution code which includes rotational mixing and internal magnetic fields. We confirm that thermohaline mixing has the potential to destroy most of the helium 3 which is produced earlier on the main sequence during the red giant stage, in stars below 1.5Msun. We find this process to continue during core helium burning and beyond. We find rotational and magnetic mixing to be negligible compared to the thermohaline mixing in the relevant layers, even if the interaction of thermohaline motions with the differential rotation may be essential to establish the time scale of thermohaline mixing in red giants.Comment: Proceedings of the Conference "Unsolved problems in stellar physics" - Cambridge, July 200

Experimental and numerical study of SiON microresonators with air and polymer cladding

A systematic experimental and numerical study of the device performance of waveguide-coupled SiON microresonators with air and polymer cladding is presented. Values of device parameters like propagation losses of the microresonator modes, the off-resonance insertion losses, and the straight waveguide to microresonator coupling are determined by applying a detailed fitting procedure to the experimental results and compared to results of detailed numerical simulations. By comparing the propagation losses of the fundamental TE polarized microresonator mode obtained by fitting to the measured spectra to the also experimentally determined propagation losses in the adjacent straight waveguide and the materials losses, it is possible to identify the loss mechanisms in the microresonator. By comparing experimental results for microresonators with air and polymethylmethacrylate cladding and a detailed numerical study, the influence of the cladding index on the bend losses is evaluated. It is demonstrated that the presence of an upper cladding can, under the right conditions, actually be beneficial for loss reduction