Group IV lasers are expected to revolutionize chip-to-chip optical communications in terms of cost, scalability, yield, and compatibility to the existing infrastructure of silicon industries for mass production. Here, we review the current state-of-the-art developments of silicon and germanium light sources toward monolithic integration. Quantum confinement of electrons and holes in nanostructures has been the primary route for light emission from silicon, and we can use advanced silicon technologies using top-down patterning processes to fabricate these nanostructures, including fin-type vertical multiple-quantum-wells. Moreover, the electromagnetic environment can also be manipulated in a photonic crystal nanocavity to enhance the efficiency of light extraction and emission by the Purcell effect. Germanium is also widely investigated as an active material in Group IV photonics, and novel epitaxial growth technologies are being developed to make a high quality germanium layer on a silicon substrate. To develop a practical germanium laser, various technologies are employed for tensile-stress engineering and high electron doping to compensate the indirect valleys in the conduction band. These challenges are aiming to contribute toward the convergence of electronics and photonics on a silicon chip.
A computational fluid dynamics-based design system with the integration of three blade design approaches, automatic mesh generator and CFD codes enables a quick and efficient design optimization of turbine components. It is applied to a Francis turbine rehabilitation project with strict customer requirements to provide over 3% increase in peak efficiency, 13% upgrade in power, and improved cavitation characteristics. Extensive turbulent flow simulations are performed for both the existing and new turbines at design and off design conditions. In order to take into account the interactions between different components, particularly the effects between the rotating and stationary parts, coupling calculations based on the implicit coupling method under multiple frames of reference are carried out for the entire turbine model. As a result, the runner and guide vanes are optimized to the greatest extent, and the stay vanes are locally modified with a possible minimum cost under the geometrical constraints of the existing machine. The performance of the new design is verified by model tests, and exceeds required improvements.
We propose a germanium fin light-emitting diode for a monolithic light source on a Si photonics chip. The germanium fins were fabricated by the oxidation condensation of silicon-germanium sidewalls epitaxially grown on silicon fins. We found that a tensile stress is applied to the pure germanium fins by the difference of the thermal expansion coefficient with that of the surrounding oxide. The electroluminescence spectra were consistent with those expected from direct recombination in germanium with a tensile stress. The strong immunity of germanium fins against high current densities would be favourable to achieve population inversions by electrical pumping.
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