2D Photonic Bandgap Structure Significantly Improves Performance of Laser Emission from a Conjugated Polymer System
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which corresponds to the time of the fluorescent event. This suggests that the defect center fluorescence is a unique, nonrandom event compared with the background signal. Zero correlation in this case also implies that only a single defect center is responsible for the event. The overall collection efficiency in this experiment is still very poor (0.0014). The researchers suggest that one improvement is to “couple an emitting dipole to a microcavity so it will emit light in a single mode.” They also said that better collection optics and samples will also improve
efficiency. The researchers hold that despite low collection efficiency with the current setup, the relative simplicity of this experiment provides a good platform for future development. JUNE LAU
Quantum Mechanical Bond Breaking Characteristics Explain Fracture Anisotropy of Silicon In the June 5 issue of Physical Review Letters, scientists from the Max Planck Institut für Metallforschung in Stuttgart and the Universidad Autonoma de
Madrid have reported quantum mechanical simulations of the bond breaking process at the tip of a crack in silicon which explain the well-documented cleavage anisotropy of this material. Cracks in silicon propagate on two types of cleavage planes and clearly prefer one particular direction on both of these planes. This experimental finding cannot be explained with arguments based on the Griffith criterion, which relates fracture resistance to the surface energy of the fracture surfaces. A more thorough theoretical analysis is difficult because cracks form on the atomic scale but extend to macroscopic dimensions. However, the enormous increase in computing power recently has now opened new opportunities for such studies. In the present case, the atoms around the crack tip were first loaded with the stress field of a macroscopic crack. The energy and all the forces on the atoms were then calculated ab initio using density functional theory. Upon increasing the externally applied load, the bond breaking process and the relaxations of the surrounding atoms were then monitored. Scientists Rubén Pérez and Peter Gumbsch said that the surprising result of the simulations was that the bond breaking at the crack tip proceeds differently for the different crack orientations. While the crack tip bonds smoothly lengthened for the easy propagation direction, the bond length remained almost unchanged for the difficult orientation until the external load reached a critical value at which the bond abruptly broke like a snapping elastic spring. The discontinuous bond breaking is preceded by a significant load sharing between several bonds at the crack tip, which effectively shields the crack tip bond from the applied load. This leads to a socalled trapping of the crack, which stabilizes it up to loads far above the Griffith value. Consequently, the macroscopically observed fracture anisotropy can be directly viewed as a result of the difference in the way the atomic bonds break.
2D Photonic Bandgap Structure Significantly Improves Performan
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