Negative Group Velocity

The group velocity for pulses in an optical medium can be negative at frequencies between those of a pair of laser-pumped spectral lines. The gain medium then can amplify the leading edge of a pulse resulting in a time advance of the pulse when it exits the medium, as has been recently demonstrated in the laboratory. This effect has been called superluminal, but, as a classical analysis shows, it cannot result in signal propgation at speeds greater than that of light in vacuum.Comment: v3 adds discussion of "rephasing", and adds a figure. v4 adds references to the early history of negative group velocity, and adds a figure; thanks to Alex Grani

Elastic stress concentration at radial crossholes in pressurised thick cylinders

Results of a parametric finite element analysis investigation of stress concentration at radial crossholes in pressurized cylinders are presented in numerical and graphical form. The analysis shows that the location of maximum stress does not generally occur at the junction between the bores, as is commonly supposed, but at some small distance up the crosshole from the junction. Maximum stress concentration factors (SCFs) are defined on the basis of the maximum principal stress, von Mises equivalent stress, and stress intensity. Three-dimensional plots of the SCF against the cylinder radius ratio b/a and the crosshole-to-main-bore-radius ratio c/a are presented. The SCFs were found to vary across the range of geometries considered with local minima identified within the parameter range in most cases. The results therefore allow designers to select optimum b/a and c/a ratios to minimize stress concentration in real problems

Socioemotional wealth and the innovativeness of family SMEs in the United Arab Emirates

Why are some family SMEs more innovative than others? We use the heterogeneity within family SMEs to explore how their socioemotional wealth (SEW) affects innovativeness. The ubiquity of smaller family firms means that their innovativeness is critical for policymakers, such as those in the United Arab Emirates, seeking innovation-led development. We conduct a multi-case study analysis of SEW and innovativeness in fourteen family SMEs based in the United Arab Emirates. Participants were from a range of sectors and across the employment size-range of family SMEs. None of the most innovative family SMEs had highly family-centric socioemotional wealth. High family-centricity was however evident in all the least innovative firms who survived on reputation and incremental customer or supplier-driven improvements. The least innovative firms were amongst the smallest but not the youngest, with firm age not influential for innovativeness. The paper proposes redressing family-centric SEW preferences to raise the innovativeness of family SMEs. This will involve longer-term decision-making that gives greater consideration to the interests of external stakeholder as well as future generations of the family

Development of high speed power thyristor: The gate assisted turn-off thyristor

A high speed power switch with unique turn-off capability was developed. This gate-assisted turn-off thyristor was rated at 609 V and 50 A with turn-off times of 2 microsec. Twenty-two units were delivered for evaluation in a series inverter circuit. In addition, test circuits designed to relate to the series inverter application were built and demonstrated

Coupled Pulsation And Translation Of A Gas Bubble And Rigid Particle

A nonlinear analytic model describing the interaction of a spherical gas bubble and spherical rigid particle is presented. Both the bubble and particle are free to translate. The model is accurate to fifth order in terms of a nondimensional expansion parameter R/d, where R is a characteristic radius and d is the distance separating the bubble and particle. Numerical simulation results are presented to demonstrate the effects of key particle parameters and an external acoustic source.Applied Research Laboratorie