Phase effects in a semiconductor laser with diffraction extraction of radiation

Phase effects arising in a semiconductor laser with diffraction extraction of radiation and a distributed Bragg reflector on the substrate side are taken into account exactly quantitatively and the possibilities of using these effects in lasers is analyzed. It is shown that the phase effects studied can be used to increase the laser efficiency substantially. Pis’ma Zh. Tekh. Fiz. 23, 7–11 (September 26, 1997)     

On stability of second-order quasi-polynomials with a single delay

In this note the stability of a second-order quasi-polynomial with a single delay is studied. Although there is a vast literature on this problem, most available solutions are limited to some particular cases. Moreover, some published results on this subject appear to contain imprecise, or even wrong, conditions. The purpose of this note is to show that by accurate application of known theories, a complete explicit characterization of stability regions can be derived in a most general case. As a byproduct of the proposed analysis, we show that in the high-order case the quasi-polynomial is delay-independent unstable whenever its delay-free version has an odd number of unstable roots (or, equivalently, a negative static gain).     

Structural Transformations during the Mechanochemical Synthesis and Heating of Co–Al Alloys

X-ray diffraction analysis and differential scanning calorimetry are used to follow the structural changes induced in cobalt by high-energy ball milling and the processes involved in the mechanochemical synthesis (MCS) of Co–Al alloys containing 3, 10, 20, and 50 at % Al. The results demonstrate that short-term (2–3 min) high-energy milling of cobalt, consisting, as a rule, of two phases, hcp and fcc, causes the high-temperature (fcc) phase to disappear. Longer term milling leads to the reverse, hcp-to-fcc phase transformation. This milling-induced transformation is attributable to the increase in the probability of stacking faults, which attains 20%. Similar structural changes occur in the initial stages of MCS at Al contents from 3 to 20 at %. The formation of fcc Co_{1 – x} Al _x solid solutions (x < 0.5) is accompanied by an increase in the probability of stacking faults. The presence of Al stabilizes stacking faults in hexagonal cobalt and accelerates the transition to the cubic phase, which dissolves the aluminum. The milling of a mixture containing 20 at % Al leads to the formation of a solid solution and, at intermediate milling times, CoAl. The latter phase disappears at sufficiently long milling times. Mechanical alloying of an equiatomic mixture (Co + 50 at % Al) yields phase-pure CoAl. Heating of the MCS alloys to 720°C stabilizes the Co_{1 – x} Al _x solid solution and the intermetallic phase CoAl. Heating to 720°C after medium-term milling leads to the formation of the metastable phase Co₃Al (L1₂ type), which disappears at higher temperatures.     

Bandwidth limits of luminescent solar concentrators as detectors in free-space optical communication systems

Luminescent solar concentrators(LSCs)have recently emerged as a promising receiver technology in free-space optical communications due to their inherent ability to collect light from a wide field-of-view and concentrate it into small areas,thus leading to high optical gains.Several high-speed communication systems integrating LSCs in their detector blocks have already been demonstrated,with the majority of efforts so far being devoted to maximising the received optical power and the system's field-of-view.However,LSCs may pose a severe bottleneck on the bandwidth of such communication channels due to the comparably slow timescale of the fluorescence events involved,a situation further aggravated by the inherent reabsorption in these systems,and yet,an in-depth study into such dynamic effects remains absent in the field.To fill this gap,we have developed a comprehensive analytical solution that delineates the fundamental bandwidth limits of LSCs as optical detectors in arbitrary free-space optical links,and establishes their equivalence with simple RC low-pass electrical circuits.Furthermore,we demonstrate a time-domain Monte Carlo simulation platform,an indispensable tool in the multiparameter optimisation of LSC-based receiver systems.Our work offers vital insight into LSC system dynamic behaviour and paves the way to evaluate the technology for a wide range of applications,including visible light communications,high-speed video recording,and real-time biological imaging,to name a few.     

The effect of heat treatment on the microfibrillar network of poly(p-phenylene benzobisthiazole)

Materials exhibiting exceptional mechanical properties can be fabricated from rigid polymers. In a spinning process, an oriented solution is solidified by the action of a nonsolvent, which induces crystallization of an oriented rigid polymer. Drying and heat treatment result in the final material having the desired properties. The effect of heat treatment on the morphology of poly(p-phenylene benzobisthiazole) (PBZT) films was studied by measurements of small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD). Films were formed by coagulation in water. The wet films were dried with supercritical CO₂ to maintain an open network structure. Coagulation in water resulted in formation of microfibrils having diameter of ∼9 nm. Heat-treatment at 600°C caused significant coalescence of microfibrils. The crystallite size was increased by heat treatment from a value that is smaller than the microfibrillar diameter indicating defects in chain packing, to a value comparable to the microfibrillar diameter.     

Symmetry properties of the reciprocity gap functional in the linear elasticity

The reciprocity gap functional for a linear elastic body with an embedded defect is considered. The functional is a bilinear functional depending on two stress states—in an elastic body with a defect and without it. It is shown that for various types of boundary conditions the functional is a symmetric one in a certain sense. Some corollaries of the symmetry properties of the functional are obtained.