Electrical pumping of a third-order mode semiconductor laser

AlGaAs-based quantum well laser structures with third-order waveguide mode emission at 775 nm are a promising route toward compact twin-photon sources at 1.55 μm based on the principle of modal phase matching between the pumping frequency and fundamental modes at half frequency in III–V semiconductor waveguides. Following the demonstration and characterization of an optically pumped third-order mode semiconductor laser, in this paper we present data of the corresponding structure under conditions of electrical pumping. By pumping electrically and optically the same sample made for current injection, identical transverse far-field angular laser mode profiles are measured and with very low parasitic losses. Although they do not follow the third-order mode emission pattern as it is expected, however this means that the different way of pumping, that of the electrical one as compared to optical pumping is not responsible for the absence of third-order mode emission. Furthermore, since the undoped optically pumped laser sample correctly emits on the third-order mode, it is concluded that the cladding layers of the structure still need to be optimized in doping and thickness, in order to reduce the internal losses for the third-order mode.     

Rapid development of scalable scientific software using a process oriented approach

Scientific applications are often not written with multiprocessing, cluster computing or grid computing in mind. This paper suggests using Python and PyCSP to structure scientific software through Communicating Sequential Processes. Three scientific applications are used to demonstrate the features of PyCSP and how networks of processes may easily be mapped into a visual representation for better understanding of the process workflow. We show that for many sequential solutions, the difficulty in implementing a parallel application is removed. The use of standard multi-threading mechanisms such as locks, conditions and monitors is completely hidden in the PyCSP library. We show the three scientific applications: kNN, stochastic minimum search and McStas to scale well on multi-processing, cluster computing and grid computing platforms using PyCSP.     

Synthesis and spectral kinetic study of photoinduced processes of photochromic nitro-substituted indoline and benzothienopyrroline spiropyrans in solutions

Nitro-substituted benzothienopyrroline spiropyrans containing substituents in the ben-zothienopyrroline moiety were synthesized. The comparative spectral kinetic study of the pho-tochromic transformations of these compounds and their indoline analogs in solvents of various polarity was carried out. On the basis of the results obtained and literature data, the photoin-duced aggregation of molecules of the merocyanine form in weakly polar solvents was suggested.     

Quantification of functional group interactions in transition states

A new version of the double mutant cycle approach has been used for the evaluation of weak noncovalent interactions in transition states.     

On the absorption of infrared radiation by electrons in semiconductor inversion layers

Resonances in the inversion layer for infrared fields normal to the interface do not occur at the electric subband splittings. They are shifted from the subband splittings by resonant screening of the infrared field. The corrections for (100) Si surfaces are shown to be comparable to the shifts ascribed to many-body effects.     

Steam‐Induced Coarsening of Single‐Unit‐Cell MFI Zeolite Nanosheets and Its Effect on External Surface Brønsted Acid Catalysis

Commonly used methods to assess crystallinity, micro‐/mesoporosity, Brønsted acid site density and distribution (in micro‐ vs. mesopores), and catalytic activity suggest nearly invariant structure and function for aluminosilicate zeolite MFI two‐dimensional nanosheets before and after superheated steam treatment. Yet, pronounced reaction rate decrease for benzyl alcohol alkylation with mesitylene, a reaction that cannot take place in the zeolite micropores, is observed. Transmission electron microscopy images reveal pronounced changes in nanosheet thickness, aspect ratio and roughness indicating that nanosheet coarsening and the associated changes in the external (mesoporous) surface structure are responsible for the changes in the external surface catalytic activity. Superheated steam treatment of hierarchical zeolites can be used to alter nanosheet morphology and regulate external surface catalytic activity while preserving micro‐ and mesoporosity, and micropore reaction rates.     

Approximate solution of a class of singular control problems

A class of singular control problems involving amplitude constraints on the controls is examined. IfL is the space of control functionsU, the control constraint setS can be identified with the unit ball inL . Now, for anyn (1, ), an analogous problem may be set up withL ⁿ forU and the unit ball inL ⁿ forS. This modified problem is necessarily nonsingular for controllable systems. It is shown that, by takingn sufficiently large, the solution to the modified problem also solves the original problem arbitrarily closely (in a sense made precise). Behavior asn is investigated.This research was supported by the Science Research Council of Great Britain and the Commonwealth Fund (Harkness Fellowship).     

Substituent effects on aromatic stacking interactions

Synthetic supramolecular zipper complexes have been used to quantify substituent effects on the free energies of aromatic stacking interactions. The conformational properties of the complexes have been characterised using NMR spectroscopy in CDCl(3), and by comparison with the solid state structures of model compounds. The structural similarity of the complexes makes it possible to apply the double mutant cycle method to evaluate the magnitudes of 24 different aromatic stacking interactions. The major trends in the interaction energy can be rationalised using a simple model based on electrostatic interactions between the pi-faces of the two aromatic rings. However, electrostatic interactions between the substituents of one ring and the pi-face of the other make an additional contribution, due to the slight offset in the stacking geometry. This property makes aromatic stacking interactions particularly sensitive to changes in orientation as well as the nature and location of substituents.     

Optimization, design and fabrication of a non-cryogenic quantum infrared detector

A study of the optimization of the detectivity of a mid infrared double heterostructure photovoltaic detector is proposed. Simple approximate analytic expressions for the dark current are compared with full numerical calculations, and give physical insight on the mechanisms dominating the dark current. The analysis is performed step by step, from a simple p–n junction to the full double heterostructure. The influence of temperature, barrier band gap energy in a double heterostructure, doping density in the active region, on diffusion and generation–recombination mechanisms is analyzed. It is shown how the performances of a double heterostructure photovoltaic detector can be improved by a controlled doping the active region. Nevertheless, its development is still limited by the difficulties occurring during device processing. For example, the use of dry etching for the processing of InAs_0.91Sb_0.09 p–i–n photovoltaic detectors induces a strong leakage current along the mesa edge. In this letter, we show an improvement of the R₀A characteristic by several orders of magnitude at low temperature by using an Ion Beam Etching (IBE) followed by a wet chemical etching. This optimized and reliable device processing allows us to demonstrate that the detector performance is actually limited by the diffusion current of holes. Finally, we discuss the ability of an n-type barrier made of InAs/AlSb super-lattice to avoid hole diffusion and to improve the R₀A characteristic of these detectors. To cite this article: B. Vinter et al., C. R. Physique 4 (2003).