Design strategies to increase the brightness of gain guided tapered lasers

The basic operating principles of gain guided tapered lasers are studied in detail by means of numerical simulations. The carrier induced lensing effect caused by spatial hole burning is described as the main mechanism limiting the device performance. The influence of the following design parameters in the maximum output power and beam brightness is analyzed: refractive index step of the ridge waveguide section, angle and length of the tapered section, and the use of beam spoilers. The results provide design guidelines for improving the device performance.     

Arbitrary truncation order three-point finite difference method for optical waveguides with stepwise refractive index discontinuities

An exact finite difference (FD) representation of the second-order derivative on three nodes is presented and used to obtain an FD algorithm that allows achieving an arbitrary truncation order. The FD weights are calculated analytically using the series that expresses the field value at a given FD node in terms of the field value and its derivatives at a neighboring node, when a stepwise discontinuity in the refractive index distribution is present between the nodes. The results obtained confirm that the proposed algorithm is accurate, efficient, and achieves the predicted improved performance.     

Melanin from epidermal human melanocytes: Study by pyrolytic GC/MS

Pigmentation of human skin is determined by the presence of melanin, the polymeric pigment that is produced in melanocytes and transferred to adjacent keratinocytes. Epidermal melanocytes produce two distinct types of melanin pigments: eumelanin, composed mainly of indole-type monomers, and pheomelanin that contains benzothiazine-type backbone. Eumelanin protects skin against UV-induced damages, whereas pheomelanin is believed to act as a potent UV photosensitizer and promote carcinogenesis. In this study, pyrolysis in combination with gas chromatography and mass spectrometry (Py-GC/MS) was applied for structural studies of the epidermal pigment isolated from the cultured human melanocytes. The analysis was preceded by investigations of DOPA-originated synthetic eumelanin and pheomelanin standards. This allowed determination of pyrolytic markers for both types of melanin pigments. To obtain additional information on the natural pigment structure, the samples were thermally degraded in the presence of tetramethylammonium hydroxide as the derivatizing agent. It was shown that the analyzed pigment from normal human epidermal melanocytes derived from moderately pigmented skin is of eumelanin type with little incorporation of a pheomelanin component. The results indicate that Py-GC/MS is a rapid and efficient technique for the differentiation of epidermal melanin types and may be an alternative to commonly used methods based on chemical degradation.     

Ab initio calculations of dispersion coefficients for nucleic acid base pairs

The results of ab initio calculations of two- and three-body dispersion coefficients for the four most important nucleic acid bases are reported. The isotropic as well as anisotropic coefficients were found by using the time-dependent Hartree-Fock approach and the aug-cc-pVDZ basis set. Single and double excitation coupled-cluster theory with noniterative treatment of triple excitations [CCSD(T)] was used to find the values of static polarizabilities which were subsequently used to estimate the values of the CCSD(T) dispersion coefficients. A comparison of these estimated CCSD(T) dispersion coefficients with coefficients found by using empirical approaches based on atomic contributions revealed that the latter are not reliable.     

Well-Defined Aminophenolate Zinc and Magnesium Complexes as Excellent Inhitiators for the ROP of Lactides

A series of molecular homo and heteroleptic zinc and magnesium compounds with aminophenolate ligands [(µ,η²-L²)ZnEt]₂ ( 1 ), [(η²-L²)Zn(µ-BnO)]₂ ( 2 ), [Zn(η²-L²)₂] ( 3 ), [Zn(η²-L³)₂] ( 4 ), [Mg(η²-L³)₂] ( 5 ) (L²-H = N-[methylene(2-hydroxy-3,5-di-tert-butylphenyl)]-N-methyl-N-cyclohexylamine, L³-H = N-[methylene(2-hydroxy-3,5-di-tert-butylphenyl)]-N-methyl-N-methyl-1,3-dioxolaneamine) have been prepared and characterized. The homoleptic complexes 3–5 are most probably a mixture of diastereoisomers that in solution show an interesting dynamics which plays an important role in their catalytic behavior. The complexes 2 – 5 are efficient initiators in ring-opening polymerization (ROP) of lactides to produce polymers with desired molecular weight and narrow polydispersity.     

Addition of γ-silyloxyallyltins on ethyl glyoxylate: evaluation of the influence of the experimental conditions on the stereochemical course of the reaction

Ethyl glyoxylate was reacted with α-substituted γ-(t-butyldimethylsilyloxy)-allyltributyltin in order to obtain selectively each diastereomer of ethyl 3-(t-butyldimethylsilyloxy)-2-hydroxyhex-4-enoate and subsequently the corresponding diols. Diastereomers syn-E, anti-E and anti-Z were obtained in good yields with good to high selectivities and the obtained results were rationalized by consideration of cyclic or open transition states in agreement with the experimental conditions and with the structure of the starting reagents.     

Conductance of alpha-helical peptides trapped within molecular junctions

Self-assembled monolayers of alpha-helical peptides on a gold surface were employed as model systems for the investigation of mediated electron transfer. The peptides contained 14, 15, 16, and 17 amino acid residues. The measurements of electron transmission through single molecules of helical peptides were performed using scanning tunneling spectroscopy (STS). The molecules were trapped between the gold tip and the substrate. Electrical contact between the molecule and the gold probe was achieved by the use of peptides containing thiol groups present at each end of the helix. The conductance behavior of the peptides was examined as a function of tip-substrate distance at fixed bias voltage. Measurements performed with peptides containing different numbers of amino acid residues indicate that the distance dependence of electron transmission through an alpha-helix is weaker than that through simple n-alkyl bridges.     

On -cobordisms of spherical space forms

Given a manifold of dimension at least 4 whose universal covering is homeomorphic to a sphere, the main result states that a compact manifold is isomorphic to a cylinder if and only if is homotopy equivalent to this cylinder and the boundary is isomorphic to two copies of ; this holds in the smooth, PL and topological categories. The result yields a classification of smooth, finite group actions on homotopy spheres (in dimensions ) with exactly two singular points.

     

Existence and uniqueness of a Nash equilibrium feedback for a simple nonzero-sum differential game

Existence and uniqueness of a Nash equilibrium feedback is established for a simple class nonzero-sum differential games on the line.