Efficient approximately 2 microm Tm3+ -doped tellurite fiber laser

Laser emission in the range of 1.88-1.99 micrim from a Tm3+ -doped tellurite fiber is demonstrated when pumped with a diode-pumped Er3+/Yb3+-doped silica fiber laser operating at 1.57-1.61 microm. This pump source excites the Tm3+ ions directly into the F43 upper laser level and yields an output power of 280 mW with a slope efficiency of 76% in a 99%-12% laser cavity arrangement and a 32 cm long fiber. This result is very close to the Stokes efficiency limit of approximately 80%. This is, to the authors' knowledge, the first demonstration of high efficiency lasing in a tellurite fiber at wavelengths longer than 1.56 microm.     

Tm(3+)/Ho(3+) codoped tellurite fiber laser

Continuous-wave and Q-switched lasing from a Tm(3+)/Ho(3+) codoped tellurite fiber is reported. An Yb(3+)/Er(3+)-doped silica fiber laser operating at 1.6 microm was used as an in-band pump source, exciting the Tm(3+) ions into the (3)F(4) level. Energy is then nonradiatively transferred to the upper laser level, the (5)I(7) state of Ho(3+). The laser transition is from the (5)I(7) level to the (5)I(8) level, and the resulting emission is at 2.1 microm. For continuous wave operation, the slope efficiency was 62% and the threshold 0.1 W; the maximum output demonstrated was 0.16 W. Mechanical Q switching resulted in a pulse of 0.65 microJ energy and 160 ns duration at a repetition rate of 19.4 kHz.     

Orbital-Overlap control in the solid-state reactivity of beta-azido-propiophenones: selective formation of cis-azo-dimers

Solid-state photolysis of 1a,b yields selectively cis-3a,b. X-ray analysis of 1a,b reveals the molecules adopt an extended structure and as such the crystal packing arrangement consists of planar, pi-stacked molecules. The shortest intermolecular distance between adjacent N-atoms is approximately 3.76 A and would lead to formation of trans-3a,b, whereas cis-3a,b is formed by dimerization between N-atoms that are approximately 3.9 A apart. We propose that the molecular orbital alignment of the adjacent nitrenes controls the solid-state reactivity.     

Waring-Goldbach Problem for Unlike Powers

In this paper, it is proved that with at most O（N65/66） exceptions, all even positive integers up to N are expressible in the form p^2 2＋p^3 3＋p^4 4＋p^5 5. This improves a recent result O（N19193/19200＋ε） due to C. Bauer.     

Ultra-Compact and Efficient Integrated Multichannel Mode Multiplexer in Silicon for Few-Mode Fibers

Space-division multiplexing (SDM) is one of the key enabling technologies to increase the capacity of fiber communication systems. However, implementing SDM-based systems using multimode fiber has been challenging with the need for compact, low-cost, and scalable mode de/multiplexer (DE/MUX). Here a novel integrated mode MUX for few-mode fibers (FMFs) is presented which can launch up to eight spatial and polarization channels. The new design is composed of a 2D multimode grating coupler (MMGC), highly compact spot size converters (SSCs), and adiabatic directional couplers (ADCs). Eight data lanes in FMFs can be selectively launched with integrated optical phase shifters. Experimental results reveal efficient chip-to-fiber coupling with peak efficiencies of −3.8, −5.5, −3.6, and −4.1 dB for LP₀₁, LP_11a, LP_11b, and LP_21b modes, respectively. Thanks to the use of an integrated subwavelength Mikaelian lens for mode-independent field size conversion with loss ≤−0.25 dB, the total footprint of the MMGC and SSCs is only 35×35 µm². The proposed design shows great potential for densely integrated photonic circuits in future SDM applications.     

Automerizations in benzenoid hydrocarbons. New mechanistic insights from the thermal rearrangement of benz[a]anthracene-5-13C.

Scrambling of the carbon atoms in benz[a]anthracene between positions 5 and 6 has been shown to occur at high temperatures; a mechanism involving thermal generation of a carbene is proposed.     

Electron promotion by surface functional groups of single wall carbon nanotubes to overlying metal particles in a fuel-cell catalyst

A remarkable promotion: Functional groups added onto single-wall carbon nanotubes (SWNTs) can significantly influence the activity of a noble metal for formic acid oxidation. Phenolate groups on SWNTs under alkaline conditions can double the activity of 20 % w/w Pd compared to unmodified SWNTs. This catalyst has 14 times higher activity than the commercial benchmark catalyst (10 % w/w Pd on Vulcan).     

A Semi-Analytical Solution for Large-Scale Injection-Induced Pressure Perturbation and Leakage in a Laterally Bounded Aquifer–Aquitard System

A number of (semi-)analytical solutions are available to drawdown analysis and leakage estimation of shallow aquifer–aquitard systems. These solutions assume that the systems are laterally infinite. When a large-scale pumping from (or injection into) an aquifer–aquitard system of lower specific storativity occurs, induced pressure perturbation (or hydraulic head drawdown/rise) may reach the lateral boundary of the aquifer. We developed semi-analytical solutions to address the induced pressure perturbation and vertical leakage in a “laterally bounded” system consisting of an aquifer and an overlying/underlying aquitard. A one-dimensional radial flow equation for the aquifer was coupled with a one-dimensional vertical flow equation for the aquitard, with a no-flow condition imposed on the outer radial boundary. Analytical solutions were obtained for (1) the Laplace-transform hydraulic head drawdown/rise in the aquifer and in the aquitard, (2) the Laplace-transform rate and volume of leakage through the aquifer–aquitard interface integrated up to an arbitrary radial distance, (3) the transformed total leakage rate and volume for the entire interface, and (4) the transformed horizontal flux at any radius. The total leakage rate and volume depend only on the hydrogeologic properties and thicknesses of the aquifer and aquitard, as well as the duration of pumping or injection. It was proven that the total leakage rate and volume are independent of the aquifer’s radial extent and wellbore radius. The derived analytical solutions for bounded systems are the generalized solutions of infinite systems. Laplace-transform solutions were numerically inverted to obtain the hydraulic head drawdown/rise, leakage rate, leakage volume, and horizontal flux for given hydrogeologic and geometric conditions of the aquifer–aquitard system, as well as injection/pumping scenarios. Application to a large-scale injection-and-storage problem in a bounded system was demonstrated.     

Computations of modes I and II stress intensity factors of sharp notched plates under in-plane shear and bending loading by the fractal-like finite element method

The fractal-like finite element method (FFEM) is used to compute the stress intensity factors (SIFs) for different configurations of cracked/notched plates subject to in-plane shear and bending loading conditions. In the FFEM, the large number of unknown variables in the singular region around a notch tip is reduced to a small set of generalised co-ordinates by performing a fractal transformation using global interpolation functions. The use of exact analytical solutions of the displacement field around a notch tip as the global interpolation functions reduces the computational cost significantly and neither post-processing technique to extract SIFs nor special singular elements to model the singular region are required. The results of numerical examples of various configurations of cracked/notched plates are presented and validated via published data. Also, new results for cracked/notched plate problems are presented. These results demonstrate the accuracy and efficiency of the FFEM to compute the SIFs for notch problems under in-plane shear and bending loading conditions.