Oxidation of Pt(111) under near-ambient conditions

The oxidation of Pt(111) at near-ambient O2 pressures has been followed in situ using x-ray photoelectron spectroscopy (XPS) and ex situ using x-ray absorption spectroscopy (XAS). Polarization-dependent XAS signatures at the O K edge reveal significant temperature- and pressure-dependent changes of the Pt-O interaction. Oxide growth commences via a PtO-like surface oxide that coexists with chemisorbed oxygen, while an ultrathin α-PtO2 trilayer is identified as the precursor to bulk oxidation. These results have important implications for understanding the chemical state of Pt in catalysis.     

Alternating cascade of spectrally different erbium-doped fiberamplifiers for link-loss-insensitive long-haul WDM transmission

This paper numerically investigates a cascade of erbium-doped fiber amplifiers (EDFAs) with different spectral characteristics in an alternating scheme. Thereby, in a transmission band with appropriate spectral properties (1544-1550 nm), the sensitivity of optically amplified long-haul WDM transmission links on the span loss between amplifiers can be reduced to that of single-channel systems. This may be as much as an order of magnitude better than in a traditional, nonalternating, WDM cascade. While passive bandpass filters are necessary to control the wavelength range, there is no need for any active gain equalization or feed-back control. The gain in the cascade can also to some extent automatically adjust to compensate for a spectral tilt of the loss. We show that the range of this automatic loss tilt compensation can be increased, up to three times its original value, by enhancing the difference of the net gain that signals at different wavelengths experience in EDFAs of different types     

Preservation of the Pt(100) surface reconstruction after growth of a continuous layer of graphene

Scanning tunneling microscopy shows that a layer of graphene can be grown on the hex-reconstructed Pt(100) surface and that the reconstruction is preserved after growth. A continuous sheet of graphene can be grown across domain boundaries and step edges without loss of periodicity or change in direction. Density functional theory calculations on a simple model system support the observation that the graphene can have different rotation angles relative to the hex-reconstructed Pt surface. The graphene sheet direction can be changed by incorporating pentagon-heptagon defects giving rise to accommodation of edge dislocations. The defect formation energy and the induced buckling of the graphene have been characterized by DFT calculations.     

A Stokes model with cavitation for the numerical simulation of hydrodynamic lubrication

We present a cavitation model based on the Stokes equation and formulate adaptive finite element methods for its numerical solution. A posteriori error estimates and adaptive algorithms are derived, and numerical examples illustrating the theory are supplied, in particular with comparison to the simplified Reynolds model of lubrication. Copyright © 2010 John Wiley & Sons, Ltd.     

A 980-nm Yb-doped fiber MOPA source and its frequency doubling

A continuous-wave (CW) master oscillator-power amplifier (MOPA) fiber source, tunable around 978 nm, was frequency-doubled to 488.7 nm. Both the laser and the amplifier were made with cladding-pumped jacketed air-clad Yb-doped fibers. The MOPA generated up to 2.7 W of power in an output beam with an M/sup 2/ value of 1.8. This was frequency-doubled in a periodically poled potassium titanyl phosphate crystal at room temperature, in a single-pass configuration. The generated blue light had a CW power of 18.1 mW, a nearly Gaussian spatial intensity profile, and an M/sup 2/ value of 1.7.     

Intensity variations in the near-band-edge recombination of GaP epitaxial layers, grown on (111) and (001) oriented substrates, as observed by cathodoluminescence imaging

In this study a comparison between GaP layers, grown by metalorganic vapour phase epitaxy on either (111) oriented GaP substrates or (001) oriented GaP substrates, is made with respect to the near-band-edge luminescence. Spectrally resolved cathodoluminescence, CL, images were recorded at ˜ 25 K with detection on either the bound exciton emission or the donor-to-acceptor pair emission. The intensity variations observed in the CL images are interpreted in terms of aggregation of impurities around threading dislocations. Contrary to the layers grown on (001) oriented substrates, the layers grown on (111) oriented substrates show a segregation effect of the acceptor constituent involved in the donor-to-acceptor pair emission.     

Laser linewidth broadening caused by a laser amplifier

It is shown that a laser amplifier connected to a laser will broaden the linewidth with a factor on the order of the laser multiplier power gain. The laser linewidth can be broadened in comparison to the free-running linewidth if the laser is connected to a laser amplifier because the noise emitted by the amplifier enters the laser and perturbs the oscillation. The broadening factor can be reduced to unity by inserting an isolator between the laser and the laser amplifier.<>     

Amplitude and frequency modulation characteristics of widelytunable GCSR lasers

The amplitude (AM) and frequency (FM) modulation responses of a grating assisted codirectional coupler with rear sampled grating reflector laser were measured over a 40-nm tuning range. The extracted modulation bandwidth varied between 3 and 4 GHz and parameters as parasitic cutoff, maximum relaxation frequency and K-factor were extracted from curve fittings to the AM response. The frequency modulation measurements revealed low chirp despite the large tunability of the device. The linewidth enhancement factor was calculated from the AM and FM measurements. The values ranges from three to nine for ten different operating wavelengths     

Long-wavelength erbium-doped fiber amplifier gain enhanced by ASEend-reflectors

We use for what we believe is the first time narrow-band end-reflectors to reduce losses through short-wavelength amplified stimulated emission (ASE) in silica-based erbium-doped fiber amplifiers operating at wavelengths above 1570 mm. The end-reflectors feed a small fraction of the ASE, up to a few tenths of a milliwatt, back into the amplifying fiber. The reflected ASE compresses the short-wavelength gain and thus reduces the ASE-losses, from, e,g., 50 mW for a launched pump power of 110 mW at 980 nm without end-reflector to 10 mW with an optimized end-reflector. We investigate possible improvements of gain (around 5 dB) and output power (up to 17 mW), and the influence of the amount and wavelength of the feedback