ALBERT

Notes: The switching dynamics between TE- and TM-polarization states is studied in a strained ridge-waveguide InGaAsP/InP laser that exhibits TE/TM bistability. Using current modulation with frequencies between 50 and 500 MHz, three types of emission are distinguished. With increasing modulation amplitude, the laser runs through a region of TE emission, a range of stochastic switching between TE and TM modes, and a third region of regular polarization switching. The minimum modulation amplitude for regular switching rises strongly with frequency while the respective switching times decrease from about 700 ps at 50 MHz down to 250 ps around 500 MHz, corresponding to gigahertz mode switching.

Notes: The effect of particulate volume fraction vp and diameter dp on the composite Young's modulus Ec is studied both experimentally, using a silica bead/epoxy system, as well as with the help of computer simulations. The experimental and simulation results show that for a given particulate size, the overall Ec vs vp curve displays a concave upward shape and not a linear shape. This superlinear trend of the data implies that the average strain normalized to the applied strain λ=ε¯p/εc transferred to the particulates increases with volume fraction. The above finding is explained in terms of a mean-field picture, where a single particle interacts with an effective medium consisting of the remaining particles embedded in the matrix. As the modulus of the effective medium surrounding a reference particle increases with vp, the modulus mismatch between the reference particulate and the medium is consequently reduced. This leads to an overall increase in the normalized average strain λ transferred to each particulate as vp is increased. The experimental results using silica particulates with various sizes dp, as well as the simulation results, show that smaller particulates provide an increased composite modulus as compared to larger particulates, at constant vp. General equations are developed, which relate the composite modulus to the average particle stress or strain, given only information about the volume fraction and the Young's modulus of each of the phases present.Through the application of these relations, it is found that smaller particulates display a greater amount of normalized average strain λ transferred than larger particulates. The effect of particulate Young's modulus Ep in combination with particulate size on the resulting Ec is also studied using simulations only. It is found that for a low particulate to matrix modulus ratio Ep/Em, the particulate size has very little influence on Ec. Moreover, the shape of the Ec vs vp curve can be well approximated by a straight line up to large values of vp. On the other hand, as the ratio Ep/Em is increased, the superlinear trend of the composite modulus Ec vs vp data is more apparent. This results in a smaller range of the Ec vs vp curve, which can be approximated by a linear function. It is also found that the extent of this linear region also decreases with particle size.

Notes: The interaction of continuous CO2 laser radiation with water is described. Tightly focused radiation at powers of up to 120 W is used to generate and maintain a conical depression in the water surface similar to the keyhole created during laser penetration welding. The observed penetration depth of up to 7 mm is explained in terms of a hydrodynamic model and observations on the interaction induced liquid flows are described. The momentum reaction to the flow of steam up out of the keyhole generates a downward flow in the liquid around the keyhole with a velocity of ∼20 cm/s under the conditions of these experiments. The experiment has also been performed under variable gravity conditions provided by the NASA KC-135 Microgravity Aircraft to observe effects associated with changes in hydrostatic pressure and buoyancy on the system.

Notes: The new Schwerionen-Synchrotron (SIS)/Experimenteller Speicher-Ring (ESR) heavy ion accelerator facility [Nucl. Instrum. Methods A 278, 7, 19 (1989)], built for basic atomic and nuclear physics at Darmstadt (Federal Republic of Germany), also provides unique possibilities to study rf accelerator and beam/target interaction physics for inertial confinement fusion driven with heavy ion beams. It is the first machine that offers this opportunity. Beam parameters and experiments planned over the next five years are discussed.

Notes: A new method to determine the proton concentration in LiNbO3 is presented. The method is based on the measurement of the diffraction efficiency of a photorefractive grating in two situations. It is first measured after recording at room temperature, and second after saturation of the fixing process at a given temperature (about 150 °C). From only these two experimental data, the value obtained for the proton concentration in our sample is H0=(4.4±0.7)×1018 cm−3. This value agrees, within the experimental error, with that obtained from the infrared absorption arising from the OH− stretching bond.

Notes: We have realized a novel wavelength division multiplexer/demultiplexer based on rib waveguides in GaInAsP/InP. The device is a strongly asymmetric codirectional coupler with periodic variation of the coupling constant and has sinc2-type characteristics. Our devices have center wavelengths around 1.3 μm, filter half-widths 10–15 nm, and far end isolation down to −17 dB. The structure is suited for integration into optoelectronic integrated circuits for applications in bidirectional optical communication systems.

Notes: Work function measurements with the so-called onset technique have been used to study the temperature-dependent behavior of the electron energy gap of YBa2Cu3O7−x superconducting films. In the temperature range from TC down to 30 K a BCS-like behavior with gap width 2Δ0 at T=0 between 38 and 62 meV is found.