How phonons govern the behavior of short, strong hydrogen bonds in urea-phosphoric acid

Recent neutron diffraction data have shown that the hydrogen atom involved in the short, strong hydrogen bond in urea-phosphoric acid migrates toward the midpoint of the hydrogen bond as the temperature increases. With the help of solid state ab initio calculations and inelastic neutron scattering, we have investigated the temperature dependence of the structural and vibrational properties of the system. The potential energy surface of the proton in the short, strong hydrogen bond and the thermal population of the energy levels therein cannot account for the observed proton migration. Ab initio molecular dynamics simulations clearly reveal the migration of the proton. This molecular dynamics result was reported recently by other authors, but they only offered a tentative explanation in terms of a resonance between high-frequency vibrations, which is not supported by the calculations presented here. We explain the proton migration in terms of phonon-driven structural fluctuations and their impact on the temperature-dependent evolution of the potential energy surface of the short hydrogen-bond proton.     

Broad-bandwidth shot-noise-limited class-A operation of a monomode semiconductor fiber-based ring laser

We demonstrate the operation of a monomode semiconductor laser with a relative intensity noise limited by the shot-noise floor, -156 dB/Hz for a typical detected photocurrent of 1 mA, over a large frequency range from 50 MHz to 18 GHz. We achieve direct control of photon lifetime to turn an initially class-B laser into a relaxation-oscillation-free class-A one while preserving strict single-mode operation. Finally, we confirm experimentally that the laser operation in the desired class-A regime allows a dramatic filtering out of the relative intensity noise.     

Control of an external-cavity laser diode operating in the regular pulse package regime with a frequency shift of the optical feedback

I demonstrate numerically that the regular pulse package regime observed in an external-cavity laser diode can be controlled by means of an adequate shift of the optical feedback frequency. This control leads to a stable pulsed behavior.     

Predicting ductile fracture of low carbon steel sheets: Stress-based versus mixed stress/strain-based Mohr–Coulomb model

Two distinct implementations of the Mohr–Coulomb failure model are used in conjunction with a non-associated quadratic plasticity model to describe the onset of fracture in low carbon steel sheets. The stress-based version corresponds to the original Mohr–Coulomb model in stress space. For the mixed stress/strain-based version, the Mohr–Coulomb failure criterion is first transformed into the space of stress triaxiality, Lode angle parameter and equivalent plastic strain and then used as stress-state dependent weighting function in a damage indicator model. Basic fracture experiments including tensile specimens of different notch radii and a punch test are performed to calibrate the material parameters of the respective models. Subsequently, the models are used to predict the crack initiation in a Hasek test and during the stamping of an anticlastic structure. Unlike for the calibration experiments, the loading history during stamping is highly non-linear. Both models can be calibrated with similar accuracy, but the strain-based model predicts the instant of onset of fracture with greater accuracy in the stamping experiment which is an advantage of the empirical damage accumulation rule.     

Transport properties of overheated electrons trapped on a helium surface

An ultra-strong photovoltaic effect has recently been reported for electrons trapped on a liquid helium surface under a microwave excitation tuned at intersubband resonance [D. Konstantinov, A.D. Chepelianskii, K. Kono, J. Phys. Soc. Jpn 81, 093601 (2012)]. In this article, we analyze theoretically the redistribution of the electron density induced by an overheating of the surface electrons under irradiation, and obtain quantitative predictions for the photocurrent dependence on the effective electron temperature and confinement voltages. We show that the photo-current can change sign as a function of the parameters of the electrostatic confinement potential on the surface, while the photocurrent measurements reported so far have been performed only at a fixed confinement potential. The experimental observation of this sign reversal could provide a reliable estimation of the electron effective temperature in this new out of equilibrium state. Finally, we have also considered the effect of the temperature on the outcome of capacitive transport measurement techniques. These investigations led us to develop, numerical and analytical methods for solving the Poisson-Boltzmann equation in the limit of very low temperatures which could be useful for other systems.     

2-μm high-power multiple-frequency single-mode Q-switched Ho:YLF laser for DIAL application

We report on the development and the demonstration of a two-wavelength single-frequency laser oscillator based on Ho:YLF crystal. This laser is especially suitable for application as a transmitter in differential absorption lidar (DIAL)/integrated path differential absorption (IPDA) measurements of atmospheric carbon dioxide (CO₂) using the R30 CO₂ absorption line at 2,050.967 nm. The oscillator consists in a fiber-coupled and free-space solid-state hybrid system and can be used in high-energy middle-rate or moderate-energy high-rate configurations. The latter produced On and Off sequentially single-frequency laser pulses with 13 mJ of energy at a repetition rate of 2 kHz and pulse duration of 42 ns. The pulse energy and frequency stabilities are specially documented in free-running, single-frequency and two-frequency seeding single-mode operations. Standard deviation is 7.7 % for pulse energy and 2 MHz for frequency stability for the two-wavelength seeding operation. Allan variance plot shows that frequency fluctuations are reduced below 70 kHz for 10 s of averaging which is suitable for accurate CO₂ DIAL or IPDA measurements.     

Cu(I)-catalyzed oxidative cyclization of alkynyl oxiranes and oxetanes

In the presence of a Cu(I) catalyst and a pyridine oxide, alkynyl oxiranes and oxetanes can be converted into functionalized five- or six-membered α,β-unsaturated lactones or dihydrofuranaldehydes. This new oxidative cyclization is proposed to proceed via an unusual allenyloxypyridinium intermediate.