Abstract:We report the fabrication and characterization of rib chalcogenide waveguides produced by dry etching with CF 4 and O 2 . The high index contrast waveguides (∆n ∼ 1) show a minimum propagation loss of 0.25 dB/cm. The high refractive nonlinearity of ∼ 100 times silica in As 2 S 3 allowed observation of a π phase shift due to self-phase modulation of an 8 ps duration 1573 nm pulse in a 5 cm long waveguide.

We report here experimental results on laser ablation of metals in air and in vacuum in similar irradiation conditions. The experiments revealed that the ablation thresholds in air are less than half those measured in vacuum. Our analysis shows that this difference is caused by the existence of a long-lived transient nonequilibrium surface state at the solid-vacuum interface. The energy distribution of atoms at the surface is Maxwellian-like but with its high-energy tail truncated at the binding energy. We find that in vacuum the time needed for energy transfer from the bulk to the surface layer to build the high-energy tail, exceeds other characteristic timescales such as the electron-ion temperature equilibration time and surface cooling time. This prohibits thermal evaporation in vacuum for which the high-energy tail is essential. In air, however, collisions between the gas atoms and the surface markedly reduce the lifetime of this nonequilibrium surface state allowing thermal evaporation to proceed before the surface cools. We find, therefore, that the threshold in vacuum corresponds to nonequilibrium ablation during the pulse, while thermal evaporation after the pulse is responsible for the lower ablation threshold observed in air. This paper provides direct experimental evidence of how the transient surface effects may strongly affect the onset and rate of a solid-gas phase transition

A range of carbon nanofoam samples was prepared by using a high-repetition-rate laser ablation technique under various Ar pressures. Their magnetic properties were systematically investigated by dc magnetization measurements and continuous wave ͑cw͒ as well as pulsed EPR techniques. In all samples we found very large zero-field cooled-field-cooled thermal hysteresis in the susceptibility measurements extending up to room temperature. Zero-field cooled ͑ZFC͒ susceptibility measurements also display very complex behavior with a susceptibility maximum that strongly varies in temperature from sample to sample. Low-temperature magnetization curves indicate a saturation magnetization M S Ϸ 0.35 emu/ g at 2 K and can be well fitted with a classical Langevin function. M S is more than an order of magnitude larger than any possible iron impurity, proving that the observed magnetic phenomena are an intrinsic effect of the carbon nanofoam. Magnetization measurements are consistent with a spin-glass type ground state. The cusps in the ZFC susceptibility curves imply spin freezing temperatures that range from 50 K to the extremely high value of Ͼ300 K. Further EPR measurements revealed three different centers that coexist in all samples, distinguished on the basis of g-factor and relaxation time. Their possible origin and the role in the magnetic phenomena are discussed.

The formation mechanism of carbon onions is investigated. The microstructure of onions formed using pulsed-laser deposition is found to depend critically on the background gas pressure. Molecular dynamics simulations show that an optimal annealing temperature of 4000 K is required to form well-ordered onions ͑concentric fullerene-like spheres͒, in agreement with experiment. The onions form from the outer layer first, and a model is presented in which the background pressure must be sufficient to allow atoms to cluster, yet low enough to allow annealing into well-ordered onions.

No abstract