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International audienceA two-dimensional axisymmetric model of the propagation of intense femtosecond laser pulses through dispersion-free transparent media is described. The effects of diffraction, nonlinear Kerr effect (instantaneous and retarded) and multiphoton ionisation are included. Numerical results concerning air and other gases are discussed. In particular, time self-compression of femtosecond pulses is predicted. Stable self-guided pulses are simulated, in agreement with recent experimental observations

The conductivity tensors of single crystals and polycrystals of RFe 2 (RϭGd,Tb,Ho,Lu) and GdCo 2 were determined in the visible and near UV ranges. The magneto-optical Kerr effect ͑MOKE͒ was studied at different temperatures and magnetic fields. The single-crystal data show more features and larger magnitudes in the MOKE spectrum than the polycrystalline data under the same experimental conditions. The theoretical optical conductivity tensors for these compounds were calculated using the tight-binding linear-muffin-tin orbital ͑TB-LMTO͒ method in the local spin-density approximation. The agreement between theory and experiment was poor except for LuFe 2 , in which the 4 f shell is completely closed.

The conductivity tensor of polycrystalline Pt 3 X (XϭMn, Co) was determined between 1.6 and 5.2 eV. Samples were arc melted, mechanically polished, and annealed at 500°C for 1 h in Ar. The complex dielectric function was measured from 1.3 to 5.2 eV at room temperature with a rotating analyzer ellipsometer. The magneto-optic Kerr effect was studied between 10 and 293 K in magnetic fields up to 3 T. We used the tight-binding linear-muffin-tin-orbital method in the local spin-density approximation to determine the band structure, density of states, and optical conductivity. Including an empirical quasiparticle self-energy and a lifetime broadening yields good agreement of experimental and calculated spectra. ͓S0163-1829͑98͒07625-5͔ II. EXPERIMENT A. Sample preparation and characterizationPt 3 X samples were arc-melted under an Ar atmosphere. Before arc-melting the chamber is purged to 20 mT and backfilled with ultra-high-purity argon ͑99.9995%͒ to about 0.75 atm five times. A Zr button is used to getter the atmosphere during the arc-melting process. After a homogeniza-

Using spectroscopic ellipsometry, we measured the pseudodielectric function of Si1−x−yGexCy alloys (0≤x≤0.48,0≤y≤0.05) grown on Si(001) using molecular beam epitaxy. For pseudomorphically strained layers, the energy shifts of the E1, E1+Δ1, E0′, and E2 transitions are determined by line shape analysis and are due to alloy composition effects, as well as hydrostatic and shear strain. We developed expressions for hydrostatic and shear shift from continuum elasticity theory, using deformation potentials for Si and Ge, for biaxial stress parallel to the (001) growth plane in a diamond or zinc blende-type crystal and applied this to the ternary Si–Ge–C alloy. The energies of E1 and its spin-orbit split partner E1+Δ1 agree fairly well with theory. The E2 transitions in Si1−xGex at around 4.3 eV depend linearly on Ge concentration. In case of relaxed layers, the E1 and E1+Δ1 transitions are inhomogeneously broadened due to the influence of misfit and threading dislocations. For a silicon cap on top of a dislocated, relaxed SiGe layer, we recovered the bulk Si dielectric function.

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Spectroscopic ellipsometry was used to measure the dielectric functions of epitaxial and bulk Ge at photon energies from 1.5 to 5.2 eV. The epitaxial Ge was grown at 400°C by molecular beam epitaxy on ͑001͒ Si substrates. The optical response and the interband critical-point parameters of Ge on Si were found to be indistinguishable from that of bulk single crystal Ge, indicating high optical quality. Dislocation density measurements using an iodine etch verified low surface defect densities. We conclude that epitaxial Ge grown on Si at relatively low temperatures is suitable for optical device applications.