A. Dos Santos

Frequency-domain interferometer for measuring the phase and amplitude of a femtosecond pulse probing a laser-produced plasma

A frequency-domain interferometer for probing the variations of the dielectric constant of a plasma with sub-100- fs temporal resolution and lambda/2000 phase resolution is described. Imaging the plasma on the entrance slit of a spectrograph provides spatial resolution along a diameter of the focal spot. The technique is used to map out the expansion of the critical density surface of a femtosecond laser-produced plasma with subnanometer spatial resolution along the laser axis.

Specific Interactions between Local Metallicolous Plants Improve the Phytostabilization of Mine Soils

At present, no efficient technique is available for cleaning up soils which are highly polluted by heavy metals. Limiting the movement of pollutants out of the contaminated area by creating a dense and persistent plant cover appears to be the more reasonable approach. In this context, phytostabilization is a technique that uses metallicolous plants to revegetate highly polluted soils. This paper presents the results of an experiment performed in situ using metallicolous ecotypes of four plant species native to the Mediterranean French region, and grown in different combinations at a polluted site over two years. The soils were highly polluted with zinc, cadmium and lead. The aim was to find the best species mixture in terms of cover, biomass and duration. The four species used were the biennial legume Anthyllis vulneraria, two perennial grasses, Festuca arvernensis and Koeleria vallesiana, and the perennial forb Armeria arenaria. Mixtures which included A. vulneraria, and especially when in combination with F. arvernensis, showed the highest values of cover and biomass. After flowering, the biennial individuals of A. vulneraria disappeared but subsequent germination and survival of seedlings occurred abundantly under the two grasses. Mixtures with A. arenaria showed the lowest values of cover and biomass. Soil nitrogen increased in the plots with A. vulneraria as well as the concentration of essential nutrients (N P K) in the aerial parts of the two grasses. In contrast, the concentration of metals (Zn Pb Cd) decreased in the aboveground biomass of the latter in the same plots. These results show that reciprocal facilitation effects can act in heavy metal polluted environments, and that phytostabilization efforts in the Mediterranean region can be improved by using mixtures including local metallicolous legume and grass species.

Cascaded highly nondegenerate four-wave-mixing phenomenon in transparent isotropic condensed media

Broad-bandwidth light pulses with different frequencies, extending from the IR to the UV, are simultaneously generated when two noncollinear, ultrafast laser pulses from a visible dual-frequency laser propagate through bulk isotropic transparent media such as common glass. This phenomenon, which is believed to have been previously unreported, can be explained by a cascade of highly nondegenerate four-wave-mixing processes and corresponds to a coherent scattering effect with geometrically minimized phase mismatch. Frequency-upconverted beams were observed up to the 11th order.

Contrasted Behaviour of an Electron Gas in MgO, Al2O3 and SiO2

We report on the observation, by use of a sub-picosecond time-resolved phase-sensitive technique, of an electron gas pumped by an intense femtosecond laser pulse into the conduction band of three different wide-band-gap oxides. The free-carriers lifetime is measured to be two orders of magnitude longer in MgO and in Al2O3 than in SiO2.

Phase dynamics of reflected probe pulses from sub-100-fs laser-produced plasmas

Using the technique of frequency-domain interferometry, we measure the phase shift between a pair of reflected probe pulses from a plasma produced by 80-fs, 1015 W/cm2 laser interaction on a solid target. A numerical model shows that one cannot interpret phase dynamics merely by adding the phase-shift contributions of the propagation in the underdense plasma and of the moving reflection turning point. The phase-shift difference between S- and P-polarized light at a given angle of incidence is found to vary with the electron density gradient scale length and to be weakly dependent on the plasma collision rate. This phase difference is used to make a snapshot measurement of the radial variations of the density scale length of an aluminum plasma.

Femtosecond multiphoton generation of the self‐trapped exciton in α‐SiO2

Nonlinear optical excitation of crystalline quartz with intense femtosecond UV pulses yields the 2.8 eV recombination luminescence of the self‐trapped exciton. The relation between the excitation and emission intensities reveals two‐ and three‐photon kinetics for photon energies of 4.4 and 4.0 eV at excitation densities below 1018 cm−3. These power laws are not sizably influenced by transient linear absorption, self‐focusing, and filamentation.

Generation of megawatt tunable pulses of duration 90 fs at a 11 kHz repetition rate: First experiments in the range 800–850 nm

Abstract We report the generation of 90 fs duration pulses with 3 MW peak power, in the near infrared around 830 nm, at a 11 kHz repetition rate. This system, which allows a large tunability is based on the amplification of the continuum in an infrared dye pumped by the yellow line of a copper vapor laser.

Diffraction of light by a corrugated magnetic grating : experimental results and calculation using a perturbation approximation to the Rayleigh method

When visible light is diffracted by a relief grating covered with a ferromagnetic film, the intensity diffracted in all orders varies as magnetization is reversed. The relative variation in diffracted intensity is measured in the transverse, but generally non-specular, Kerr geometry (reflection case, magnetization perpendicular to diffraction plane). The effect is modelled using a perturbation approximation to the Rayleigh method. The theoretical results are in good agreement with experiment.

Observation of X-ray fluorescence resulting from photoionization by a femtosecond laser-produced plasma X-ray source

We demonstrate the first observation of X-ray fluorescence resulting from inner-shell photoionization by a subpicosecond laser-produced plasma X-ray source. 8 keV electrons produced by a 100 fs laser at 3*1016 W cm-2 intensity create K-shell vacancies in a Ti layer. This generates a 4.51 keV K alpha X-ray pulse which is used to photoionize a Ca sample. Fluorescence of Ca at 3.69 keV (K alpha ) and 4.01 keV (K beta ) is observed as a result of Ti K alpha photoionization.

Filament amplification in a liquid

We study the amplification of femtosecond filaments in a liquid amplifying medium, Sulphorhodamine 640 in an ethanol solvent. Evidence of filament diameter expansion is demonstrated.