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Nous présentons des résultats récents de laser X collisionnel transitoire pompé en incidence rasante à haut taux de répétition. Ils ont été obtenus à partir du laser de pompe Titane :Saphir 30 TW disponible sur l'installation européenne du LLC à Lund (Suède). Nous avons démontré lors de cette expérience qu'il était possible d'obtenir en routine près de 3 microjoule par impulsion à 18,9 nm avec seulement 1 J d'énergie de pompe infrarouge. Nous avons plus particulièrement étudié l'influence de l'angle de rasance sur cible de l'impulsion laser qui vient pomper le plasma, quelques centaines de picosecondes après la première impulsion responsable de sa création. Un système d'imagerie XUV à haute résolution nous a en effet permis d'obtenir des informations précieuses sur la pupille de sortie du laser X, comme l'énergie totale dans la tache mais aussi la distance d'émission par rapport à la cible, et encore les dimensions horizontale et verticale de la source.

Nous avons étudié les propriétés non linéaires de l'émission harmonique en utilisant une technique d'autocorrélation d'ordre supérieur du faisceau laser. Dans certaines conditions de génération (éclairement, pression, focalisation), les profils d'autocorrélation présentent un comportement inattendu. Nous utilisons ensuite la bonne stabilité mécanique de notre dispositif expérimental (quelques nm) afin de mesurer, pour la première fois, les traces d'autocorrélation du 1erordre des harmoniques d'ordre élevé.

Les propriétés de cohérence ainsi que la très courte durée des impulsions de la génération d'harmonique d'ordre élevé ont été appliquées à des applications de type pompe/sonde utilisant l'interférométrie dans le domaine XUV. Nous avons pour la première fois mesuré dans deux expériences différentes les densités électroniques de plasmas par interférométrie fréquentielle et spatiale dans le domaine XUV avec une résolution temporelle de quelques dizaine de femtosecondes.

By tightly focusing ultra-short pulses from a Ti : sapphire terawatt laser onto a high-Z metallic target, hard x-ray pulses of short duration are produced. In most of our previous work concerning x-rays, a 150 mJ laser pulse with a 110 fs duration has been used. Using mostly tin and tantalum targets, hard x-rays in the 10-60 keV range have been produced and used in differential absorption imaging around the [MAT] absorption edge of a contrast agent and also in imaging employing gated viewing for suppression of scattered radiation. In order to increase the x-ray yield (shortening the acquisition time) an increase in the laser repetition rate is desirable while still staying in the [MATH] energy regime. We have used a 1 kHz repetition-rate laser delivering 35 fs pulses in order to work towards these goals. We have clear evidence of hard x-ray generation above 30 keV, even for low laser pulse energies. We also studied the effect of a fs prepulse. The medical imaging capability of the source was explored. The use of a prepulse has been optimized in order to improve the image quality as well as the overall x-ray generation yield.

Les sections efficaces d'ionisation des états excités de l'hélium 1s2p 1P et 1s3p 1P ont été mesurées au voisinage du seuil d'ionisation en observant la saturation de l'ionisation. Les états excités de l'hélium sont préparés par l'absorption résonnante d'une harmonique d'ordre élevée produite par un laser picoseconde accordable. Ces états sont ensuite ionisés par un faisceau sonde en absorbant un photon. La modification de la fréquence du faisceau sonde, du proche infrarouge à l'ultraviolet, nous a permis de déterminer la dépendance de la section efficace d'ionisation en fonction de l'énergie de l'électron arraché. Les résultats expérimentaux confirment quantitativement les travaux théoriques effectués auparavant.

Laser wakefield accelerators offer the possibility of compact electron acceleration. However one of the key outstanding issues with the results reported to date is the electron beam stability. Experiments on two laser systems reveal that the contrast ratio between the ASE pedestal and main pulse is an important factor in determining the quality of the electron beam. With a high contrast ratio (108) the electron beam profile is a well collimated single beam having a low pointing instability (<10?mrad?rms). With a lower contrast (106) the beam profile contains multiple beamlets which exhibit a large pointing instability (50?mrad?rms). A high contrast ratio not only improves the beam pointing stability (6?mrad) but also stabilizes the electron beam energy reproducibility (5%).

The temporal contrast is classified into two main regimes, the nanosecond-scale and the picosecond-scale contrast prior to the main pulse. The Lund terawatt laser system is shown to be improved on the nano- and picosecond-scale by a factor of 10 and 50, respectively, when it was optimized for contrast but not for energy. Calculations are also presented to emphasize the role of angular dispersion on the picosecond contrast. Finally we show a compromise between the duration and contrast of femtosecond laser pulses amplified in an optical parametric (chirped pulse) amplifier.

Abstract:: Rotationally resolved spectral lines in the C - X (1, 0) band of carbon monoxide are investigated under high resolution using a coherent vacuum ultraviolet laser source, continuously tunable near 107 nm. Transition frequencies are determined by calibrating against a reference standard of iodine lines, recorded with saturation spectroscopy in the visible range, yielding an absolute accuracy of 0.003 cm-1 in the vacuum ultraviolet. Improved molecular constants for the excited state are derived and no effects of perturbation are found at the present level of accuracy. Line broadening measurements result in information on the excited state lifetime of the C 1 Σ + , v = 1 state for five natural isotopomers of carbon monoxide: τ( 12 C 17 O ) = 280 ps, τ( 12 C 18 O ) = 210 ps, τ( 13 C 16 O ) = 295 ps, τ( 13 C 17 O ) = 160 ps, and τ( 13 C 18 O ) = 150 ps. Within the accuracy of the present measurements no effects of J-dependent lifetimes were observed, for neither of the isotopomers. In addition direct time domain measurements of the lifetime of the C 1 Σ + , v = 0 and v = 1 states of the main isotopomer are performed in a pump-probe experiment using a picosecond VUV-laser, yielding τ( 12 C 16 O ) = 1780 ps for v = 0 and τ( 12 C 16 O ) = 625 ps for v = 1. For C 1 Σ + , v = 0 in 12C16O and 13C16O the same lifetime is found; this lifetime matches experimental values of the oscillator strength and hence supports previous results showing pure radiative decay in this state; the error margins however do not exclude some low level of predissociation. The measurements indicate that the C 1 Σ + , v = 0 state of the 13C18O isotopomer is predissociated with an estimated yield of 17% (i.e. above the level of predissociation for 12C16O.) From the combined data predissociation yields upon excitation of the C 1 Σ + , v = 1 state are derived, lying in the range 0.84-0.91 for the five less abundant isotopomers; for the main 12C16O isotopomer a strongly deviating predissociation yield of 0.65 is deduced.

In this paper, we present and compare the two different XUV interferometric techniques using high-order harmonics that have been developed so far. The first scheme is based on the interference between two spatially separated phase-locked harmonic sources while the second uses two temporally separated harmonic sources. These techniques have been applied to plasma diagnostics in feasibility experiments where electron densities up to a few 1020 e−/cm3 have been measured with a temporal resolution of 200 fs. We present the main characteristics of each technique and discuss their respective potentials and limitations.

We present two interferometry schemes in the extreme ultraviolet, based on either the wave-front division of a unique harmonic beam (1st scheme) or two spatially separated, phase-locked harmonic sources (2nd scheme). In the first scheme using a Fresnel bimirror interferometer, we measure the degree of spatial coherence of the 13th harmonic generated in xenon, as a function of different parameters. A high degree of coherence, larger than 0.5, is found for the best conditions in almost the full section of the beam. Then, we demonstrate that the second scheme can be used for interferometry measurements with an ultrahigh time resolution. The 11th harmonic is used to study the spatial variation of the electron density of a laser-produced plasma. Electronic densities higher than 2.1020 cm−3 are measured.

Intense continua of electromagnetic radiation of very brief duration are formed in the interaction of focused ultra-short terawatt laser pulses with matter. Two different kinds of experiments, which have been performed utilizing the Lund 10 Hz titanium-doped sapphire terawatt laser system are being described, where visible radiation and X-rays, respectively, have been generated. Focusing into water leads to the generation of a light continuum through self-phase modulation. The propagation of the light through tissue was studied addressing questions related to optical mammography and specific chromophore absorption. When terawatt laser pulses are focused onto a solid target with high nuclear charge Z, intense X-ray radiation of few ps duration and with energies exceeding hundreds of keV is emitted. Biomedical applications of this radiation are described, including differential absorption and gated-viewing imaging.

Imaging of technical and biological objects using hard X-rays from a laser-produced plasma source is demonstrated. Magnification radiography and single-shot imaging of biological samples are feasible with the source, which utilised focused radiation from a short-pulse terawatt laser. Differential imaging with element specificity and a new projection geometry for X-ray radiography are proposed.

We have investigated how high-order harmonics generated in rare gases depend on the atomic density. The peak and the profile of the atomic density in the interaction region were measured as a function of the backing pressure and the distance from the nozzle by a differential interferometry technique. The conversion efficiency for the harmonics in the plateau was found to increase approximately quadratically over the entire range of peak pressures investigated (3–80 mbar). The intensity of the harmonics in the cutoff region, in contrast, increased only until an optimum peak pressure was reached, beyond which it decreased. This optimum peak pressure was found to be dependent on both the laser intensity and the process order. To understand this effect, we have performed extensive propagation calculations of both the fundamental and the harmonic fields, using ionization rates and dipole moments from a tunnel ionization model. We obtained good agreement with the experimental results. The observed effect is attributed to ionization-induced defocusing of the fundamental laser beam, which reduces the peak intensity obtained in the medium and shortens the extent of the plateau.

A method for time-resolved recording of light scattering in thin, highly scattering media is described. Subpicosecond pulses from a high-power Ti:sapphire laser are used, and single-shot recordings of the scattered light are made with a fast streak camera. The method is applied to the study of light scattering in paper, and a 1-ps resolution is demonstrated. The dependence of the light scattering on the basis of weight and density of the paper has been studied. A white-light continuum generated from the high-power pulses by the use of self phase modulation in water is used to study the wavelength dependence of the scattering process. A model for the propagation of light in paper has been developed and used in Monte Carlo simulations. The experimental results are used for testing this model, and absorption and scattering parameters are determined from that comparison.

We have measured radiative lifetimes in the 5s n p1P1 (n=5–29) and 5s n f1F3 (n=4–11) Rydberg series in neutral strontium. The measurements were performed with single-step laser excitation starting from the ground state or from a metastable state populated by collisions. The decay photons were detected using delayed coincidence technique or a transient recorder. The presence of configuration interaction in the 5s n p1P1 series can be observed aroundn=8. The perturbation in the 5s n f1F3 series is not reflected in the behaviour of the lifetime values.

Using timeresolved laser spectroscopy lifetime values as well as hyperfine coupling constants have been determined for the 4s2np2P3/2 (n=7–11) states in69Ga and71Ga. Theoretical calculations of hyperfine structures based on manybody perturbation theory have been performed in the same sequence (n = 4–8) for both finestructure components. The theoretical values are compared with existing experimental data.

We report the near-field imaging characterization of a 10 Hz Ni-like 18.9 nm molybdenum soft-x-ray laser pumped in a grazing incidence pumping (GRIP) geometry with a table-top laser driver. We investigate the effect of varying the GRIP angle on the spatial behavior of the soft-x-ray laser source. After multiparameter optimization, we were able to find conditions to generate routinely a high-repetition-rate soft-x-ray laser with an energy level of up to 3 μJ/pulse and to 6×10^17 photons/s/mm^2/mrad^2/(0.1% bandwidth) average brightness and 1×10^28 photons/s/mm^2/mrad^2/(0.1% bandwidth) peak brightness.

We demonstrate that high-order harmonics generated by short, intense laser pulses in gases provide an interesting radiation source for extreme ultraviolet interferometry, since they are tunable, coherent, of short pulse duration, and simple to manipulate. Harmonics from the 9th to the 15th are used to measure the thickness of an aluminum layer. The 11th harmonic is used to determine the spatial distribution of the electron density of a plasma produced by a 300-ps laser. Electronic densities higher than 2–10^20 electrons/cm^3 are measured.

Next generation intense, short-pulse laser facilities require new high repetition rate diagnostics for the detection of ionizing radiation. We have designed a new scintillator-based ion beam profiler capable of measuring the ion beam transverse profile for a number of discrete energy ranges. The optical response and emission characteristics of four common plastic scintillators has been investigated for a range of proton energies and fluxes. The scintillator light output (for 1 MeV > Ep< 28 MeV) was found to have a non-linear scaling with proton energy but a linear response to incident flux. Initial measurements with a prototype diagnostic have been successful, although further calibration work is required to characterize the total system response and limitations under the high flux, short pulse duration conditions of a typical high intensity laser-plasma interaction.