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We report the successful demonstration of a hybrid system that combines pulsed laser deposition (PLD) and magnetron sputtering (MS) to deposit high quality thin films. The PLD and MS simultaneously use the same target, leading to an enhanced deposition rate. The performance of this technique is demonstrated through the deposition of titanium dioxide and bismuth-based perovskite oxide Bi2FeCrO6 (BFCO) thin films on Si(100) and LaAlO3 (LAO) (100). These specific oxides were chosen due to their functionalities, such as multiferroic and photovoltaic properties (BFCO) and photocatalysis (TiO2). We compare films deposited by conventional PLD, MS and PLD combined with MS, and show that under all conditions the latter technique offers an increased deposition rate (+50%) and produces films denser (+20%) than those produced by MS or PLD alone, and without the large clusters found in the PLD-deposited films. Under optimized conditions, the hybrid technique produces films that are two times smoother than either technique alone.

We report the imaging of tendon with Interferometric Second Harmonic Generation microscopy. We observe that the noncentrosymmetric structural organization can be maintained along the fibrillar axis over more than 150 μm, while in the transverse direction it is ∼1–15 μm. Those results are explained by modeling tendon as a heterogeneous distribution of noncentrosymmetric nano-cylinders (collagen fibrils) oriented along the fibrillar axis. The preservation of the noncentrosymmetric structural organization over multiple tens of microns reveals that tendon is made of domains in which the ratio between fibrils with positive and negative polarity is unbalanced. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on shadowgraphic measurements showing the first space- and time-resolved snapshots of ultraintense laser pulse-generated fast electrons propagating through a solid target. A remarkable result is the formation of highly collimated jets ( $l20\ensuremath{-}\ensuremath{\mu}\mathrm{m}$) traveling at the velocity of light and extending up to 1 mm. This feature clearly indicates a magnetically assisted regime of electron transport, of critical interest for the fast ignitor scheme. Along with these jets, we detect a slower ( $\ensuremath{\approx}c/2$) and broader (up to 1 mm) ionization front consistent with collisional hot electron energy transport.

We have analyzed the acceleration of laser-generated protons, produced at the rear surface of flat-cone targets irradiated by an ultra-intense (I∼5×1019 W/cm2) short (400 fs) laser pulse. We used different target sizes and shapes in order to find the optimum target layout. We find that for targets with a too narrow cone structure, the production of the hot electrons, driving the proton acceleration, is located prior to the accelerating rear surface of the target, resulting in a reduced maximum proton energy.

Recent theoretical and experimental studies suggest the possibility of enhancing the efficiency and ease of laser acceleration of protons and ions using underdense or near critical plasmas through electrostatic shocks. Very promising results were recently obtained in this regime. In these experiments, a first ns pulse was focused on a thin target to explode it and a second laser with a high intensity was focused on the exploded foil. The delay between two lasers allowed to control the density gradient seen by the second laser pulse. The transition between various laser ion acceleration regimes depending on the density gradient length was studied. With a laser energy of a few Joules, protons with energies close to the energies of TNSA accelerated protons were obtained for various exploded foils configurations. In the high energy regime (~180 J), protons with energies significantly higher than the ones of TNSA accelerated protons were obtained when exploding the foil while keeping a good beam quality. These results demonstrate that low-density targets are promising candidates for an efficient proton source that can be optimized by choosing appropriate plasma conditions. New experiments were also performed in this regime with gas jets. Scaling shock acceleration in the low density regime to ultra high intensities is a challenge as radiation losses and electron positron pair production change the optimization of the shock process. Using large-scale Particle-In-Cell simulations, the transition to this regime in which intense beams of relativistic ions can be produced is investigated.

Les processus dynamiques se produisant lors de transitions de phase ultra-rapide peuvent etre deduits a partir de mesures de diffraction ou d'absorption de rayonnement X. Les lasers femtosecondes ont recemment ete utilises pour etudier la dynamiques de la matiere au moyen d'une pompe optique et d'une sonde X: du rayonnement X K alpha produit par interaction laser plasma. Nous presentons nos plus recents resultats concernant le developpement d'un syteme de spectroscopie d'absorption du rayonnement X (XAS) basee sur une source laser-plasma large bande dans la gamme 1-5 nm permettant d'atteindre une resolution temporelle femtoseconde. Le systeme est concu pour sonder les dynamiques electroniques ayant lieu durant la transion de phase semiconducteur-metal du dyoxide de vanadium (V02) lorsque celle-ci est initiee par une impulsion laser femtoseconde. Dans la presente experience, un spectre large bande proche du seuil L du vanadium (511 eV) et du seuil K de l'oxygene (525 eV) du V02 a ete genere et mesure avec un haut rapport signal sur bruit (100), une grande resolution spectrale (AE/E = 4.2 x 10 -3 ), et une resolution temporelle de 1,2 ps.

High harmonics produced in aligned molecules contain structural information of bound-state electronic states. Our recent work has shown that high harmonic generation in aligned molecules is dependent on the symmetry and structure of the highest occupied molecular orbital (HOMO) [ J. Itatani, D. Zeidler, J. Levesque, et al., Phys. Rev. Lett. 94 123902 (2005)]. We show that it is possible to reconstruct the shape of the HOMO by applying a computerized tomography algorithm to the harmonic spectra obtained at different molecular alignments [ J. Itatani, J. Levesque, D. Zeidler, et al., Nature 432 867 (2004)]. In the present paper we review these findings and explain them using a simple but powerful analytical model.

We ionized He, Ne and Ar in the tunnelling regime with a linearly polarized laser pulse and measured the ion-recoil momentum distribution on an axis perpendicular to laser polarization for two different wavelengths (l = 800 nm and l = 1800 nm). We observed a significantly narrower distribution for 800 nm than for 1800 nm. Classical simulations of the electron wave packet's evolution under the combined influence of the ion's Coulomb potential and the laser field show that the narrowing is caused by Coulomb focusing of the electron wave packet during recollisions. The narrowing is sensitive on the longitudinal momentum of the wave packet after tunnelling, which suggests a way of measuring it. Measurements at 800 nm in circular polarization-for which no recollision occurs-do not exhibit this narrowing of the perpendicular momentum distribution. Simulations for circular polarization show another aspect of Coulomb focusing: the Coulomb potential also affects the wave packet's transverse momentum as it leaves the saddle point immediately after tunnelling.

The laminarity of high current multi-MeV proton beams produced by irradiating thin metallic foils with ultra-intense lasers has been measured. For proton energies >10 MeV, the transverse and longitudinal emittance are, respectively, −4 eV s, i.e. at least 100-fold and may be as much as 10 4 -fold better than conventional accelerators beams. The ion beam source size is measured to be 10 MeV. Magnetic stripping of the co-moving electrons out of the beam after a few cm of debunching is not observed to induce emittance growth.

Single-electron wavefunctions, or orbitals, are the mathematical constructs used to describe the multi-electron wavefunction of molecules. Because the highest-lying orbitals are responsible for chemical properties, they are of particular interest. To observe these orbitals change as bonds are formed and broken is to observe the essence of chemistry. Yet single orbitals are difficult to observe experimentally, and until now, this has been impossible on the timescale of chemical reactions. Here we demonstrate that the full three-dimensional structure of a single orbital can be imaged by a seemingly unlikely technique, using high harmonics generated from intense femtosecond laser pulses focused on aligned molecules. Applying this approach to a series of molecular alignments, we accomplish a tomographic reconstruction of the highest occupied molecular orbital of N2. The method also allows us to follow the attosecond dynamics of an electron wave packet.

This paper summarizes the research this team has performed over the past few years investigating laboratory electrical breakdown discharges in the presence of a plasma cylinder created by a single ultrashort laser pulse. This work is part of a feasibility study about the control of lightning using laser systems. Our experimental investigations have included discharges (i) in modest (30 cm) air gaps mediated by streamers, and (ii) in large (several meters) ambient air gaps for which the discharge took place through the formation of a leader, the mechanism relevant to large scale natural discharges such as lightning. In order to understand the observations, various physical models have been used, the main results of which are discussed in this paper. To cite this article: F. Vidal et al., C. R. Physique 3 (2002) 1361–1374.

Time-resolved visible spectroscopy of plasmas produced by laser breakdown of air using femtosecond Nd laser pulses (50–300 mJ, 500 fs) reveals features not observed with nanosecond laser pulses. Emission is initially dominated by molecular lines, specifically the second positive system of N2 and the first negative system of N2+. This is followed by continuum emission with a growth time of ∼3 ns and a decay time of ∼30 ns. Atomic lines of N and O emerge from the decay of the continuum and last up to 1 μs; only faint ionization lines are observed. Several of the atomic lines are initially strongly broadened, narrowing over a period of 100 ns.

With the aim of elucidating the role of thermal effects in air in the leader propagation, the properties of the ionized air (charged species densities, electric field, and air temperature) in a limited region near the tip of the positive leader were calculated using a two-dimensional axisymmetric numerical model. Charged-particle densities and the electric field were treated consistently. Compared with the geometrical field, the actual electric field appears to be much more uniform and is strongly reduced at the tip due to the plasma shielding. As a consequence of the field damping, thermal effects are determined to play a negligible role outside the leader boundary.

International audience; Although bipolar jets are seen emerging from a wide variety of astrophysical systems, the issue of their formation and morphology beyond their launching is still under study. Our scaled laboratory experiments, representative of young stellar object outflows, reveal that stable and narrow collimation of the entire flow can result from the presence of a poloidal magnetic field whose strength is consistent with observations. The laboratory plasma becomes focused with an interior cavity. This gives rise to a standing conical shock from which the jet emerges. Following simulations of the process at the full astrophysical scale, we conclude that it can also explain recently discovered x-ray emission features observed in low-density regions at the base of protostellar jets, such as the well-studied jet HH 154.

A deposition system combining pulsed laser deposition (PLD, cooled Ga target) and a source of atomic nitrogen was developed to grow epitaxial gallium nitride on sapphire. The layers obtained with this system were characterized using high-resolution X-ray diffraction, atomic force microscopy (AFM), Hall effect, and secondary ion mass spectroscopy (SIMS). It is found that the crystal quality greatly depends on the atomic nitrogen flux incident on the substrate during growth. After optimization of the atomic nitrogen-to-gallium flux ratio, samples showing very narrow GaN(0 0 0 2) rocking curves (full width at half maximum, FWHM=80 arcsec) have been synthesized at a low substrate temperature (Ts=750°C). Surface analysis of these thin films, using AFM, also show a very low roughness (Rrms=14 A).

The triggering and guiding of leader discharges using a plasma channel created by a sub-joule ultrashort laser pulse have been studied in a megavolt large-scale electrode configuration (3–7 m rod-plane air gap). By focusing the laser close to the positive rod electrode it has been possible, with a 400 mJ pulse, to trigger and guide leaders over distances of 3 m, to lower the leader inception voltage by 50%, and to increase the leader velocity by a factor of 10. The dynamics of the breakdown discharges with and without the laser pulse have been analyzed by means of a streak camera and of electric field and current probes. Numerical simulations have successfully reproduced many of the experimental results obtained with and without the presence of the laser plasma channel.

Results on the deposition of aluminum nitride thin films on silicon (111) substrates by pulsed laser deposition are reported. A KrF excimer laser was used to ablate in vacuum an AlN target. The intensity and the repetition rate of the laser as well as the substrate temperature were varied and their effects on film properties were determined. The AlN films were characterized by several techniques such as in situ laser interferometry, X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy and transmission electron microscopy. At a deposition temperature of 500 °C and a laser intensity of 1 × 10 8 W/cm 2 , highly oriented AlN layers are deposited. Increasing the laser intensity leads to the presence of metallic Al in the films. As temperature is increased from 500 to 920 °C, the growth rate is found to decrease but the crystalline structure as well as the smoothness of the films are both significantly improved. It is also found that a reduction of the growth rate by varying the repetition rate from 30 to 10 Hz leads to a further improvement of the crystalline quality of the AlN layers.

Using the nonlinear propagation properties of ultrashort laser pulses in air, we were able to produce long ionized filaments that served to guide spark discharges. With a laser pulse energy of 20 mJ, one or two ionized filaments were created and could guide streamer discharges over 2 m air gaps, where the electric field was fairly uniform and had an average value of 0.6 MV/m. Such a guiding effect was observed for times of 1–3 μs after the laser pulse created the ionized filaments. Longer delays (10–15 μs) were recorded at higher laser pulse energy, with a larger number of filaments. Images of the early stages of the discharge of a uniform air gap show that the laser-produced ionized filaments do not initiate the discharge process but act rather as preferred channels where the leader growth is accelerated. In the end, these straight conductive channels carry the arc current as the voltage in the gap breaks down.

The propagation of high-power short-pulse laser beams over considerable distances in air is studied both experimentally and via numerical simulations. Filaments are formed after 5–10 m and their propagation over distances in excess of 200 m is reported for the first time. The lateral dimensions of the filaments are found to range from about 100 μm to a few millimeters in diameter. The early values of plasma electron density have been inferred to be a few times 1016 cm−3 using longitudinal spectral interferometry. For 500 fs pulses and a wavelength of 1053 nm, the energy in the filament can be quite high initially (∼8 mJ) and is found to stabilize at about 1.5–2 mJ, after about 35 m. A simple model based on the nonlinear Schrodinger equation coupled to a multiphoton ionization law appears to describe several experimental results quite well.

The efficiency of the conversion from laser energy into aluminium K-shell line energy has been measured for different interaction regimes of a sub-picosecond laser pulse with controlled preformed plasmas. We compare the cases of thermal plasmas for which transient ionization effects are dominant to non-thermal plasmas in which both transient and non-Maxwellian effects are present. In a first experiment, a low-intensity green laser pulse (400 fs, ) is normally incident on a preformed plasma previously generated by a similar laser pulse. In a second experiment a high-intensity p-polarized green pulse (400 fs, ) is sent with a large incidence angle on the preformed plasma. We have observed in these experiments that the conversion efficiency (He- or Li-like lines from aluminium) scales as , where is the duration of the x-ray emission measured at the full width half maximum, and that this scaling was not affected by non-Maxwellian effects.

The formation and evolution of density channels created by the interaction of a short bright (600 fs, $5\ifmmode\times\else\texttimes\fi{}{10}^{18}\mathrm{W}{\mathrm{cm}}^{\ensuremath{-}2}\ensuremath{\mu}{\mathrm{m}}^{2}$) laser pulse with a preformed plasma are studied by means of experiments and 2D particle-in-cell (PIC) simulations. Hollow density channels are observed by interferometry, and a fast radial expansion $(5\ifmmode\times\else\texttimes\fi{}{10}^{8}\mathrm{cm}/\mathrm{s})$ is measured. Magnetic fields around 50 MG inferred from Faraday rotation measurements suggest the occurrence of self-focusing. The PIC simulations support this hypothesis and show ion depletion during the laser pulse as well as radial expansion in agreement with experiments.

We have compared the acceleration of high-energy ions from the rear-surface of thin foils for various contrast conditions of the ultra-intense laser pulse irradiating the targets. The experiments were performed using the LULI 100 TW facility. We used Al targets of variable thicknesses and the laser pulse contrast ratio ahead of the main pulse was varied using either a fast Pockels cell or a single or double plasma mirror. The latter was installed at an intermediate field position, in between the focusing optics and the target, so that its effect was optimized. By improving with these two methods the laser pulse contrast, we have observed that we could significantly reduce the thickness of the target used for proton acceleration and at the same time increase both the cut-off energy of the accelerated protons and the energy conversion efficiency of the process.

An ultrahigh vacuum deposition–reflectometer system was constructed to allow the deposition and subsequent in situ reflectance measurement of extreme-ultraviolet multilayer mirrors. In this article various aspects of the deposition chamber and the reflectance measurement setup are discussed. To demonstrate the capabilities of this system, Y/Mo multilayer mirrors were studied in detail by varying deposition conditions and measuring their subsequent in situ reflectance. By optimizing the deposition conditions, reflectances of 21.3%, 34.7%, and 46.1% were obtained for the Y/Mo mirrors at wavelengths of 7.9, 9.7, and 11.4 nm, respectively.

Relativistic electrons are produced, with energies up to 20MeV, by the interaction of a high-intensity subpicosecond laser pulse (1 {mu}m , 300 fs , 10{sup 19} W/cm{sup 2} ) with an underdense plasma. Two suprathermal electron populations appear with temperatures of 1 and 3MeV. In the same conditions, the laser beam transmission is increased up to 20{percent}{endash}30{percent}. We observe both features along with the evidence of laser pulse channeling. A fluid model predicts a strong self-focusing of the pulse. Acceleration in the enhanced laser field seems the most likely mechanism leading to the second electron population. {copyright} {ital 1997} {ital The American Physical Society}

In a velocity map imaging spectrometer, we measured the electron momentum distributions from the ionization of O molecules with 800 nm wavelength, 40 fs laser pulses at a peak intensity of W cm. The molecules were aligned at 0, 45 and 90 relative to the laser polarization prior to ionization. We show that for all alignments the low momentum region – populated by direct electrons which do not recollide with the parent ion – is consistent with the ionized orbital being filtered and projected onto the continuum electron wave packet. In the high momentum region – populated by rescattered electrons – we observe that the pattern created by diffraction of the recolliding wave packet by the ion core disappears as the alignment gets closer to the laser field axis. We find that a two-slit diffraction model agrees well with the results for molecules aligned at 90, but only partially predicts the decrease in the diffraction signature for smaller alignment angles.

We report the imaging of tendon with Interferometric Second Harmonic Generation microscopy. We observe that the noncentrosymmetric structural organization can be maintained along the fibrillar axis over more than 150 μm, while in the transverse direction it is ∼1-15 μm. Those results are explained by modeling tendon as a heterogeneous distribution of noncentrosymmetric nano-cylinders (collagen fibrils) oriented along the fibrillar axis. The preservation of the noncentrosymmetric structural organization over multiple tens of microns reveals that tendon is made of domains in which the ratio between fibrils with positive and negative polarity is unbalanced.

SHG microscopy, used to image noncentrosymmetric structural organization, cannot measure the relative SHG phase between the structures. Using I-SHG microscopy, we show that the relative phase between structures in tendon and muscle can be imaged.

This paper is about the dc and microwave characterization of the first high electron mobility transistors (HEMT's) realized using X-ray lithography processing. This full field patterning technology is proposed as a way to produce transistors with sub-200 nm gate lengths and high operating frequency. Fully operational devices are demonstrated and characterized. The dc parameters are determined to be comparable to those obtained on similar devices processed with electron beam lithography. A maximum intrinsic transconductance of 575 mS/mm is obtained. This shows that the X-ray radiation does not cause any significant damage to the substrates and that the process developed is viable. Devices with different geometries were tested at microwave frequencies. A current gain cutoff frequency of 70 GHz is measured for the device with the shortest gate-length whereas a maximum oscillation frequency as high as 105 GHz is measured for devices having the lowest gate resistance.

Retrieving the characteristics of hot electrons produced in the interaction between solid targets and ultra-intense (I?>?1018?W?cm?2) laser pulses is essential for achieving?progress in our understanding of the interaction physics, which is?key for optimizing numerous downstream applications. Until now, various methods have been used, direct or indirect, but no correlation and no assessment of their respective merits were performed. Here we compare results retrieved from four different diagnostics, direct or indirect, as well as local or non-local, i.e. spectrometry of electrons, spectrometry of the protons accelerated by the electrons and optical probing of these beams expanding into vacuum from the targets. We show that measurements obtained locally at the target rear surface are consistent with those far away from the target and that one can use the diagnostics of the co-moving proton beams to retrieve information about electrons.

The pulsed laser deposition (PLD) technique has been used to produce giant magnetoresistance (GMR) Co x Cu 1− x granular alloy films. An MR as large as 8% has been found for the annealed film with x Co = 0.18 at 4.2 K in a magnetic field of up to 1 T. GMR and its thermal evaluation are shown to depend on the density and size distribution of the Co precipitates. The influence of the characteristics of the PLD technique on the GMR effect is also discussed.

Space–time‐resolved Thomson scattering was used to characterize the electron and ion temperatures in laser produced plasmas of various atomic numbers (Z=6, 13, 32, 73, 92). A Nd:glass laser with intensity between 1013 and 5×1014 W/cm2 was used to produce plasmas of relevance to laser fusion and x‐ray laser studies. A comparison of the electron temperature and density deduced from Thomson scattering, x‐ray spectroscopy, and lasnex hydrocode simulations is presented.

This paper reports on the first achievement of sub-200nm gate length GaAlAs/GaAs HEMTs produced by x-ray lithography. The chemical amplified SAL601 resist process is optimized with a post-exposure bake (PEB) at 95°C for the replication of very narrow and isolated trenches (gates). The fabricated transistors have gate lengths of 180nm.

The stress evolution of plasma enhanced chemical vapor deposition a‐SiC:H films was studied by increasing the annealing temperature from 300 to 850 °C. A large stress range from −1 GPa compressive to 1 GPa tensile was investigated. Infrared absorption, x‐ray photoelectron spectroscopy, and elastic recoil detection analysis techniques were used to follow the Si‐C, Si‐H, and C‐H absorption band evolutions, the Si2p and C1s chemical bondings, and the a‐SiC:H film hydrogen content variations with the annealing temperatures, respectively. It is pointed out that the compressive stress relaxation is due to the hydrogenated bond (Si—H and C—H) dissociation, whereas the tensile stress is caused by additional Si—C bond formation. At high annealing temperatures, a total hydrogen content decrease is clearly observed. This total hydrogen loss is interpreted in terms of hydrogen molecule formation and outerdiffusion. The results are discussed and a quantitative model correlating the intrinsic stress variation to the Si...

Time‐resolved spectroscopy (with a 2 psec temporal resolution) of plasmas produced by the interaction between solid targets and a high contrast subpicosecond table top terawatt (T3) laser at 1016 W/cm2, is used to study the basic processes which control the x‐ray pulse duration. Short x‐ray pulses have been obtained by spectral selection or by plasma gradient scalelength control. Time‐dependent calculations of the atomic physics [Phys. Fluids B 4, 2007, 1992] coupled to a Fokker–Planck code [Phys. Rev. Lett. 53, 1461, 1984] indicate that it is essential to take into account the non‐Maxwellian character of the electron distribution for a quantitative analysis of the experimental results.

We have analyzed the coupling of ultraintense lasers (at {approx}2x10{sup 19} W/cm{sup 2}) with solid foils of limited transverse extent ({approx}10 s of {mu}m) by monitoring the electrons and ions emitted from the target. We observe that reducing the target surface area allows electrons at the target surface to be reflected from the target edges during or shortly after the laser pulse. This transverse refluxing can maintain a hotter, denser and more homogeneous electron sheath around the target for a longer time. Consequently, when transverse refluxing takes places within the acceleration time of associated ions, we observe increased maximum proton energies (up to threefold), increased laser-to-ion conversion efficiency (up to a factor 30), and reduced divergence which bodes well for a number of applications.

The process of high-order harmonic generation leads to the production of a train of attosecond-duration extreme ultraviolet (XUV) pulses, with one pulse emitted per optical half-cycle. For attosecond pump-probe experiments, a single, isolated attosecond pulse is preferable, requiring an almost continuous spectrum. We show experimentally and numerically that the addition of a second laser field, and later a third, at a noncommensurate frequency relative to the driving field can modify the subcycle shape of the electric field, leading to the appearance of additional spectral components between the usual odd harmonics and in some cases a quasicontinuum. We perform a parametric study of the frequency ratio between the two first laser fields, the result of which is in good agreement with theoretical selection rules. We also show numerically that using three laser frequencies from an optical parametric amplifier can achieve a single attosecond pulse from a 24-fs laser pulse.

Amorphous silicon carbide films (a–SixC1−x :H) deposited by the argon- or helium-diluted PECVD technique were studied as a function of their composition. Microstructural investigations were mainly achieved by means of FTIR and XPS techniques. Nuclear techniques were used to obtain precise information on the film hydrogen content. The Si–H IR-absorption band was deconvoluted in different monohydride and dihydride silicon environments. The existence of SiH2 bonds in the Si-rich composition was evidenced. From the analysis of the C–H and Si–H absorption bands it is shown that hydrogen atoms are preferentially bonded to carbon atoms. The deconvolution of the Si2p core level peak suggests that above a composition of x ∊ 0.5, the noncarburized (Si, Si, H) local environment contribution increases to the detriment of the hydrocarburized (Si, C, H) environments. From the evolution of the C1s peak, it can be deduced that there is a change in the carbon atom bonding states when the film composition is varied. These results are correlated and discussed in terms of the local bonding environments and their evolution with film composition.

Electrons of 2.3 MeV energy and light in the wavelength range from 2λ0 to 1.28λ0 scattered in the direction of incident laser have been observed from the plasmas of exploding thin foils created by a carbon dioxide laser. The correlation measurements of electrons and light, performed independently, pointed to a common physical origin for these near the quarter‐critical density region of the plasma where low phase velocity plasma waves are driven. This also indicated that low‐density forward Raman scattering with high phase velocity plasma waves may not be principally responsible for the energetic electrons. In order to interpret the experimental results a simple computer calculation has been performed obtaining the final energies of electrons trapped in freely propagating electron plasma waves with increasing phase velocity.

With a new ultrahigh-vacuum deposition/ref lectometer system, Mo/Y multilayer mirrors were deposited by dc-magnetron sputtering and characterized in situ with synchrotron radiation. The Mo/Y multilayer mirrors, measured before exposure to air, had near-normal-incidence reflectances as high as 46% at wavelengths near 11.4 nm. After several days of exposure these samples typically had a relative reflectance loss of ~10% as a result of oxidation of the top Mo layer. The best Mo/Y multilayers were fabricated with base pressures below the low 10(-9) Torr range and after a bakeout to reduce water vapor in the chamber.

The acceleration of protons, induced by electrons generated by a short-pulse laser, is experimentally investigated when varying the density of the plasma target the laser is interacting with. The experimental results are compared with particle-in-cell (PIC) simulations for which the target conditions are inferred from hydrodynamic simulations. High-energy protons are observed only for the two extreme configurations, namely solid-density foils and near- critical-density plasmas having large gradients. Cold solid foils, however, yield the highest energy protons and best proton beam profiles. As suggested by simulations, near-critical-density plasmas could be optimized to further increase the proton energy.

Time resolved monochromatic imaging diagnostic is used to study in lasing plasma the effects of laser irradiation non-uniformity on line focused plasma conditions. Significant large scale non-uniformities are observed with a conventional focusing system. A new focusing arrangement (SWA) is used to produce a more uniform linear plasma. Quantitative measurements of the influence of the local laser intensity variations on the ionization balance and on the recombination time history are presented and discussed.

Molecular structure is usually determined by measuring the diffraction pattern the molecule impresses on x-rays or electrons. We used a laser field to extract electrons from the molecule itself, accelerate them, and in some cases force them to recollide with and diffract from the parent ion, all within a fraction of a laser period. Here, we show that the momentum distribution of the extracted electron carries the fingerprint of the highest occupied molecular orbital, whereas the elastically scattered electrons reveal the position of the nuclear components of the molecule. Thus, in one comprehensive technology, the photoelectrons give detailed information about the electronic orbital and the position of the nuclei.

We have compared the acceleration of high-energy ions from the rear surface of thin foils for various prepulse contrast conditions of the ultraintense laser pulse irradiating the targets. The experiments were performed using the Laboratoire pour I'Utilisation des Lasers Intenses 100-TW laser facility with 15-20 J energy pulses of >1018 W/cm2 intensity irradiating aluminum targets of variable thicknesses. The laser pulse contrast ratio ahead of the main pulse (prepulse) was varied using either a fast Pockels cell or a single or double plasma mirror (PM). The latter was installed at an intermediate field position, in between the focusing optics and the target. This way, the effect of reducing the laser prepulse by the PM was optimized. With both methods improving the laser pulse contrast, we have observed that one can significantly reduce the thickness of the target used for proton acceleration and, at the same time, increase 1) the maximum energy of the accelerated protons and 2) the energy conversion efficiency of the process. This offers interesting perspectives from high-energy (5-10 MeV) ion acceleration using sub-petawatt laser facilities. A full characterization of contrast enhancement due to the PM, as well as a detailed analysis of the accelerated proton beams, is presented.

This paper deals with the characterization of novolak positive resists for X-ray lithography. Exposure curves and resist pattern profiles of the Olin Hunt HEBR214 and WX242 resists are obtained experimentally using a laser plasma X-ray source and are also calculated with a simulation program called XLIMLAS. Finally, comparisons of tungsten and WX242 resist etch rates using RIE with CF 4 are also reported.

The emissivity of the rear of a shocked foil is spectrally and temporally resolved by coupling a transmission grating (1000 I/mm) and an X-UV streak camera (with a low density Csl photocathode), providing a high temporal resolution over a large spectral range. The shock is generated with two techniques: direct illumination of a single Al foil with a 0.26 μm wavelength laser (ablation pressure ≈ 50 Mbars) or by colliding an Al foil with a laser accelerated CH foil (generated pressures greater than 100–200 Mbars). Different thicknesses of Al are used in single foil experiments, and different initial spacing and impacted foil thicknesses are used in double foil experiments. Double foil experiments indicate that targets can be optimized for high pressure generation, and single foil experiments show that there is a radiative heat wave, around 200 eV, due to the heating of the foil by X rays emitted in the ablated region during the laser pulse. Double foil experiments have been compared with 2-D hydrodynamic Lagrangian simulations and single foil experiments have been compared with 1-D hydrodynamic Lagrangian simulations taking into account radiative heat transfer.

A 12 μm polyester foil is accelerated by a 0.26 μm wavelength laser and collides with a 15 μm thick molybdenum foil. The accelerating pressure is 45 Mbar (laser intensity≊3– 4×1014 W/cm2) and gives to the polyester foil a velocity of about 160 km/sec. The measurement of the shock pressure induced in the impacted foil is made with an improved step technique. When the initial spacing between the two foils is too large compared to the focal spot radius, i.e., larger than 20–30 μm, the different experimental results cannot be reproduced with one‐dimensional simulations; this is only possible by using a two‐dimensional Lagrangian code that has been developed and that takes into account the strong deformation of the accelerated foil. Finally, even with the low level of x‐ray heating due to the ablation plasma, multihundred megabar pressures can be obtained within a very short time.

We report on the first experiments of high-order harmonic generation done with the 100 Hz high-energy optical parametric amplifier (OPA) of the Advanced Laser Light Source. Using krypton and argon as targets, we show that the OPA's signal beam - with a wavelength range from 1200 nm to 1600 nm, 1.3 mJ to 0.8 mJ of pulse energy and 100 fs pulse duration - can generate fully tunable XUV radiation down to a wavelength of 15 nm. We have also started to investigate the use of the OPA pulses for molecular imaging. Inducing molecular alignment with 800 nm, 70 fs pulses, we have measured the high harmonics spectra generated with 1300 nm pulses from nitrogen molecules oriented at various angles with respect to the ionizing field, in order to study for the first time the technique of molecular orbital tomography with a laser wavelength different than 800 nm., Ultrafast X-Ray Sources and Detectors, August 26-27, 2007, San Diego, CA, USA, Series: Proceedings of SPIE

In the last few years, intense research has been conducted on laser‐accelerated ion sources and their applications. These sources have exceptional properties, i.e. high brightness and high spectral cut‐oft high directionality and laminarity, short burst duration. These proton sources open new opportunities for ion beam generation and control, and could stimulate development of compact ion accelerators for many applications. We have studied the variations of the proton acceleration characteristic time with target and laser parameters. We used these variations to correct one of the model recently developed to predict maximum energies of laser‐accelerated protons for low energy, short duration laser pulses. We have also developed an ultra‐fast laser‐triggered micro‐lens that allows tunable control of the beam divergence as well as energy selection, therefore solving two of the major problems that these proton beams were facing. We used PIC simulations to explain the focusing and energy selection mechanisms, ...

The comparative efficiency and beam characteristics of high-energy ions generated by high-intensity short-pulse lasers (approximately 1-6 x 10(19) W/cm2) from both the front and rear surfaces of thin metal foils have been measured under identical conditions. Using direct beam measurements and nuclear activation techniques, we find that rear-surface acceleration produces higher energy particles with smaller divergence and a higher efficiency than front-surface acceleration. Our observations are well reproduced by realistic particle-in-cell simulations, and we predict optimal criteria for future applications.

International audience; The past few years have seen remarkable progress in the development of laser-based particle accelerators. The ability to produce ultrabright beams of multi-megaelectronvolt protons routinely has many potential uses from engineering to medicine, but for this potential to be realized substantial improvements in the performances of these devices must be made. Here we show that in the laser-driven accelerator that has been demonstrated experimentally to produce the highest energy protons, scaling laws derived from fluid models and supported by numerical simulations can be used to accurately describe the acceleration of proton beams for a large range of laser and target parameters. This enables us to evaluate the laser parameters needed to produce high-energy and high-quality proton beams of interest for radiography of dense objects or proton therapy of deep-seated tumours.