Yves Beaudoin

Ultrafast x‐ray sources*

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.

Production of a high-density and high-temperature plasma with an intense high-contrast subpicosecond laser

We report on the observation of high-electron-density (ne ≈ 15nc where nc is the critical density, 4 × 1021 cm−3 for λ = 0.53 μm) and high-temperature (Te ≈ 400 eV) plasma obtained when an intense high-contrast 400-fs laser pulse interacts with a solid Al target. After the laser intensity is increased from 1016 to 5 × 1017 W/cm2, the kiloelectron-volt x-ray emission is brighter by approximately an order of magnitude and is still produced from very dense plasma (6 × 1022 cm−3).

Time-resolved kiloelectron-volt spectroscopy of ultrashort plasmas.

We describe a technique for spectrally and temporally resolving the kilo-electron-volt emission fromultrashort plasmas produced from solid targets with a tabletop terawatt 400-fs laser. The firsttime-resolved Al spectra (near 8 Å) obtained with a 2-ps time resolution are presented. The results clearly demonstrate that the resonance emission width decreases as the plasma density increases. The ultrafast K(α) emission component is also measured in our experimental conditions.

Ultrafast x-ray emission from ultrashort plasmas

We present recent results of our effort to develop an efficient, user-friendly, table-top ultrafast X-ray source. The factors affecting the duration and the intensity of the X-ray emission in the keV range are studied. Time-dependent calculation of the atomic physics coupled to a Fokker- Planck code is used for a quantitative analysis of the experimental results.

Laser reflectometry in situ measurement of lead zirconate titanate film growth.

A convenient method is described for optical characterization of thin films during growth. The method has been demonstrated on lead zirconate titanate (PZT) films deposited by pulsed laser ablation for various temperatures. The optical constants of the PZT films as well as the film growth rate were determined in situ by fitting (with three free parameters) the calculated reflectance as a function of film thickness to the experimental reflectance curve as a function of deposition time, as obtained by unpolarized laser reflectometry. The fitted parameters are the uniform complex PZT refractive index and the layer thickness (assumed proportional to time), with the complex refractive index of the platinum substrate being measured previously. These results compare well with the subsequent ellipsometric measurements made to assess the precision of the reflectometry technique.

Deposition of silicon carbide thin films by pulsed excimer laser ablation technique in the 25-700°C deposition temperature range

Laser ablation deposition technique was used to deposit silicon carbide thin films on both Si(100) and quartz substrates. The deposition was accomplished by ablating SiC sintered ceramic targets, using a KrF (248 nm) excimer laser. At a laser intensity of about 1 X 109 W/cm2, substrate temperatures in the (25-700) degree(s)C range were investigated. When the deposition temperature is varied from 27 to 650 degree(s)C, (i) the density of a-SiC films increases from 2.6 to 3.0 g cm-3, while their mean roughness value (for a film thickness of about 1 micrometers ) slightly changes from 0.44 to 0.5 nm; (ii) the optical transmission of a-SiC films is significantly improved (the absorption coefficient at 632.8 nm wavelength was reduced by a factor of about 5); and (iii) their Si-C bond density, as determined by FTIR spectroscopy, increases from (13.1 +/- 1.3) to (23.4 +/- 2.4) 1022 bond cm-3. The increased number of Si-C bonds is correlated to the increase of the optical transmission. Over all the investigated deposition temperature range, the a-SiC films were found to be under high compressive stress around a mean value of about 1.26 GPa. The control of the stress of a-SiC films was achieved by means of post- thermal annealings and the annealed a-SiC films were successfully used to fabricate x-ray membranes.

Laser-induced inner shell vacancies in doubly ionized argon

The creation of laser-induced inner shell vacancies in doubly ionized Ar atoms is reported. First, theoretical calculations are performed for different laser systems and as much as 30% of doubly ionized Ar atoms in the [Ne]3s3p5 configuration with a 3s shell vacancy at optical and near-infrared frequencies are predicted. Analysis of the results shows that the mechanism for creation of the inner shell vacancy is a non-sequential double ionization process. Finally, the radiation spectrum of Ar in the field of an intense Ti:sapphire/Nd:glass laser system at 1053 nm has been measured, and the line corresponding to the fluorescence emission due to radiative filling of the created inner vacancy in the 3s shell is observed.

X-ray conversion efficiency in short-pulse laser plasmas

We present a comparative study of the x-ray emission produced by laser plasma sources in short ((tau) L equals 0.6 ps) and long ((tau) L equals 0.6 ns) pulse regimes for copper and tantalum targets. The experiments at (tau) L equals 0.6 ps show that the x-ray conversion efficiency (eta) is still increasing with laser intensity between 2 X 1015 W/cm2 and 5 X 1016 W/cm2 for both sub-keV (0.1 - 0.75 keV) and keV (0.75 - 2 keV) ranges. In addition, we found that the optimum values (eta) at (tau) L equals 0.6 ps are at least 2 - 4 times lower than those at (tau) L equals 0.6 ns.

Spectroscopic analysis of short-pulse laser-produced plasmas

Experimental spectra of hot dense plasmas of aluminium produced by the interaction of a subpicosecond laser with solid targets at 10 16 and 5 × 10 17 W/cm 2 are analyzed and discussed. A detailed analysis of the K -shell spectra is given through time-dependent calculations of atomic physics postprocessed to Fokker-Planck calculations of the laser-matter interaction. The non-Maxwellian character of the electron distribution function is shown. An evaluation of the electronic density and of the ion temperature 7 i will be presented through Stark line broadening calculations. An X-ray spectrum from a Tantalum target also will be presented along with a preliminary interpretation.

Laser plasma interactions at electron-relativistic laser intensities

A review is given (intended for the non-expert) of the field of ultrashort laser pulses at ultra high intensities and their interactions with plasmas. The review covers progress and basic concepts for theory, modelling and experiments, with emphasis on the background aspects, the basic experimental considerations and some possible applications.