F. Canova

Bright, point X-ray source based on a commercial portable 40 ps Nd:YAG laser system

AbstractWe present some experimental results on X-ray spectra obtained from plasmas produced using a compact Nd:YAG lasersystem. The beam was focused on different targets~Cu,Al,Ge,...!and both high resolution and low resolution X-rayspectra were recorded.Keywords: ps laser; X-ray plasma sources; X-ray spectroscopy 1. INTRODUCTIONLaser-plasma sources are very powerful laboratory-scaleX-raydeviceswherewavelengthtunabilityisfeasiblebecauseof the comparatively free choice of target material, laserwavelength, and intensity. They have already been success-fully applied for radiobiology ~Turcu et al., 1994a; Masiniet al., 1999; Milani et al., 1999; Bortolotto et al., 2000!,X-ray microscopy~Batani et al., 2000, 2002; Desai et al.2003;Polettietal.,2004!,micro-lithography~Bijkerketal.,1992; Turcu et al., 1994b!. For all these applications theknowledgeofX-rayspectraisveryimportant,eitherdirectlyorbecauseenergydepositionandradiationdosearestronglyenergy dependent.For instance in micro-lithography;1-keVX-ray photonsare required to achieve efficient radiation transfer throughthe X-ray mask membrane and sufficient energy depositiononto the resist coated Si wafer.In radiobiology, laser-plasma X-ray sources were used toirradiate biological specimens: V-79 Chinese hamster cells~Turcu et al., 1994a!, and study DNA damage, DNA repairand repair inhibition, or Saccharomices Cerevisiae yeastcells ~Masini et al., 1999; Milani et al., 1999! and studymetabolic damages. In the first case, X-rays at hn’1.2 keVwere produced using L-shell emission from Cu targets,which could guaranty penetration of X-rays to the cellnucleus, so that absorbed dose was really related to DNAdamage. In the second case, it was essential to produce lowenergy X-ray photons, which would mainly deposit the doseatthecellwall,inordertoproducedamagesatthemetaboliclevel without touching the cell nucleus.Finally in X-ray microscopy, the spectrum is importantbecause only X-rays in the “water window” ~between 22and 44 A ! will contribute to the formation of a clean image,being absorbed by biological material but not by water. Thepresence of other X-ray lines will diminish the image con-trast dramatically.Besides all this, the study of X-ray spectra from plasmasis very important in itself and it is a well developed field inphysics. Indeed on one side, X-ray spectra are very impor-tant for atomic physics because by recording X-ray emis-sion lines, it is possible to access the energy levels in atomsand in multi-charged ions~Rosmej et al., 1997; Stepanovet al., 1997; Vergunova et al., 1997; Biemont et al., 2000!.On the other side, X-ray spectroscopy is a key technique inplasma diagnostics ~Koenig et al., 1997; Magunov et al.,1998; Batani et al., 1999! since from measurements ofX-ray spectra we can calculate the plasma parameters: elec-tron density, electron temperature, ion charges, and evendeduce important information on plasma opacities and onthe exact shape of the electron distribution function~includ-ing the presence of fast electrons!A key problem for applications and for X-ray spectros-copy, would be the development of very compact X-raysources which would be cheap ~and be reproduced in many

Complete characterization of damage threshold in titanium doped sapphire crystals with nanosecond, picosecond and femtosecond laser pulses

The major bottleneck for the development of robust and cost-effective femtosecond amplification systems is the uncertainty concerning the damage threshold of Ti: Sapphire crystals. Up to now, Ti: Sapphire is the only material that supports the generation of temporally short pulses (few femtosecond) at high repetition rates, and overcoming this bottleneck will represent a major advance in laser performance for all the femtosecond community. Currently, when pumped at 532nm, the uncertainty on Ti:Sapphire damage threshold, is about a factor of ten. The empirically estimated threshold is 10J/cm2 but for safety reasons the femtosecond laser community (especially the companies producing the lasers) uses the conservative value of 1J/cm2. Such a low pumping fluency means low extraction efficiency during the amplification process and a great waste of pumping energy, the most expensive part of a Ti:Sapphire amplifier. In order to remove this bottleneck, we launch a complete analysis of all the factors that influence the damage threshold in Ti:Sapphire Crystals. Our program is to first measure the bulk threshold to define the upper threshold limit, and the influence of Ti ion concentration in the crystal garnet. Then, we will analyze all the surface effects that influence the value of the threshold. These effects depend on the polishing, on the cleaning process, as well as the type of anti-reflective coating. Only a complete understanding of all the mechanisms involved in threshold limitation will allow us to produce Ti:Sa crystals with the best performances. The study of the characteristics of the Ti:Sapphire damage threshold will not be complete and reliable without a complete characterization of the pump beams (temporal and spatial modulations), and this analysis will be done with nanosecond and picosecond pulses at 532nm. Finally, to complete the exploration of the the behavior of the titanium doped sapphire crystal, we will characterize the damage threshold with femtosecond pulses, at 800nm to reach the deterministic dielectric threshold and validate fundamentals models and simulation results. To our knowledges this is the first time that such a complete characterization is done for Ti:Sapphire laser crystals. We will present the first conclusions about the experiments as well as the methods we will employ in our systematic analysis.

Wavefront correction and aberrations pre-compensation in the middle of Petawatt-class CPA laser chains

We describe preliminary experiences to validate correction of wavefront aberrations in middle of laser chain. This technique allows correction of aberrations from first part, and the pre-compensation of aberrations built in second part of laser.

97% Top hat efficiency, 4 J/cm 2 damage threshold compression gratings

High diffraction efficiency all-dielectric pulse compression grating is reported with a close to 100% flat top over more than 20 nm spectral width around 800 nm wavelength and more than 4 J/cm2 damage threshold.

Efficient aberrations pre-compensation and wavefront correction with a deformable mirror in the middle of a petawatt-class CPA laser system

In this paper, we describe the experimental validation of the technique of correction of wavefront aberration in the middle of the laser amplifying chain. This technique allows the correction of the aberrations from the first part of the laser system, and the pre-compensation of the aberrations built in the second part. This approach will allow an effective aberration management in the laser chain, to protect the optical surfaces and optimize performances, and is the only possible approach for multi-petawatt laser system from the technical and economical point of view. This approach is now possible after the introduction of new deformable mirrors with lower static aberrations and higher dynamic than the standard devices.

Wavefront correction and aberrations pre-compensation in the middle of Petawatt-class CPA laser systems

We describe preliminary experiences to validate correction of wavefront aberrations in middle of laser chain. This technique allows correction of aberrations from first part, and the pre-compensation of aberrations built in second part of laser.

Damage threshold of sapphire in short and long pulse regime

The work presents the determination of the laser-induced damage threshold (LIDT) fluence of sapphire under various experimental conditions concerning the material irradiation (fs, ps and ns temporal regimes) and material preparation (surface state). The results may be used for optimising laser micromachining processes and also for studying laser crystal damage in high peak power femtosecond Ti:Sapphire laser chains.

Use of a high spatial resolution wavefront analyser to predict intensity hot spot in a femtosecond laser chain

We demonstrate the use of a new wavefront analyser called MIROMA in a laser chain to measure the residual spatial modulations of the beam induced by footprints of a deformable mirror and prevent optical damages.