Olivier Gobert

Ablation thresholds of metals with femtosecond laser pulses

The laser ablation threshold experiments were performed on pure metals with the fs Gaussian laser beam focused to 41.5 mm spot diameter onto metal surfaces. Three different ablation thresholds were distinguished. The multi-shot ablation threshold for Cu with 70 fs pulse was found to be 0.018 J/cm2 and of one order to magnitude lower than that one observed previously. In the fluence range of 0.018- 0.2 J/cm2 the ablation rate was approximately equal to 0.01 nm/pulse. The threshold dependence on the pulse duration was demonstrated in the range of 70 fs-5 ps for Cu. As the laser pulse width increased, the ablation threshold had the tendency to be higher. The ablation rate dependence on laser fluence for the other metals under study in our experiments with 70 fs was similar to that of CU.

Femtosecond laser ablation of metals: precise measurement and analytical model for crater profiles

Laser ablation of Cu, Al, Fe, Zn, Ni, Pb, and Mo by short pulse laser (800nm wavelength, 70fs pulse duration, 0.01-28 J/cm2 fluence range) in air was studied. Three different ablation thresholds were distinguished in all metals. The lowest ablation threshold was of one order of magnitude lower than the one observed previously. In the fluence range of 0.018-0.18 J/cm2 the ablation rate was ≈0.01 nm/pulse. A dependence of the threshold on the pulse duration was demonstrated in the range of 70 fs- 5 ps for cupper. As the laser pulse duration increased, the ablation threshold had the tendency to be higher. A periodic structure was observed at the bottom of the crater in all metals. The spacing d of the patterned structure was determined to be d=300±40 nm for 0.07 J/cm2 and d=600±40 nm for 0.22 J/cm2. The spacing depended on the laser fluence rather than on laser wavelength.

Bessel beam transformation by anisotropic crystals

Transformation of Bessel beams by biaxial and uniaxial crystals is investigated experimentally and theoretically. Experimental observations show beam symmetry changing and formation of complex intensity patterns, depending on the orientation of the crystal. These patterns can appear as a regular system of peak intensities. Results of numerical calculations support the experimental findings.

Formation of Self-Organized Regular Nanostructures upon Femtosecond Laser Ablation

At the bottom of ablation craters produced in many materials, e.g. dielectric and silicon crystals, by the impact of femtosecond laser radiation, regular periodic structures are observed with a feature size at the order of a few 100 nanometers, much smaller than the incident wavelength. Their orientation depends strongly on the laser polarization but not on any intrinsic crystalline parameters. An increasing number of shots results in higher contrast, better developed structures, indicating a positive feedback. The region around the impact is shown, by micro Raman spectroscopy, to undergo phase transformations like under high pressure. The structure spacing appears to depend crucially on the depth of the perturbed volume, i.e. the incident (and absorbed) energy. All observations suggest that the structures form by self-organization from instabilities induced in the material by the laser input. A general picture suggests that the irradiation results in a rapid, non-equilibrium destabilization of the crystal structure, which should not be confused with melting as a classical thermodynamic process (i.e. temperatures defined as equilibrium properties). Relaxation from this instability results in the self-assembly of the observed structures. Theoretical simulations demonstrate the feasibility of this model, which also is corroborated by comparison to other unstable situations.

Microablation of pure metals: laser plasma and crater investigations

Crater shapes and plasma plume expansion in the interaction of femtosecond, picosecond and nanosecond laser pulses with various pure metal in air and noble gases at atmospheric pressure were studied. The craters formed at the surfaces were measured by an optical microscope profilometer with 0.01 micrometers depth and 0.5 micrometers lateral resolutions. The measurements of laser plasma expansion were carried out with ICCD camera with 3 micrometers spatial and 1 ns temporal resolutions. These measurements were made in 0-100 ns time delay range and at different wavelengths in 200-850 nm optical spectral range. Laser ablation efficiencies, crater profiles, plasma plume shapes at different time delays, rates of plasma expansion in both longitudinal and transversal directions to the laser beam were obtained. Experimental results were analyzed from the point of view of different theoretical models of laser beam interaction with plasma and metals. The laser pulse duration range used in our study was of particular interest, as it includes the characteristic time of electron-phonon relaxation in solids, that is, of the order off one picosecond. Thus, we could study the different regimes of laser ablation without and with laser beam/plasma plume interaction. It was found that for nanosecond pluses the laser beam absorption, as well as its scattering and reflection in plasma, were the limiting factors for efficient laser ablation and precise material processing with sharply focused laser beams.

Xe(L) x-ray emission from laser-cluster interaction

We have measured absolute L x-ray emission yields from (Xe)n clusters (with n in the range 10^5 – 10^7 atoms/cluster) irradiated by 60 femtoseconds 800 nm IR and 160 femtoseconds 400 nm UV laser pulses of 10^15 – 10^17 W cm-2 peak intensity. Measurements have been performed as a function of cluster size (backing pressure) and laser peak intensity. Identification of spectroscopic features as well as x-ray emission yield variation with laser wavelength and intensity are in strong contradiction with results and interpretation from previous studies.

Efficient broadband 400 nm noncollinear second-harmonic generation of chirped femtosecond laser pulses in BBO and LBO

Smith and later Liu et al have detailed an analysis of the three wave-mixing ultra-broadband phase matching (PM) condition using the concept of pulse-front tilt (PFT). We applied their model to design an original set-up for high power achromatic fs noncollinear (NC) second harmonic generation (SHG) with PFT in Beta-Barium Borate (BBO) and Lithium Triborate (LBO).

Wavelength dispersion measurement of electro-optic coefficients in the range of 520 to 930 nm in rubidium titanyl phosphate using spectral interferometry

Rubidium titanyl phosphate (RTP) is widely used for electro-optical applications at low switching voltages. RTP is nonhygroscopic and does not induce piezoelectric ringing up to the megahertz range. It has large electro-optic (EO) coefficients and a high damage threshold. We present here the EO coefficient wavelength dispersion measurements in the [550,950] nm spectral range using a method based on spectral interferometry. These data are necessary for, among other things, a quantitative modelization of an EO carrier-envelope phase shifter.

Local Spectral Compression method for CPA lasers

Measurement of femtosecond pulses without any reference requires three successive steps: a linear filter, a non linear interaction (e.g. SHG) and a spectral measurement of the output [1]. As previously demonstrated, the linear filtering can be performed using a pulse shaper, such as an acousto-optic programmable dispersive filter (AOPDF) [2]. Since a significant number of CPA amplifiers now include a pulse shaper in their front-end, it is highly convenient to use such a device not only to correct but also to characterize the amplified pulses by adding at the back-end a nonlinear element and a spectral detector. In this paper, we demonstrate experimentally a new phase-only measurement technique called Local Spectral Compression (LSC) on a CPA laser system (CEA Saclay) by inserting an AOPDF in front of the last three-pass amplifier and a nonlinear BBO crystal at the output. The second harmonic signal is recorded with a spectral detector.

Subpicosecond dynamics of intense laser-cluster interaction: keV x rays and highly charged ion production

We have performed studies of keV x-ray production from (formula available in paper) rare gas clusters submitted to intense IR laser pulses. Up to (formula available in paper) per pulse at a moderate atomic density have been observed. High resolution spectroscopy studies in the case of (formula available in paper) clusters have also been performed, gibing unambiguous evidence of highly charge ions with K vacancies production. We have determined the photon energies and the absolute photon emission yields as a function of several physical parameters governing the interaction: size and atomic number of the clusters, peak intensity of the laser. Unexpectedly low laser intensity thresholds have been measured. The result obtained indicate nevertheless that x-rays may be emitted before cluster explosion on a subpicosecond time scale, and that several mechanisms must be involved in the first stage of the production of the hot nanoplasma induced from each cluster.