Guillaume Petite

Femtosecond, picosecond, and nanosecond laser microablation: Laser plasma and crater investigation

Crater shapes and plasma plume expansion in the interaction of sharply focused laser beams with metals in air at atmospheric pressure were studied. Laser ablation efficiencies and rates of plasma expansion were obtained. The best ablation efficiency was observed with femtosecond laser pulses. It was found that for nanosecond pulses the laser beam absorption, its scattering and reflection in plasma were the limiting factors for efficient laser ablation and precise material sampling with sharply focused laser beams. The experimental results obtained were analyzed with relation to different theoretical models of laser ablation.

Optics of subwavelength gradient nanofilms

Propagation and tunneling of light through subwavelength photonic barriers, formed by dielectric layers with continuous spatial variations of dielectric susceptibility across the film are considered. Effects of giant heterogeneity-induced non-local dispersion, both normal and anomalous, are examined by means of a series of exact analytical solutions of Maxwell equations for gradient media. Generalized Fresnel formulae, visualizing a profound influence of gradient and curvature of dielectric susceptibility profiles on reflectance/transmittance of periodical photonic heterostructures are presented. Depending on the cutoff frequency of the barrier, governed by technologically managed spatial profile of its refractive index, propagation or tunneling of light through these barriers are examined. Nonattenuative transfer of EM energy by evanescent waves, tunneling through dielectric gradient barriers, characterized by real values of refractive index, decreasing in the depth of medium, is shown. Scaling of the obtained results for different spectral ranges of visible, IR and THz waves is illustrated. Potential of gradient optical structures for design of miniaturized filters, polarizers and frequency-selective interfaces of subwavelength thickness is considered.

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.

Broadband femtosecond infrared parametric amplification in β-BaB 2 O 4

Infrared femtosecond pulses of 100 μJ of energy are generated in a two-stage, triple-pass, type I β-barium borate parametric amplifier. A focusable intensity of 5 × 1012 W/cm2 with an f/20 optic is demonstrated.

Dose, dose rate and irradiation temperature effects in β-irradiated simplified nuclear waste glasses by EPR spectroscopy.

The influence of dose, dose rate and irradiation or annealing temperature on the formation of paramagnetic centers has been investigated by electron paramagnetic resonance (EPR) spectroscopy on a series of β-irradiated 4-, 5- and 6-oxide glasses in order to study the use of external β-irradiation relative to simulate actual conditions during the storage of nuclear waste glasses. An unusual saturation of defect concentration occurs above 104 Gy. There is no dose rate dependence for the irradiation in a range between 2×103 and 2×104 Gy/s. The increase of irradiation temperature leads to an increase of Zr3+, a decrease of Fe3+ concentration, the formation of magnetic clusters and a strong decrease in the total concentration of paramagnetic defects. In addition, annealing and irradiation temperatures do not produce the same effects on the concentration of the various defects.

Time-resolved studies of carriers dynamics in wide band gap materials

Abstract Lasers delivering high intensity ultrashort pulses are adequate tools to study the dynamics of irradiation induced modifications in wide band gap materials. We present the results of two complementary time-resolved pump-probe optical experiments. We use a technique of interferometry in the frequency domain to probe the instantaneous modification of the refractive index of the material induced by a pump pulse. The presence of free carriers in the conduction band induces a negative phase shift of the probe pulse which is proportional to the excitation density. Comparison between different oxides reveals an extremely contrasted behaviour: the photoexcited carriers are trapped in 150 fs in SiO 2 , while they remain quasi-free for more than 50 ps in MgO and Al 2 O 3 . These lifetimes are closely connected to the formation of radiation induced defects. In particular, using a conventional time-resolved absorption technique in the UV (5.2–5.6 eV), we have studied the intrinsic defects formation kinetics in SiO 2 . Our results show that the formation of self-trapped excitons (STE) is occuring in the ultrashort time of 150 fs, in agreement with the free carrier lifetime measured in the above experiment. At low temperature, the STE recombine radiatively, while at temperature above the luminescence quenching temperature, a small proportion of the STE is converted into permanent defects (O vacancies).

A structural approach by MAS NMR spectroscopy of mechanisms occurring under β-irradiation in mixed alkali aluminoborosilicate glasses

Two series of mixed Na/K and Na/Li aluminoborosilicate glasses have been irradiated with electrons of 1.8 MeV at doses close to 109 Gy. Si, B and Al glass former environment changes under irradiation have been probed by MAS NMR spectroscopy. It was shown that the mixed alkali effect acts on the middle range compositions by operating with a selective blockage in irradiated glasses, depending on the alkali nature and concerning either modifier or charge compensator alkalis.

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.

Effect of Sm-, Gd-codoping on structural modifications in aluminoborosilicate glasses under β-irradiation

Two series of Sm-, Gd-codoped aluminoborosilicate glasses with different total rare earth content have been studied in order to examine the codoping effect on the structural modifications of beta-irradiated glasses. The data obtained by Electron Paramagnetic Resonance spectroscopy indicated that relative amount of Gd3+ ions located in network former position reveals non-linear dependence on Sm/Gd ratio. Besides, codoping leads to the evolution of the EPR signal attributed to defects created by irradiation: superhyperfine structure of boron oxygen hole centres EPR line becomes less noticeable and resolved with increase of Gd amount. This fact manifests that Gd3+ ions are mainly diluted in vicinity of the boron network. By Raman spectroscopy, we showed that the structural changes induced by the irradiation also reveal non-linear behaviour with Sm/Gd ratio. In fact, the shift of the Si-O-Si bending vibration modes has a clear minimum for the samples containing equal amount of Sm and Gd (50:50) in both series of the investigated glasses. In contrast, for single doped glass there is no influence of dopants content on Si-O-Si shift (in case of Gd) or its diminution (in case of Sm) occurs which is explained by the reduction process influence. At the same time, no noticeable effect of codoping on Sm3+ intensity as well as on Sm2+ emission or on Sm reduction process was observed.

Micro-Raman and EPR studies of β-radiation damages in aluminosilicate glass

Abstract Structural changes under β-irradiation have been investigated by means of micro-Raman and electron paramagnetic resonance (EPR) spectroscopies on a calcium aluminosilicate glass with the composition 48.22 SiO 2 , 12.64 Al 2 O 3 , 2.48 Na 2 O, 3.45 Cs 2 O, 0.46 Rb 2 O, 32.75 CaO. The Raman spectra of this sample show no significant structural changes up to 4×10 9 Gy. This result could be correlated to the fact that alkaline and alkaline earth migration under β-irradiation are reduced for this glass composition because these ions are trapped close to aluminium ions for charge compensating reasons. EPR spectra of these irradiated glasses indicate the creation of three paramagnetic centres (Al–OHC, Oxy and E ′ defects) which increase with dose.