Stéphane Laville

Temporal characterization of femtosecond laser pulses induced plasma for spectrochemical analysis of aluminum alloys

Abstract This paper reports studies on time-resolved space-integrated laser induced breakdown spectroscopy (LIBS) of plasmas produced by ultrashort laser pulses at atmospheric pressure, on aluminum alloy targets. The temporal behavior of specific ion and neutral emission lines of Al, Mg and Fe has been characterized. The results show a faster decay of continuum and spectral lines, and a shorter plasma lifetime than in the case of longer laser pulses. Spectroscopic diagnostics were used to determine the time-resolved electron density, as well as the excitation and ionization temperatures. In comparison with plasmas produced by ns laser pulses, the plasma generated by ultrashort pulses exhibits a faster thermalization. Analytical performances of fs-LIBS were also evaluated. Linear calibration curves for minor elements (Mg, Fe, Si, Mn, Cu) presented in aluminum alloys were obtained. The limits of detection are in the parts per million (ppm) range and are element-dependent.

Investigation of resonance-enhanced laser-induced breakdown spectroscopy for analysis of aluminium alloys

Resonance-enhanced laser-induced breakdown spectroscopy (RELIBS) was investigated with the aim to improve the limit of detection of trace elements in the context of elemental analysis of aluminium alloys. A Q-switched Nd:YAG laser pulse (7 ns, 1064 nm) was used for ablation of the samples and was followed, after a suitable delay, by an Optical Parametric Oscillator (OPO) laser pulse (7 ns), tuned at 396.15 nm, to resonantly excite the aluminium host atoms. In particular, the Mg I 285.21 nm and Si I 288.16 nm lines were observed in the acquisition spectral window. We investigated the influence of the main experimental parameters, namely, the excitation wavelength, the interpulse delay and the ablation and excitation fluences, on the signal-to-noise ratio for the Mg I 285.21 nm line. We found that, at low ablation fluences, typically less than a few J cm−2, the Mg signal at 285.21 nm achieved using RELIBS was significantly enhanced when compared to LIBS using the same ablation fluence. At fluences higher than 8 J cm−2, the effect of the excitation pulse became unnoticeable and similar results were observed for both approaches. The optimum conditions were achieved for an interpulse delay of about 30 ns, an ablation fluence of about 3.8 J cm−2 and an excitation fluence of about 1.1 J cm−2. The corresponding absolute LoDs were 0.7 and 50 fg, for Mg and Si, respectively, using RELIBS. When using LIBS, they were 4 and 128 fg, instead. Finally, the applicability of RELIBS in the context of a minimally destructive elemental analysis is discussed.

Investigation of the State of Local Thermodynamic Equilibrium of a Laser-Produced Aluminum Plasma

In this work, the assumption of local thermodynamic equilibrium (LTE) for a laser-induced plasma in ambient air is examined experimentally using two different laser systems, namely an infrared short-pulse Ti:Sapphire laser and an ultraviolet long-pulse XeCl excimer laser. The LTE assumption is investigated by examining the plasma produced at a laser fluence of 10 J/cm2 from aluminum targets containing iron and magnesium impurities. The excitation temperature is deduced from Boltzmann diagrams built from a large number of spatially integrated neutral iron lines distributed from 3.21 to 6.56 eV. It is shown that at any time after the end of the laser pulse, the neutral excited states are in excellent Boltzmann equilibrium. Detailed investigation of Boltzmann equilibrium further validates previous temperature measurements using less accurate diagrams. However, observations of ion lines provide some evidence that the ionized species do not obey Saha equilibrium, thereby indicating departure from LTE. This could be explained by the fact that the plasma cannot be considered as stationary for these species.

Quantitative analysis of metallic traces in water-based liquids by UV-IR double-pulse laser-induced breakdown spectroscopy

Double-pulse laser-induced breakdown spectroscopy (DP-LIBS) was performed for quantitative analysis of three metallic trace elements: Fe, Pb and Au in aqueous solutions. The plasma was generated using a UV (266 nm) frequency-quadrupled Q-switched Nd:YAG laser (7 ns) and then reheated by a 1064 nm Q-switched Nd:YAG laser (7 ns) in a quasi-collinear geometrical configuration. In order to improve the reproducibility of LIBS measurements, a circulation cell was used, providing a reproducibility of about 4% with a laser repetition rate of 0.3 Hz. The influence of the inter-pulse delay and the fluence of the second laser pulse on the signal-to-noise ratio (SNR) for the analytical lines was investigated and optimized. Analytical figures of merit of DP-LIBS and conventional single-pulse LIBS (SP-LIBS) were evaluated by establishing the calibration curves for the Fe I 358.12 nm, Pb I 405.78 nm and Au I 267.60 nm lines. The signal was greatly enhanced in DP-LIBS while the noise level did not vary as much. An improvement of the relative limit of detection of about 10 was achieved using DP-LIBS when compared to UV SP-LIBS in all cases. Measurement of the electron density as a function of time indicates that the plasma plume lifetime is longer in DP-LIBS. Similar trends in the excitation temperature were not observed for reasons that we attribute to larger uncertainties related to the Boltzmann plot method.

Influence of the laser pulse duration on laser-produced plasma properties

In the framework of laser-induced plasma spectroscopy (LIPS) applications, time-resolved characteristics of laser-produced aluminium plasmas in air at atmospheric pressure are investigated for laser pulse durations ranging from 100 fs to 270 ps. Measurements show that for delays after the laser pulse longer than ~100 ns, the plasma temperature increases slightly with the laser pulse duration, while the electron density is independent of it. In addition, as the pulse duration increases, the plasma radiation emission lasts longer and the spectral lines arise later from the continuum emission. The time dependence of the continuum emission appears to be similar whatever the duration of the laser pulse is, while the temporal evolution of the line emission seems to be affected mainly by the plasma temperature. Finally, as far as spectrochemical applications (such as LIPS) of laser-produced plasmas are concerned, this study highlights the importance of the choice of appropriate temporal gating parameters for each laser pulse duration.

Improving laser-induced breakdown spectroscopy (LIBS) performance for iron and lead determination in aqueous solutions with laser-induced fluorescence (LIF)

The combination of laser-induced breakdown spectroscopy (LIBS) and laser-induced fluorescence (LIF) was investigated to improve the limit of detection (LoD) of trace elements in liquid water, while preserving the distinctive on-line monitoring capabilities of LIBS analysis. The influence of the main experimental parameters, namely the ablation fluence, the excitation fluence, and the inter-pulse delay was studied to maximize the fluorescence signal. The plasma was produced by a 266 nm frequency-quadrupled Q-switched Nd:YAG laser and the trace elements under investigation were then re-excited by a nanosecond optical parametric oscillator (OPO) laser, delivering pulses in the sub-mJ energy range, and tuned to strong absorption lines of the trace elements. The reproducibility of the measurements was improved using a home-made flow-cell, and relative standard deviations as low as 6.7% for a series of 100 shots were attained with a repetition rate of 0.7 Hz. Using the LIBS-LIF technique, we demonstrated LoDs of 39 ppb and 65 ppb for Pb and Fe, respectively, accumulating over 100 laser shots only, which correspond to an improvement of about 500 times with respect to LIBS.

Influence of the laser pulse duration on spectrochemical analysis of solids by laser-induced plasma spectroscopy.

Quantitative analysis of aluminum and copper alloys by means of laser-induced plasma spectroscopy (LIPS) has been investigated for three representative laser pulse durations (80 fs, 2 ps, and 270 ps). The experiments were carried out in air at atmospheric pressure with a constant energy density of 20 J/cm2. Because the decay rate of the spectral emission depends on the laser pulse duration, the optimum detection requires an optimization of the temporal gating acquisition parameters. LIPS calibration (sensitivity and nonlinearity) and the limit of detection (LOD) are discussed in detail. While the LOD of minor elements embedded in alloy samples obtained by sub-picosecond or sub-nanosecond laser pulses are both time and element dependent, provided an appropriate temporal window is chosen, the optimum LODs (several parts per million (ppm)) prove to be independent of the laser pulse duration. Finally, it is found that for elements such as those detected here, gated LIPS spectra using picosecond or sub-picosecond laser pulses provide much better LOD values than non-gated spectra.

Numerical simulations of ultrashort laser pulse ablation and plasma expansion in ambient air

Abstract Using a self-consistent one-dimensional Cartesian Lagrangian fluid code, we modeled the ultrashort laser pulse ablation of solid aluminum and the subsequent plasma expansion in ambient air. A laser fluence of approximately 10 J/cm 2 is considered. The code axial plasma temperature and density are strongly inhomogeneous and the maximum radiation emission generally occurs in the front of the plasma. The code average plasma temperature is in good agreement with the experiments for all times, while larger discrepancies with respect to the experiments are observed at late times for the plasma density. Experimental results are in reasonable agreement with the condition of thermodynamic equilibrium, which is an important assumption in the model.

Ablation of aluminum thin films by ultrashort laser pulses

In this study, various results are presented for laser ablation experiments on aluminum and silicon, made in ambient air by means of subpicosecond laser pulses. These results include threshold fluences for plasma formation and for the appearance of various spectral lines, and the single shot fluence required to remove aluminum layers of various specific thicknesses (ranging from 10 to 500 nm) deposited on a silicon substrate. The threshold fluence for plasma formation is of the order of 0.1 J/cm2. Threshold fluences for the appearance of the spectral lines considered vary from 0.1 to about 5 J/cm2. Finally, our results suggest that for high fluences, even for ultrashort laser pulses, the ablation depth is essentially determined by a long-range process, such as thermal conduction in the solid, rather than by the short range optical depth.

Laser-ablated volume and depth as a function of pulse duration in aluminum targets.

The ablated depth and volume per laser pulse from an aluminum target were measured for pulse durations that ranged from 80 fs to 270 ps at an average fluence of approximately 100 J/cm2 and a wavelength of 0.8 microm. The ablated volume showed a flat maximum for subpicosecond pulses and a minimum for approximately 6 ps. The crater diameters were rather constant up to pulse durations of approximately 6 ps and increased for larger pulse durations. As a result, the ablated depth also showed a plateau for subpicosecond pulses but decreased monotonically with pulse duration. A physical interpretation of these results and their consequences for laser applications are discussed.