J. Margot

Direct evaluation of the sp3 content in diamond-like-carbon films by XPS

The analysis of the X-ray photoelectron spectra (XPS) of the C 1s core level of pulsed laser deposited diamond-like carbon thin films, obtained at different laser intensities is presented. These spectra are deconvoluted into two different contributions, at 284.4 eV and 285.2 eV, which are respectively attributed to sp2 and sp3 hybridized carbon atoms. From the deconvoluted spectra, the sp3 content in the films is evaluated. It is found to increase from 33% to about 60% as laser intensity is varied from 0.9×108 to 7.1×108 W/cm2. These measurements have been compared to those obtained by the analysis of the C KLL X-ray excited Auger electron spectra. The two methods provide the same qualitative variation of the sp3 content with laser intensity. However, the XPS of the C 1s core level yields systematically higher sp3 content values. These differences are attributed to the presence of an sp2 rich outer layer on the surface of the DLC films, as confirmed by angle-resolved XPS. The analysis of the C 1s peak is shown to be a very simple and direct method to evaluate the sp3 content in unhydrogenated DLC thin films.

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.

Effects of Ti–W codoping on the optical and electrical switching of vanadium dioxide thin films grown by a reactive pulsed laser deposition

Thin films of thermochromic VO2, V1−xWxO2 and V1−x−yWxTiyO2 (x=0.014, and y=0.12) were synthesized onto quartz substrates using a reactive pulsed laser deposition technique. The films were then characterized by x-ray diffraction and x-ray photoelectron spectroscopy. The W and Ti dopant effects on the semiconductor-to-metal phase transition of VO2 were investigated by measuring the temperature dependence of their electrical resistivity and their infrared transmittance. Remarkably strong effects of Ti–W codoping were observed on both the optical and electrical properties of V1−x−yWxTiyO2 films. The IR transmittance was improved, while the transition temperature could be varied from 36°C for W-doped VO2 film to 60°C for Ti–W codoped VO2 film. In addition, at room temperature, a higher temperature coefficient of resistance of 5.12%∕°C is achieved. Finally, both optical and electrical hysteresis are completely suppressed by Ti–W codoping the VO2 films.

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.

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.

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.

Dependence of gold nanoparticle production on pulse duration by laser ablation in liquid media

The dependence on laser fluence and laser pulse duration of size, size distribution and concentration of gold nanoparticles synthesized by laser ablation in liquid media was investigated. It was demonstrated that increasing laser energy from 1 to 5 mJ/pulse enhances the ablation rate by a factor of 100. The behavior of the ablation rate, hence of the nanoparticle concentration, as a function of pulse duration (varied from 40 fs to 200 ps) was found to strongly differ from that in air, which can be explained by photoionization and important losses of laser energy in the femtosecond regime. The optimal pulse duration for maximum ablation rate in liquid media was found to be equal to 2 ps.

Electromagnetic surface waves for a new approach to the investigation of plasmas produced at electron cyclotron resonance (ECR)

The authors propose and put to use a new approach to studying magnetised plasmas sustained by a high frequency (HF) field, particularly aiming at examining the case of discharges achieved at electron cyclotron resonance. This approach considers the methodology and the formalism of the modeling of cylindrical plasma columns produced by electromagnetic surface waves and extends them to the case where these discharges are submitted to an axial, static magnetic field B0. It leads to a variety of waves that are guided by the plasma column, these waves differing in particular by the spatial distribution of their electric field intensity. This distribution plays on the power transfer from the HF field to the plasma and it influences the spatial density distribution of excited atoms. This led them to analyse, as a function of B0, the respective effects of the wave attenuation coefficient, wave polarization and HF power required to maintain an electron-ion pair in the discharge upon the plasma density and upon the electric field for the gas breakdown.