Optical emission and nanoparticle generation in Al plasmas using ultrashort laser pulses temporally optimized by real-time spectroscopic feedback

Abstract

International audienceWith an interest in pulsed laser deposition and remote spectroscopy techniques, we explore here the potential of laser pulses temporally tailored on ultrafast time scales to control the expansion and the excitation degree of various ablation products including atomic species and nanoparticulates. Taking advantage of automated pulse-shaping techniques, an adaptive procedure based on spectroscopic feedback is applied to regulate the irradiance and enhance the optical emission of monocharged aluminum ions with respect to the neutral signal. This leads to optimized pulses usually consisting in a series of femtosecond peaks distributed on a longer picosecond sequence. The ablation features induced by the optimized pulse are compared with those determined by picosecond pulses generated by imposed second-order dispersion or by double pulse sequences with adjustable picosecond separation. This allows to analyze the influence of fast- and slow-varying envelope features on the material heating and the resulting plasma excitation degree. Using various optimal pulse forms including designed asymmetric shapes, we analyze the establishment of surface pre-excitation that enables conditions of enhanced radiation coupling. Thin films elaborated by unshaped femtosecond laser pulses and by optimized, stretched, or double pulse sequences are compared, indicating that the nanoparticles generation efficiency is strongly influenced by the temporal shaping of the laser irradiation. A thermodynamic scenario involving supercritical heating is proposed to explain enhanced ionization rates and lower particulates density for optimal pulses. Numerical one-dimensional hydrodynamic simulations for the excited matter support the interpretation of the experimental results in terms of relative efficiency of various relaxation paths for excited matter above or below the thermodynamic stability limits. The calculation results underline the role of the temperature and density gradients along the ablated plasma plume which lead to the spatial distinct locations of excited species. Moreover, the nanoparticles sizes are computed based on liquid layer ejection followed by a Rayleigh and Taylor instability decomposition, in good agreement with the experimental findings

Analytical performances of laser-induced micro-plasma of Al samples with single and double ultrashort pulses in air and with Ar-jet: A comparative study

“…With 50 fs pulses, the total plasma intensity with double pulses of 20 µJ (220 µJ) was 100 times higher than the one with a single pulse of the same pulse energy (120 µJ), and 50 times higher than the one with a single pulse but with a doubled pulse energy (140 µJ). Our observations on ultrashort double pulse ablation have been confirmed in similar studies in [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36].…”

“…The analysis of our results and those obtained in [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] has revealed the fundamental role of the electron-lattice relaxation time ( el .  1-10 ps) for laser ablation regime and the resulting double pulse LIBS-features.…”

[INVITED] Control of femtosecond pulsed laser ablation and deposition by temporal pulse shaping

“…Considering the ablation process complexity, the irradiation-detection loop is driven by an adaptive approach. The optimization procedure is based on an evolutionary strategy and is used to progressively adapt the temporal shape of the laser pulse to induce a particular user-defined result recorded by the ablation characterization device [17].…”

“…A pulse tailoring unit in the time domain is used for Concerto system relying on Fourier synthesis [39], i.e. the spectral filtering of the pulse frequency components in a dispersive system [17,43] A spatial light modulator (SLM) consisting in a 640 pixels liquid crystal array is used to modulate the phase of the laser pulses prior to amplification. It allows a high spectral resolution and a shaping window superior to 20 ps.…”

“…Indeed, temporal pulse shaping has been successfully applied to improve thin films deposition [4,15,16] or to optimize plasma excitation, e.g. on aluminium, and modify the amount of nanoparticles [17]. As simplified counterparts, double pulses may also have great interest by generating and modifying plume characteristics [18] and deposited thin films [19].…”