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The laser ablation characteristics of yttria-stabilized zirconia ͑YSZ͒ have been investigated as a function of the target microstructure and dopant level for different nanosecond-͓ArF, KrF, and XeCl excimers; Nd:YAG ͑yttrium aluminum garnet͒ ͑fourth harmonic͔͒ and femtosecond-laser sources ͓Ti:sapphire ͑fundamental and third harmonic͔͒. Particle ejection, which compromises the quality of coatings prepared by pulsed laser deposition ͑PLD͒, was analyzed in detail. Nanosecond-laser pulses cause a severe thermomechanical surface cracking and exfoliation of micron-sized fragments on a microsecond to millisecond time scale in the case of 8 -9.5 mol % Y 2 O 3 -doped, fully stabilized zirconia ͑8YSZ and 9.5YSZ͒ targets. As a consequence of the intrinsic material brittleness, fully stabilized YSZ coatings deposited by PLD contained particles for all tested conditions. Lower doped partially stabilized zirconia ͑3YSZ͒ exhibits a superior fracture toughness attributed to a laser-induced partial transition to the monoclinic phase, detected by Raman spectroscopy, which enables the deposition of particle-free dense thin films by conventional PLD using nanosecond-UV laser radiation at moderate fluences of 1.2-1.5 J / cm 2 . The ablation dynamics of ultrashort laser pulses differ fundamentally from the nanosecond regime as evidenced, e.g., by time-resolved shadowgraphy and light scattering experiments. Femtosecond pulses prevent the exfoliation of micron-sized fragments but result invariably in a pronounced ejection of submicron particles. The resulting PLD coatings are porous and reveal a large surface roughness as they consist of an agglomeration of nanoparticles. Femtosecond-NIR pulses provide a factor of 2.5-10 higher material removal rates compared to nanosecond-and femtosecond-UV pulses. The ablation metrics, i.e., threshold fluence and effective absorptivity, mainly depend on the laser wavelength while the pulse duration, target microstructure, and dopant level are of minor importance. Evidence is presented that incubation effects play a significant role in nanosecond-and femtosecond-laser ablations of YSZ enabling material removal at comparatively low fluences for sub-bandgap photon energies.

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