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The influence of sample temperature on the laser ablation process and analytical capabilities of laser induced breakdown spectroscopy (LIBS) was studied systematically. We were interested in estimating the analytical capabilities of a LIBS probe which was developed for in situ elemental analysis of molten metals during the production of metal parts by additive manufacturing. The sample surface was heated locally (4 mm diameter spot) in an argon atmosphere using a powerful continuous wave fiber laser while a nanosecond laser ablated only a small spot (0.3 mm diameter) in the high temperature area. This procedure can be utilized for high temperature LIBS studies of any samples including materials with high melting temperatures (tungsten, aluminum oxide etc.). Low alloy steel samples were heated up to their melting points and LIBS experiments were carried out in a wide temperature range (25–1600 °C). LIBS plasma imaging revealed that the size and brightness of the expanding plume did not depend on solid sample temperature but these parameters increased manifold for molten steel ablation. Comparison of plasma emissivity, temperature and electron density did not reveal any meaningful differences in the case of LIBS measurements for room temperature (25 °C), hot solid (1050 °C) and molten (1550 °C) targets, and thus enhanced plasma emissivity was attributed to greater ablation rate. The surface temperature impact on LIBS analytical capabilities was studied by comparing calibration curve linearity, limits of detection and the root mean square error of cross-validation for chromium, silicon and manganese. High temperature of the sample surface favored greater atomic/ionic line emission but molten sample LIBS analysis revealed poor limits of detection for all elements, which was explained by strong fluctuations of the laser ablation process at the liquid metal surface. [ABSTRACT FROM AUTHOR]

Single-shot IR femtosecond-laser ablation of gold surfaces in ambient air and liquid isopropyl alcohol was studied by scanning electron microscopy characterization of crater topographies and time-resolved optical emission spectroscopy of ablative plumes in regimes, typical for non-filamentary and non-fragmentation laser production of nanoparticle sols. Despite one order of magnitude shorter (few nanoseconds) lifetimes and almost two orders of magnitude lower intensities of the quenched ablative plume emission in the alcohol ambient at the same peak laser fluence, craters for the dry and wet conditions appeared with rather similar nanofoam-like spallative topographies and the same thresholds. These facts envision the underlying surface spallation as one of the basic ablation mechanisms relevant for both dry and wet advanced femtosecond laser surface nano/micro-machining and texturing, as well as for high-throughput femtosecond laser ablative production of colloidal nanoparticles by MHz laser-pulse trains via their direct nanoscale jetting from the nanofoam in air and fluid environments. [ABSTRACT FROM AUTHOR]

Double pulse generation mode for nanosecond ytterbium fiber laser was developed. Two sequential 60-200 ns laser pulses with variable delay between them were generated by acousto-optic modulator opening with continuous diode pumping. A custom radio frequency generator was developed to produce two sequential "opening" radio pulses with a delay of 0.2–1 µs. It was demonstrated that double pulse generation did not decrease the average laser power while providing the control over the laser pulse power profile. Surprisingly, a greater peak power in the double pulse mode was observed for the second laser pulse. Laser crater studies and plasma emission measurements revealed an improved efficiency of laser ablation in the double pulse mode. [ABSTRACT FROM AUTHOR]