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Characterization of underwater laser induced plasma for surface modifications and nanoparticle production

Underwater laser induced plasma in comparison to laser induced plasma in vacuum or in gas background offer various advantages in respect to particle production of nano and micro size or surface modification [1-3]. In situ application of various optical diagnostic methods is useful in order to achieve better control growth conditions, regarding many parameters which influence the growth. In this work some preliminary studies were performed using various materials irradiated by nanosecond Nd-YAG laser. The experimental apparatus is shown in figure 1. Two modes of operation were used. In the first mode target was fixed within liquid which could be mixed by rotating magnet or not and in the second mode target was rotating within the liquid by means of magnetic stirrer in other to prevent drilling (for soft target material). Nd-YAG laser with the wavelength of 1064 nm and pulse width of 4 ns was used to irradiate simple tar-gets (titanium, GaAs) or complex (ACP) at repetition rate from 5 to 10 Hz. Liquid layer above the target was usually few mm. Laser beam was focused with a 33 cm lens. Laser energy at the target was about 140 mJ. Optical laser conditions were kept similar to experiments performed in vacuum or in the gas background [4]. During irradiation by focused laser on the target bright light spot is observed. That light was fed by an optical fibre to various spectrometers for spectral and temporal analyses. Laser treated target surface was inspected by means of an optical microscope in respect to number of pulses and com-pared to laser ablation in vacuum. Examples are shown in figure 2. Generally, comparing to laser ablation in vacuum, here drilling does not occur even after prolonged exposition to laser pulses and in the spot center, a peculiar “desert like” surface modification is observed. Already after several laser pulses transmittance of the liquid changes visually indicating generation of a small size particles. Direct in situ observation of transmittance can therefore be suitable control parame-ter (not shown in figure 1). The optical transmittance of produced colloids is further analysed as a func-tion of ageing (segregation). For microscopy (figure 3), droplets of samples of colloids were dried on appropriate substrate. Further comparison of underwater laser induced plasma parameters and target conditions to laser induced plasma in vacuum will be presented and discussed at the conference. References: [1] A.Kruusing, Underwater and water-assisted laser processing: Part 2—Etching, cutting and rarely used methods, Optics and Lasers in Engineering 41 (2004) 329–352 [2] P. Bruggeman and C. Leys, Non-thermal plasmas in and in contact with liquids, J. Phys. D: Appl. Phys. 42 (2009) 053001 (28pp) [3] G.W. Yang, Laser ablation in liquids: Applications in the synthesis of nanocrystals. Progress in Materials Science 52 (2007) 648-698 [4] N. Krstulović, N. Čutić, S. Milošević, Cavity ringdown spectroscopy of collinear dual-pulse laser plasmas in vacuum , Spectrochimica Acta B 64 (2009) 171-177

laser induced plasma; surface modifications; nanoparticles

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