Numerical Investigation of the Plasma Formation in Distilled Water by Nd-YAG Laser Pulses of Different Duration

摘要

Water breakdown studies by Nd-YAG laser pulses of duration 100 fs, 30 ps and 6 ns at wavelength 1064 nm are pre- formed to investigate the physical mechanisms which couple the laser energy into the medium. Calculations are carried out applying a modified kinetic model of water breakdown previously developed by Kennedy (1995) to investigate the correlation between threshold intensity of breakdown and laser pulse length. The modifications considered the introduction of diffusion and recombination loss processes which might take place under the experimental conditions applied in these calculations. The validity of the model is tested by comparing the calculated threshold intensities and the experimentally measured ones where good agreement is shown. The study of the time evolution of the electron density clarifies the correlation between the pulse length and dominant ionization mechanism. The analysis of the spatial distribution of the electron density along the radial and axial distances of the focal spot showed that the size of the formed plasma increases with the increase of the pulse length. On the other hand, studies of self-focusing effect illustrated that under the investigated experimental conditions the effect of this process has an effective contribution only at laser pulses of the order of femtosecond scale when the laser beam is focused by a lens of a focal length ≥8.0 cm. This result in turns assures that using femtosecond pulses in ophthalmic microsurgery could be a safe tool from the retinal damage.