الفهرس 
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Abstract
A Ti : sapphire laser system operating at central wavelength of 800 nm with 40 fs pulse duration and 10 mJ pulse energy with repetition rate of 1 kHz was used to study the optical breakdown of water and air. The laser beam is focused into the medium using a microscope objective (NA = 0.30). The experiments are carried out to analyze the following phenomena associated with laser induced optical breakdown: i- The luminescence and size of the plasma generated by 40 fs single laser pulse in air as well as 170 ps in distilled water with different laser pulse energies. This has been done utilizing the time integrated photography.ii- The shock wave in atmospheric air and distilled water produced by 40 fs single pulses using the time resolved pump probe technique. iii- The dynamics of cavitation bubbles induced by 170 ps and 40 fs laser pulses in distilled water using the same, time resolved pump probe, technique. The bubble end point technique is used to measure the breakdown threshold in distilled water for both 170 ps and 40 fs laser pules.Our result showed that, the plasma produced in air, by 40 fs laser pulses, is concentric with the focal volume and its dimensions gradually increase with increasing the pulse energy from 20µ J to 95 µJ. The plasma shape becomes almost oval attaining a homogenous luminescence over all its size. On the other hand, the plasma formation (with the 170 ps laser pulse in water, obtained at different energies) is found to be consistent with the moving breakdown model given by Docchio et al, 1988112. It is also observed that, the plasma is very small and nearly confined around the central focal point. To assure the precision of the shock wave measurements, the measured radii of the shock wave generated in air by 40 fs laser pulses are compared with those predicted from Sedov91 theory for the case of an ideal spherical and cylindrical shock wave. This comparison clarifies that the propagation of the generated shock wave agrees to some extent with the Sedov theory. In addition, the result of the measurements revealed that the breakdown obtained by the ultrashort laser pulses (40 fs) generate a localized shock waves where its propagation distance did not exceed 50 µm in radius over the energy range tested experimentally (11.5 µJ to 91 µJ per pulse). Moreover. the measurements of the shock wave radii enabled us to determine the shock wave propagation velocity as well as its pressure. These are determined at the different values of the laser pulse energy over periods of dele: time varies between 400 ps to 15 ns. The shock wave velocity and pressure values are found to depend on both the laser pulse energy and the delay time. On the other hand, for water. different results are obtained where the measured shock wave propagation velocity is found to haw a value slightly higher than the sound velocity (1483 m/s). This was shown over the whole laser pulse energies used in this analysis. The shock waves are observed to have a cylindrical shape and rapidly decaying with time in comparison with those obtained by ps and ns laser pulses. In addition, its spatial behavior (propagation distance) is found to be very limited reaching only about 26 urn at a delay time of IS ns and pulse energy of 8 µJ. An interesting finding is obtained for the measured cavitation bubble radii with 40 fs laser pulses, where a noticeable decrease of maximum bubble radii are observed compared with those measured using 170 ps pulse. In the former case, even with increasing the laser pulse energy to ten times of its threshold value (5.15µJ ). the maximum radius of the formed cavitation bubble (50 µJ) is found to be less than that obtained (57 µJ) with laser pulse energy (10.31 µJ) just enough to produce optical breakdown in water by 170 ps laser pulse. Another important result that revealed from these measurements is that. the time taken for the bubble formation and collapse in the case of 40 fs laser pulse appears to be much shorter than that obtained by longer pulse (170 ps). In addition. the shape of the cavitation bubble formed by these two laser pulses (40 fs and 170 ps) is highly distinguishable. This might be due to the fact that in fs pulse scale the energy absorption in the focal region varies with both time and space this results in an elongation of the central focal region and in turn leads to an elongation of the observed cavitation bubble. from this study one may conclude that. mechanical effects such as shock wave generation and cavitation bubble expansion are greatly reduced for shorter laser pulses. This could be due to the less energy required to produce breakdown with the shorter pulse duration, also, as a result of the heat dissipated in the focal region leading to its vaporization for femtosecond pulses this means that a large fraction of the pulse energy is used to evaporate the focal volume for shorter pulses and thus less energy is available for mechanical process.