Features of nanosecond laser-induced optical breakdown at air-water interface

Abstract

Summary form only given. Time-resolved shadow imaging has been developed for the investigation of optical breakdown induced at air-water interface by Nd:YAG laser pulses. For this purpose, laser pulses of 4-ns and 18-ns duration (with wavelengths 532 and 1064 nm) and pulse energies of 200 mJ were focused at the air-water interface. Plasma formation and shock wave emission were investigated by taking series of photographs with increasing time interval between the optical breakdown and the exposure of the photograph. Formation and propagation of cylindrical and combination of cylindrical and spherical shock waves were observed. The photographs were used to determine the shock front as a function of time as well as the shock wave velocity and pressure as a function of time or position. Although each frame was taken during a different event, the shock wave emission can well be followed, because the reproducibility of the events is good. The optical delivery system allowed for the realization of different focusing angles and was designed to move the spot from near surface air layer into a cuvette containing water. Features of optical breakdown at air-water interface are due to the breakdown phenomena occurs in the near surface air layer as well as in the water. Thus, there are two spatially different sources of shock waves and this shock fronts have been registered on the photographs taken at different time delay. Other features are due to the structure of the optical breakdown region - the plasma channel of the extended laser spark in near surface layer of liquid, which is distinct from the similar region for the optical breakdown in the bulk of water with a large distance between focus and cuvette walls. It appears that the geometry of breakdown region and the structure of the shock waves emitted on the times scale smaller than 1 /spl mu/s, are very sensitive to the laser pulse duration as well as to the wavelength of radiation. The complex geometry of optical breakdown region yields the observable shock waves are the envelope of shock front that can be constructed from the spherical shock fronts emitted at the different time instants. At the same time we used the model of spherical explosion for evaluation of the shock wave velocity and pressure as a function of time that is necessary, but the initial stage in description of this phenomena. The estimation of the parameters of shock front envelope at the most informative time scale (smaller 1 /spl mu/s) is an area of essential interest and is a subject of the further research.