Study of the temporal and spatial dynamics of plasmas induced in liquids by nanosecond Nd:YAG laser pulses. 2: Plasma luminescence and shielding.

We describe and analyze the temporally and spatially resolved luminescence and attenuation characteristics of plasmas induced in liquids commonly accepted as models for ocular media by high-irradiance Nd:YAG laser pulses of nanosecond duration. Measurements of plasma generated in different irradiation conditions, performed with the aid of a streak camera, show that, when expansion of the plasma column toward the incoming beam occurs, it coincides with quenching of the luminescence exhibited by plasma regions located closer to the focus, where breakdown has first occurred. Evidence for this effect is given both with impurityfree media, where plasma expansion occurs in a continuous and regular way, and with impurity-rich media, where plasma columns are composed either of unevenly distributed plasma regions or of single plasmas with irregular breakdown starting patterns. Experimental findings have been analyzed using a model of spatially distributed plasma shielding, in combination with a previously developed model of moving breakdown, that well interprets, in liquids, the spatially dependent breakdown starting times.