Characteristics and formation mechanism of filamentary plasma string induced by single picosecond laser pulse in sapphire

Characteristics and formation mechanism of filamentary plasma string induced by single picosecond laser pulse in sapphire are studied experimentally and numerically. Relative brightness and spatial distribution of the filamentary plasma string are characterized by time-resolved luminescence images. The whole filamentary plasma string is composed of a leading plasma string with stronger brightness and a tailing plasma string with weaker brightness. The numerical analysis shows that the different characteristics of filamentary plasma string are related to the two types of spatiotemporal evolution stages. The pivotal role of avalanche ionization for different spatiotemporal evolution stages is revealed. The filamentary plasma string induced by single pulse has a guiding significance for the subsequent pulse nonlinear propagation and the material modification; all the above provides basic information for the multi-pulse filamentation and the laser-induced filamentation processing of sapphire.