Plasma formation during the interaction of picosecond and nanosecond laser pulses with BK7 glass

Abstract

This presentation deals with the dynamics of the high-irradiance short-pulse laser exposure of dielectric materials in normal atmosphere. Time-resolved recordings of plasma ignition and expansion have been obtained for picosecond (tP = 35 ps) and nanosecond pulses (tP = 10 ns) with identical pulse energy densities at the target. In the case of picosecond pulses, a rather homogeneous plasma layer is created, which remains closely attached to the target surface, reaching a depth of a few tens of micrometres during the pulse duration, tP. Ahead of this layer, rapidly expanding plasma jets could be visualized by shadowgraphy. After laser irradiation the plasma core expands while the jets decay. In contrast to the commonly known theory of laser-supported detonation waves in atmosphere, the plasma expansion up to a time of several tP is mainly driven by radiative energy transport. In contrast, for nanosecond pulses at comparable energy densities, the plasma expansion is largely determined by hydrodynamic processes. Detailed experimental studies have been carried out and the results are compared with numerical results.