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.

Effect of laser parameters on laser ablation and laser-induced plasma formation: A numerical modeling investigation

“…The plume expansion velocity has been measured in several papers, for the range of laser irradiance studied here (10 9 -10 10 W/cm 2 ), and typical values in the order of 10000 m/s are indeed reported, increasing with laser irradiance [37,57 -59]. The velocity also decreases gradually with time [37,59], in correspondence with our model predictions. Consequently, the corresponding plume dimensions are measured in the mm range after 100 ns, increasing with laser irradiance [30,59,60].…”

“…Calculations are always carried out until 100 ns. The reason is that the model is one-dimensional, hence it assumes forward expansion of the vapor plume, which is only a reasonable assumption for the early stage, e.g., until an expansion distance of about 1 mm (depending on the laser beam spot size) [18,37,48,49]. At later stages, expansion in the radial direction will become important, so that our model will have to be extended to two dimensions (radial symmetry), in order to describe the expansion process.…”

“…[36,66] a temperature of about 25,000 K was obtained, for a laser irradiance of 5 Â 10 8 W/cm 2 , and in Ref. [37] a temperature of 400,000 K was reported for a laser irradiance of 4.6 Â 10 10 W/cm 2 , with a 10 ns pulse at 532 and 1064 nm, and glass samples. At such high calculated temperature values for 10 9 and 10 10 W/cm 2 , as illustrated in Fig.…”

“…Moreover, in Ref. [37] relatively high electron number densities of about 2 -6 Â 10 25 m À 3 were achieved for a Nd:YAG laser of 532 and 1064 nm with 10 ns pulse duration, at a laser irradiance of 4.6 Â 10 10 W/ cm 2 . Finally, in Ref.…”

“…In Ref. [37] comparison is made between a pulse of 10 ns and 35 ps, with a Nd:YAG laser at 532 and 1064 nm, for high laser irradiance, and it is found that the plume temperature and electron density, as well as the plume expansion velocity, increase drastically for the ps-pulse, compared to the ns-pulse [37]. In Ref.…”