A comparative study of the interaction between microhole sidewall and the plasma generated by nanosecond and femtosecond laser ablation of deep microholes

High-aspect-ratio microholes have many industrial applications, but are difficult to produce. Femtosecond (fs) and nanosecond (ns) laser ablation may produce potential manufacturing solutions. However, the laser-induced plasma–microhole sidewall interaction has not been well understood for laser ablation of deep microholes, which may significantly affect the hole size and/or quality. This interaction has been investigated in this paper. Due to the huge challenges involved in direct experimental observations, a physics-based model is applied as the research tool, which has been verified by measurements from literatures on laser ablation of flat targets (without deep microholes) that are relatively easy to perform. The study shows that under the same laser pulse fluence, the fs laser-induced plasma generates larger transient peak heat flux to the sidewall than the ns laser pulse. However, the high-heat-flux region moves up very rapidly in the hole, and hence the sidewall temperature is not significantly raised and sidewall melting does not occur under the studied conditions. On the other hand, for the ns laser pulse, the induced plasma maintains a relatively high heat flux to the sidewall near the hole bottom for a much longer time, which yields obvious sidewall melting and surface vaporization. The results are consistent with the previously observed sidewall surface morphology for microholes drilled by fs and ns laser pulses in air with the same pulse energy.