Inverse dependence of heat accumulation on pulse duration in laser surface processing with ultrashort pulses

Abstract

Laser pulse duration is one of the important parameters that is able to dramatically change ablation efficiency. Shorter laser pulse duration leads to deeper transport of absorbed energy in subsurface material layers. Ultra-fast and non-equilibrium processes are able to remarkably affect residual energy, which is accumulated in the deeper subsurface layers after ablation. On the other hand, undesirably high heat accumulation leads to material overheating with surface oxidation, appearance of liquid phase and, as a result, low processing quality. In this paper, the influence of laser pulse duration on the heat accumulation value was experimentally evaluated for the first time. Heat accumulation was measured with a highly sensitive fast infrared detector. The results were presented in comparison with surface structure analysis, material ablation rate and optical reflectivity. Unexpected inverse dependence of heat accumulation on laser pulse duration was detected. An optimal laser pulse duration near 1 ps corresponds to the maximum heat accumulation value and also highest ablation efficiency in contrast to longer laser pulses. The same optimal pulse duration value was detected for low frequency (250 kHz) and for high frequency (1 MHz) laser surface processing regimes. The maximum heat accumulation value was expected at the longest laser pulses. The results are discussed from the point of view of the two-temperature model.