Shot-to-shot correlation of residual energy and optical absorptance in femtosecond laser ablation

In this paper, we perform a shot-to-shot detailed study of how residual thermal energy correlates to the optical absorptance change due to laser-induced surface structural modifications in multi-shot femtosecond laser ablation. We observe an overall enhancement for residual thermal coupling and absorptance in air. Surprisingly, residual thermal coupling in air shows a non-monotonic dependence on pulse number and reaches a minimum value after a certain number of pulses, while these behaviors are not seen in absorptance. In vacuum, however, both suppression and enhancement are seen in residual energy coupling although absorptance is always enhanced. To explain these observations, we suggest that air plasma plays a dominant role in thermal coupling at a relatively low number of applied pulses, while the formation of a cavity plays a dominant role at a high number of pulses.