Evidence of Pressure Waves in Confined Laser Ablation

Abstract

The selective laser structuring of thin molybdenum films from the glass substrate side using ultra-short laser pulses displays a by up to a factor of 10 higher ablation efficiency compared to the direct laser ablation from the film side. Moreover, holes can be created without burrs, melt formations or micro cracks. A recent simulation of the glass side ablation process, also known as confined laser ablation, suggests that the laser-matter interaction in the molybdenum leads to a confined ultrafast thermal expansion during heating and melting, which generates a pressure wave that causes the metal film to bulge. However, this model disagrees with the general understanding that the confinement of an expanding gas is the driving ablation mechanism. In order to verify this new model, a pump-probe microscopy setup is utilized to investigate the propagation of a pressure wave in the glass substrate. The results show that pressure waves can be detected already at fluences that are not sufficient for evaporation and that they are generated during heating and melting. These observations support the model of ultrafast thermal expansion during heating and melting generating a pressure wave that causes the metal film to bulge.