Material ejection during drilling with a long pulse excimer laser

A shadowgraphic imaging technique is used for studying the interaction between the laser beam and the target material during laser drilling. The used laser is a XeCl excimer laser with a nearly diffraction-limited beam and 175 ns pulse length. We studied how and when the material is removed during drilling and we studied the transparency of the vapour of the target material for the laser beam. The shadowgraphic images show the development of a shockwave whose expansion is in agreement with theory. The removal of material at different times both during and after the laser pulse is studied. During the laser pulse, vapour of most target materials is transparent for the laser wavelength. The vapour of target material is not fully transparent for materials with a strong absorption line at or close to the laser wavelength. The images taken after the laser pulse show that removal occurs by vaporisation as well as melt ejection. The material removal for a series of pulses during the drilling process shows differences in material ejection depending on the hole depth. We showed that the larger part of the material is removed after the end of the laser pulse.