Method for determination of the volume of material ejected as molten droplets during visible nanosecond ablation

A novel method is presented for determining the volume of molten material ejected from a substrate as a result of visible pulsed-laser ablation. A 100-microm-wide pulsed-laser light sheet (tau approximately 5 ns, lambda = 532 nm) was used in conjunction with a CCD camera to provide high-speed cross-sectional images of single-pulse ablation of aluminum with a visible nanosecond laser source. Computational analysis of the two-dimensional gray-scale images was used to determine the total volume of material ejected from the substrate in the form of molten droplets. Ablation with dual-wavelength (511- and 578-nm) pulses of 30-ns duration was characterized under various fluence conditions (0-25 J cm(-2)), allowing a quantitative threshold for explosive melt ejection in aluminum to be established at approximately 10 J cm(-2). The temporal evolution of the ejected material showed that, for an incident fluence of approximately 40 J cm(-2), molten-droplet ejection commenced at approximately 400 ns and ceased after approximately 2 micros.