On the saturation effect in the picosecond near ultraviolet laser ablation of polyimide

The saturation effect in the ablation rate is observed in the laser ablation of polyimide at a wavelength of 355 nm and duration of 30 ps. The fluence dependence of the reflectivity, emission intensity, and photoacoustic signal have been measured to provide some understanding of the mechanism for the saturation. A mechanism invoking the blocking of the trailing part of the light pulse by both the plume and the excited polymer generated by the leading part of the pulse explains all the observation satisfactorily. However, the possibility of blocking by only the excited polymer followed by nonstatistical disposal of energy in the ejection process cannot be ruled out.