For the optimization of the laser induced material processing, an understanding of the complete ablation process is required. The ablation of vapor, melt and particles typically starts on a time scale of a few hundreds of picoseconds up to a few hundreds of nanoseconds after the initial excitation and may continue over a period of several microseconds. Using ICCD cameras (ICCD: intensified charged coupled device) this time regime can be examined. The ICCD cameras usually can reach a temporal resolution up to 200 picoseconds by the rapid electronic switching of the gain of an array of photomultipliers. The ICCD camera can be triggered electronically at any desired time by an internal delay generator, so the ablation process can be displayed at any times.
ApplicationsCopyright: Chair for Laser Technology LLT
Using an external light source, for example a laser diode, the ablation process can be investigated by the method of the, so-called, shadowgraphy. Ablated particles shield the light and become visible as a shadow on the ICCD camera.
Ablation of silicon
The ablation of silicon with a femtosecond pulse with a pulse duration of 80 femtoseconds and a fluence of 20 J/cm² was investigated at the LLT. Thereby, a plasma and an expanding material vapor that is released from the irradiated surface can be identified on a time scale of a few nanoseconds. After a few hundred nanoseconds, an ablation of material vapor and big melting drops takes place. Ablated particles can be observed up to a few microseconds.
Removal of dyes
Further, the ablation of solutions of organic dyes of an irradiated surface with a pulse duration of 80 femtoseconds and a fluence of 10 J/cm² has been investigated. The ablation of a material vapor is observed to about 600 ns. From 600 ns the removal of a contiguous melt film and melt droplets is observed. Up to several microseconds, the propagation of the melt film can be observed.Copyright: Chair for Laser Technology LLT