Characterization of GHz burst mode femtosecond laser ablationWednesday (24.06.2020) 08:40 - 09:00 Room 1
Ablation using femtosecond laser pulses makes it possible to realize the material processing with precise removal and little thermal damage for diverse materials, thus is used in the wide fields of scientific researches and industrial applications. Recently, in order to explore more efficient ablation process with higher quality, the ablation using a GHz burst mode consisting of a number of femtosecond laser pulses with an interval of an extremely short time has been investigated. [1,2] The GHz burst mode femtosecond laser pulse enables to ablate the target material before the residual thermal energy deposited by previous pulses diffuses away from the processed area. In this context, the average temperature inside the target material is decreased, since the thermal energy contained in the particles ejected by the ablation from the target material is physically carried away (ablation cooling). Consequently, ablation process can be induced at smaller ablation threshold fluence compared with conventional ablation process by continuous irradiation of single pulses. In this study, we investigate the ablation characteristics of various materials irradiated with the GHz burst mode femtosecond laser pulses.
Figure 1 shows the measured waveform of the GHz burst mode operation used in this study. Ten pulse trains with a pulse interval of 205 ps (frequency 4.88 GHz) are generated with uniform pulse energy except for the last pulse. Each pulse train is successively emitted at 200 kHz of repetition rate for the burst mode operation. Figure 2 shows the dependence of the ablation depth on the laser power. The ablation process was carried out for the high-purity oxygen-free copper target by the GHz burst mode shown in Fig. 1 and conventional irradiation scheme of femtosecond laser pulses (non-burst mode) for comparison. In the case of the GHz burst mode, the ablation occurs even at 25 mW of the input laser power, which is smaller than the ablation threshold by the non-burst mode. This result indicates that the GHz burst mode ablation suppresses diffusion of the thermal energy generated in the processing area of the cupper target to enhance the ablation efficiency.